Home Mosquito Management Integrated Mosquito Management Larvicides and Larviciding
The following is taken wholly from Chapter 5 (Larvicides and Larviciding) in the Florida Mosquito Control White Paper developed by the Florida Coordinating Council on Mosquito Control. 2009. Florida Mosquito Control: The state of the mission as defined by mosquito controllers, regulators, and environmental managers. University of Florida; Vero Beach, FL, USA
This chapter focuses on the method of controlling juvenile mosquitoes while in life stages (larvae & pupae) which only occur in water. To safely alter our aquatic environments, even temporarily, for the purpose of controlling mosquitoes, requires a good working knowledge of both the target species and larvicides, which include commercial pesticides and natural predators. Products and techniques currently used in Florida are discussed in detail, and the benefits and risks of each are considered. Commercial pesticide information includes summaries and information provided by manufacturers. Minor differences between various formulations of the same or similar active ingredients are detailed so that the competency of each product may be compared. The old days of smothering everything with one pesticide such as waste oil are gone, and mosquito control is rapidly approaching an age of prescription applications where a competent operator will apply one or a combination of larvicides in an environmentally friendly manner under a given set of conditions.
Commercial pesticide sections summarize data found in manufacturers’ current product literature and labels. Two of many additional sources of information on mosquito larvicides are:
The University of Florida published a handbook (Dean and Nesheim 1998) on correct pesticide applications which covers in depth many topics presented here.
Larviciding is a general term for killing immature mosquitoes by applying agents, collectively called larvicides, to control mosquito larvae and/or pupae. Larval Source Management (LSM) involves both the modification of water habitats, often referred to as Source Reduction (see Chapter 4), and the direct application of larvicides to control mosquito production. Most mosquito species spend much of their life cycle in the larval
stage when they are highly susceptible to both predation (see Chapter 7) and control efforts. They often are concentrated within defined water boundaries, immobile with little ability to disperse, and accessible. Adult mosquitoes, in contrast, fly in search of mates, blood meals, or water sources for egg laying and are often inaccessible, not concentrated, and widely distributed. Therefore, effective larviciding can reduce the number of adult mosquitoes available to disperse, potentially spread disease, create a nuisance, and lay eggs which leads to more mosquitoes.
The effective control of larvae and/or pupae is a basic principle of Integrated Pest Management (IPM). Effective IPM involves understanding the local mosquito ecology and patterns of arbovirus transmission and then selecting the appropriate mosquito control tools. The most common methods of IPM include Environmental Management, or Source Reduction (Chapter 4), Larviciding, and Adulticiding (Chapter 6). Other mosquito control principles include Biocontrol (Chapter 7), as well as additional methods not discussed here such as herbiciding and hand removal of aquatic plants. These methods may be used to control immature mosquitoes indirectly, usually when there is an obligatory association between the larvae/pupae and specific host plants. In Florida, Mansonia and Coquellittidia mosquitoes are associated with aquatic plants.
Common examples of highly concentrated broods include immature Aedes taeniorhynchus and Ae. sollicitans in saltmarsh pools, Psorophora columbiae in flooded pastures, and species such as Culex nigripalpus in wastewater treatment sites. In these situations, most Florida mosquito control programs larvicide as a management practice because it both minimizes the area in which control procedures must be applied and reduces the need for adult control. At these times, larviciding has a high impact on local population numbers with minimal application efforts. At other times, larviciding may be less rewarding because small numbers of larvae and pupae are widely and unevenly distributed. Examples include Culiseta melanura in bay tree swamps, Mansonia species and Cq. perturbans in large freshwater marshes with patchy host plant distribution, and Anopheles quadrimaculatus in large, overgrown grassy retention ponds.
Planning a LSM strategy is crucial to a highly effective control program. The first step begins with adult and larval surveillance. Once surveys have been conducted, it is then important to map out and prioritize potential larval habitats. Treatment thresholds, often based on the number of larvae encountered at a site, should be established to justify larviciding, and action plans appropriate for the sites should be developed.
It is important to select the appropriate control agent and formulation based on performance and other factors. It is critical to have a thorough knowledge of the biology of the targeted species in order to determine the appropriate larvicide, the timing of the application, and the amount of product to be applied. For example, Ae. taeniorhynchus tend to “ball up” when feeding as 3rd instars (Nayar 1985). The larvae are unevenly distributed and the density where they do occur is much higher than at other times in their development when they tend to be more evenly dispersed in salt marsh pools. This situation may call for an application rate higher what is normally used, but never exceeding the maximum allowed on the label. Larvicides may be chosen which exhibit a selective mode of action and have a minimal residual activity or which are not selective and exhibit long-term control. Many larvicides can be applied from either the ground by truck, boat, and hand held devices or by air with fixed wing and rotary wing aircraft, however, some products are not suitable for aerial application. Follow-up efficacy checks are important to ensure a successful larviciding program, and rotation of products should be incorporated into any IPM program.
There is no perfect larvicide for every situation, and each larvicide has its strengths and weaknesses. Larvicides may be grouped into two broad categories: biorational pesticides and conventional, broad-spectrum pesticides. The latter will be discussed in sections 5.2.3 thru 18.104.22.168.
The term “biorational” gained popularity in the climate of environmental awareness and public concern (Williamson 1999). It refers to pesticides of natural origin that have limited or no adverse effects on the environment or beneficial organisms. In order for a synthetically produced pesticide to be classified as a biorational, it must be structurally identical to a naturally occurring compound. Biorational pesticides are comprised of two major categories: 1) Microbial agents (e.g., bacteria) http://www.pesticidebook.com/pdfs/chapter24_pages293-295.pdf and 2) Biochemical agents (e.g., pheromones, hormones, growth regulators, and enzymes).
Schuster and Stansly (2006) more recently defined a biorational pesticide as any type of insecticide active against pest populations but relatively innocuous to non-target organisms, and, therefore, non-disruptive to biological control. An insecticide can be "innocuous" by having low or no direct toxicity on non-target organisms or by having short field residual, thereby minimizing exposure of natural enemies to the insecticide. By this definition, all larvicides registered for use in Florida, when applied according to label instructions, might be considered biorational. There is actually no legally clear, absolute definition of a biorational pesticide (Williamson 1999). The U.S. Environmental Protection Agency (EPA) considers biorational pesticides to have different modes of action than traditional pesticides (http://ipmworld.umn.edu/chapters/ware.htm), with greater selectivity and considerably lower risks to humans, wildlife, and the environment. The EPA lists several larval control agents as “biopesticides” (http://www.epa.gov/oppbppd1/biopesticides/ingredients/index.htm). The terms “biorational” and “biopesticide” overlap but are not identical.
Stories of prodigious numbers of mosquitoes occupy a special place in Florida’s history (Patterson 2004) beginning with 16th century explorers. An 1888 yellow fever epidemic in Jacksonville set in motion the formation of the Florida State Board of Health (FSBH) in 1889. The Florida Anti-Mosquito Association was founded in 1922. The first mosquito control legislation was passed, and the Indian River Mosquito Control District was established in 1925 (Anonymous 1948, Patterson 2004).
Larviciding became prominent when implemented as an area-wide malaria control procedure in the early 1900s, but by then it had been used as a control technique for over a century in Florida (Floore 2006). From the earliest days, two types of larval control were employed: Larviciding as a temporary control method and ditching as a permanent control method (see Chapter 4 on Source Reduction). Larviciding using waste oil or diesel oil products was implemented to control mosquitoes in the early 1800s (Howard 1910). Paris green dust, an arsenical insecticide, was developed as a larvicide in 1865 and, along with undiluted diesel oil, was used through the 1960s (Anonymous 1970). In 1958, the FSBH developed its own Paris-green granular formulation as a general purpose larvicide (Mulrennan 1958). The FSBH went on to develop its own “Florida Mosquito Larvicide” in the 1960s which contained 99% mineral oil (unpublished 24-C label 1967).
After 1945, dichloro-diphenyl-trichloroethane (DDT), a chlorinated hydrocarbon compound, was used as both an adulticide and a larvicide in Florida (Anonymous 1970, Patterson 2004). Mosquitoes became resistant to DDT, and its use was discontinued in the late 1950s. As resistance to DDT increased, malathion, an organophosphate (OP) compound, was used increasingly to control both larval and adult mosquitoes. Soon, resistance to malathion was observed in saltmarsh mosquitoes (Rathburn and Boike 1967). The FSBH then implemented a policy limiting the use of malathion to adulticiding in areas where OP larvicides were not used. Resistance (see Chapter 10) has been a concern of Florida mosquito control agencies (Boike and Rathburn 1968) for many years. Rogers and Rathburn (1964) summarized early agency attitudes toward larviciding: “Although larviciding alone is not regarded as a practical procedure for mosquito control in Florida … the great value of larvicides is fully appreciated.” Attitudes have changed, and by 2006 most mosquito control agencies in Florida had incorporated larviciding as one of their mosquito management practices.
During the ten years that have elapsed since the first edition of this document, a number of larviciding formulations are no longer registered and likely will never again be available as tools for mosquito control agencies. These products include pyrethrum, diflubenzuron, Bonide Mosquito Larvicide (oil), and BVA Chrysalin (oil). Laginex AS (active ingredient Lagenidium giganteum) has not been enthusiastically accepted in Florida or elsewhere in the United States. Some agencies may list predatory minnows which they purchase for larval control as line items in their larvicide budgets, but these fish are considered biocontrol agents. Biocontrol is discussed in Chapter 7. Industry consolidation has placed the stewardship of the remaining larvicides into the hands of fewer manufacturers. Mosquito control professionals must be diligent with applications and guard against the loss of the remaining control agents.
The regulation of larvicides and larviciding is provided for by a set of federal and state acts, statutes, and rules. Oversight includes both regulation of the pesticides themselves and regulation of pesticide applications. The principal controlling law is the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). Chapter 487 Florida Statutes (F.S.), known as “The Florida Pesticide Law”, Chapter 388 F.S. known as “The Mosquito Control Act” and associated Rules outlined in Chapters 5E-2 and 5E-13 of the Florida Administrative Code constitute the State’s authority (http://www.flaes.org/aesent/index.html). The Florida Department of Agriculture and Consumer Services (FDACS), Bureau of Entomology and Pest Control, is tasked with ensuring compliance and regulates and licenses the pest control industry and mosquito control programs.
In accordance with FIFRA and the Florida Pesticide Law, FDACS has established a Pesticide Review Council (PRC) to advise “the Commissioner of Agriculture regarding the sale, use and registration of pesticides and advises government agencies, including the State University System, regarding their responsibilities pertaining to pesticides” (http://www.flaes.org/pesticide/pesticidereviewcouncil.html). The Council serves as a statewide forum for the coordination of pesticide related activities to eliminate duplication of effort and maximize protection of human health and the environment. The PRC consists of eleven scientific members and operates under the authority of Chapter 487 F.S.
The FDACS Division of Agricultural Environmental Services (AES) administers various state and federal regulatory programs concerning environmental and consumer protection issues. These responsibilities include state mosquito control program coordination, agricultural pesticide registration, testing, and regulation, pest control regulation, and feed, seed, and fertilizer production inspection and testing. The AES, Bureau of Pesticides, Pesticide Registration Section “registers federally accepted (FIFRA) pesticides” (http://www.flaes.org/pesticide/pesticideregistration.html) that are distributed, sold, or offered for sale in Florida. Pesticides not requiring federal approval must be registered in Florida to assure adherence with State law. Emergency exemptions from federal registration also are reviewed and processed by the Pesticide Registration Section and submitted to the EPA for action. Special registration actions for new active ingredients, special local needs, significant new uses, and experimental use permits are processed through the Section. To accomplish their mission, members of the Section consult with specialists within FDACS and other state and federal agencies, commissions, and councils.
The Scientific Evaluation Section (SES) of the FDACS, AES, Bureau of Pesticides, includes scientists with expertise in geology, soil science, hydrology, mammalian and ecological toxicology, chemistry, and chemical fate modeling (http://www.flaes.org/pesticide/scientificevaluation.html). The SES provides technical support and has five core functions/programs:
The SES functions and interacts with other stakeholders to ensure the safety of the State of Florida. Many new mosquito control insecticide formulations are evaluated by the Florida Agricultural and Mechanical University, John A. Mulrennan, Sr. Public Health Entomology Research and Education Center (PHEREC) in Panama City.
Chapter 388 F.S. provides the authority for mosquito control activities. The statute includes a provision that public lands may be designated as environmentally sensitive and biologically highly productive, thereby requiring special arthropod control plans for mosquito control activities on those “designated” lands. Many state and federal land management authorities [e.g., Florida Department of Environmental Protection (FDEP), Florida Division of Forestry (FDOF), Florida Fish and Wildlife Conservation Commission (FFWCC), U.S. Fish and Wildlife Service (USFWS)] and regional water management districts designate their conservation lands similarly and have corresponding control plans in place.
The control plans are initially proposed by the mosquito control agency for individual parcels and negotiated with the public land manager until mutually agreed upon. Either party may propose further amendments. There is no overarching agreement that certain control chemicals are approved for all such public lands. For example, in 1987, the Florida Park Service and various mosquito control agencies adopted control plans for many state parks (personal communication, Dana C. Bryan, Environmental Policy Coordinator, Office of the Director, Florida Park Service, December 2006). At that time, products containing Bacillus thuringiensis israelensis (Bti) and methoprene were widely approved for use. Bacillus sphaericus (Bs) had not yet been developed commercially and hence was not included in arthropod control plans. Many subsequent plans include Bs in addition to Bti and methoprene. See Chapters 9 and 13 for additional discussions of mosquito control agency interactions with other government entities.
Mosquito larvicides registered for use in Florida are discussed below within the following classification system:
Insecticide labels usually bear a precautionary signal word. The necessity for a signal word on labels (http://www.epa.gov/oppfead1/labeling/lrm/chap-07.htm) and which word is assigned is dependent upon the results of six separate acute toxicity studies which are performed with each product formulation.
There are a variety of products and formulations within each larvicide classification. Specific formulations are different from manufacturer to manufacturer. Application rates and suggested treatment sites may differ as well.Individual product labels and material safety data sheets (MSDS), usually downloadable from manufacturers’ web sites, should
be consulted for specific information, habitat dependent application rates, and restrictions, if any. FDACS should be consulted to ensure that a specific product is labeled for use in Florida.
The initial identification of a natural juvenile hormone (JH I) in insects occurred in 1967 and was followed rapidly by the discovery of JH II and JH III (Henrick 2007). JH is involved in the regulation of physiological processes in insects including mating and metamorphosis. Research was initiated in 1968 to determine if insect pests could be selectively controlled – without environmental concerns – by developing synthetic mimics of the natural JH. Since JH does not occur in vertebrates, it was expected that selective insecticides could be developed. Sacher (1971) reported on a group of chemicals that mimic juvenile hormone activity. These chemicals appeared to block naturally occurring ecdysone from initiating molting processes and inducing metamorphosis in mosquito larvae. Staal (1975) discussed several methoprene analogs that interfere with normal insect growth and maturation. Abnormal larval growth patterns plus malformed or smaller than normal forms were observed. The first IGR, which contained several methoprene isomers, was registered in 1975 (Henrick 2007). Methoprene products currently are the only IGRs registered for use in Florida.
Methoprene (Isopropyl (2E, 4E, 7S)-11-methoxy -3,7,11 -trimethyl-2,4-dodecadienoate) is a terpenoid compound. Technical methoprene is an amber or pale yellow liquid with a faint fruity odor (http://extoxnet.orst.edu/pips/methopre.htm), which is slightly soluble in water and is miscible in organic solvents. Methoprene is a synthetic mimic and a true analog of naturally occurring JH found in mosquitoes and in other insects.
JH is found throughout the larval stages of a mosquito, but it is most prevalent during the early instars. As mosquito larvae mature, the level of naturally occurring JH steadily declines until just prior to the 4th instar molt, when larvae develop into pupae. This time is a sensitive period when all the physical features of the adult begin to form. Methoprene is absorbed through the insect’s outer "skin" or cuticle and may be incidentally ingested or enter the body through other routes. The level of applied methoprene (parts per billion) in the larvae’s water environment must be higher than the level of juvenile hormone circulating in the larvae’s body in order for the disruption of endocrine processes to occur. Therefore, the application of methoprene larvicides is most efficacious during late 4th instar. Treated larvae reach the pupal stage and then cannot emerge to become adults. Since pupae do not eat, they eventually deplete body stores of essential nutrients and starve to death. Incomplete adult emergence is an indicator of methoprene efficacy.
Methoprene is listed (http://www.epa.gov/oppbppd1/biopesticides/ingredients/index.htm by the EPA as a biopesticide. Methoprene based larvicides are General Use Pesticides (GUPs). Methoprene-based larvicides have undergone extensive studies both prior to and after registration to determine risk to humans and non-target organisms. When used according to label directions, methoprene is considered extraordinarily safe for humans and almost all non-target organisms. Methoprene does not produce nondiscriminatory, rapid toxic effects often associated with central nervous system toxicants. The lethal effects of methoprene are based on the disruption of the insect’s endocrine system mediated developmental processes, such as metamorphosis and embryogenesis. Consequently, control of mosquito larvae is relatively slow.
Methoprene is effective in a wide variety of both fresh and saltwater habitats. It is relatively selective for target species, and lingering mosquito pupae serve as a food for fish and other predators. The IGR is particularly effective against Aedes larvae. Methoprene does not bioaccumulate; it degrades into simpler compounds. Since ultraviolet light deactivates methoprene, many formulations incorporate activated charcoal or other dark inert substances to prolong product life. Early methoprene manufacturing products included two mirror-image molecules called r- and s-isomers. The racemic isomer (r-methoprene) is not active on mosquitoes. Improved manufacturing techniques allow current formulations to contain only active s-methoprene isomers. Methoprene labels bear the “CAUTION” signal word.
Microbial larvicides are formulated to deliver a natural toxin to the intended target organisms. Bacteria are single-celled parasitic or saprophytic microorganisms that exhibit both plant and animal properties and range from harmless and beneficial to intensely virulent and lethal. Bacillus thuringiensis (Bt), is the most widely used agricultural microbial pesticide in the world, and the majority of microbial pesticides registered with the EPA are based on Bt. The Bt serovar kurstaki (Btk) is the most commonly registered microbial pesticide, and this variety has activity against Lepidoptera (butterflies and moths) larvae. It was originally isolated from natural Lepidopteran die-offs in Germany and Japan. Bt products have been available since the 1950s. In the 1960s and 1970s, the World Health Organization (WHO) encouraged and subsidized scientific discovery and utilization of naturally occurring microbes. As a result of those early studies and a whole body of subsequent work, two lines of mosquito control products have been developed: crystalline toxins of two closely related gram-positive, aerobic bacteria – Bacillus thuringiensis israelensis (Bti) and Bs. Mosquito control agents based on Bt are the second most widely registered group of microbial pesticides. Highly successful Bti products have expanded the role of microbial agents into the public health arena (de Barjac 1990). Reviews of microbial agents may be found in Lacey 1985, Lacey 2007, and Singer 1985.
Bacillus thuringiensis is a bacterium which occurs naturally in soils and aquatic environments globally. In 1976, Goldberg and Margalit (1977) isolated Bti from Culex pipiens collected in an Israeli riverbed. In 1977, de Barjac designated this Bt strain as H14, noting that it is toxic to mosquito and black fly larvae. Over the last three decades, a number of other strains have been investigated, some with desired larvicidal effects. Two strains, SA3A and FM65-52, are currently utilized for commercial products.
The active ingredients in Bti formulations are delta-endotoxin (d-endotoxin) crystals separated from bacteria near the end of manufacturing processes. These toxic crystals are incorporated into various products which allow their release into water so that they may be ingested by mosquito larvae. The d-endotoxin crystals are activated by the alkaline environment and enzymes of the mosquito midgut. The alkaline gut environment allows hydrolysis of the crystal’s protein coating and the release of pro-toxins. Gut enzymes then activate the pro-toxins and facilitate their binding to the gut epithelium of the mosquito larva. Cells rupture and are destroyed at the binding sites, leading to a loss of body fluids which results in death. This rapid action typically controls larvae in 4-24 hours.
Bacillus thuringiensis israelensis is listed by the EPA as a biopesticide (http://www.epa.gov/oppbppd1/biopesticides/ingredients/index.htm). Bti based larvicides have undergone extensive risk studies both prior to and after registration. Bti products are GUPs and are safe for non-target organisms in the environment. The crystalline d-endotoxins are not activated in the acidic guts of humans or other animals or in the alkaline guts of animals which do not contain the enzymes necessary for activation and binding of released pro-toxins. This specificity accounts for the highly selective nature of Bti larvicides which is limited to Dipterans, notably mosquitoes, black flies, and some midges. Bti controls all larval instars provided they are still feeding. It is effective on most mosquito species in a very wide variety of habitats; Bti formulations are thus ideally suited for IPM. Bti product labels show the potency of the product as the number of International Toxic Units (ITU) available. This value is more meaningful than the weight percent of the active ingredients, as it characterizes the formulation’s effectiveness. ITU values are determined by a standardized laboratory bioassay which uses 4th instar Culex quinquefasciatus. Prepared volumes of toxins are applied to living mosquito larvae and the resulting mortality data provide a numerical measure of activity. Bti labels bear the “CAUTION” signal word.
Bacillus sphaericus is a naturally occurring spore-forming bacterium found throughout the world in soil and aquatic environments. Kellen and Myers (1964) isolated Bs from Culiseta incidens larvae in California. Early studies were conducted on Bs strains isolated by the Pasteur Institute, while the commercial products discussed below are based on strain 2362 isolated in Nigeria. Lacey (2007) reported that serovarieties with the most pronounced larvicidal activity are 1593 and 2362. Some strains produce a protein d-endotoxin at the time of sporulation which is toxic to many species of mosquito larvae upon ingestion. Bacillus sphaericus acts in a manner similar to Bti, except it has been shown to recycle in intact Culex cadavers, thus maintaining some residual activity (Becker et al. 1995). Once larvae ingest these Bs d-endotoxins, they are partially digested (their protein envelope is dissolved) in the alkaline gut, enabling the release of pro-toxins. These pro-toxins in turn are activated by enzymes and attach to the gut wall where they begin to disrupt, paralyze, and rupture the gut.
The activity of Bs d-endotoxins differs from that of Bti in several important ways. Bs toxins are attached to a living bacterial spore while the Bti toxins are not. The toxins of Bs and Bti bind to chemically different receptor cell sites. They are not related immunologically and are thought to have completely different molecular modes of action. Operationally, the most important differences between the toxins are speed of action and persistence in the larval habitat. Bs toxins are much slower acting than Bti toxins and can be more persistent. Bs has a slower settling rate, and the spores can invade the body cavity of the larvae where they have the capability to germinate, grow, and produce toxins. This process is known as recycling.
Bs is listed (http://www.epa.gov/oppbppd1/biopesticides/ingredients/index.htm) by the EPA as a biopesticide. Bs based larvicides are GUPs, which have undergone extensive risk studies both prior to and after registration. The crystalline d-endotoxins are not activated in the acidic guts of humans or other animals or in the alkaline guts of animals which do not contain the enzymes necessary to activate the pro-toxins. This specificity accounts for the highly selective nature of Bs larvicides; they do not target as wide a range of mosquito species as do Bti products. Formulations containing Bs. are most active against Culex and Anopheline larvae and less active against some Aedes larvae. Formulation effectiveness depends on the mosquito species and environmental conditions including water quality. In general, the best immediate results with Bs are obtained when applications are made to larvae in the 1 to 3 instars. Larval mortality may be observed as soon as a few hours after ingestion, but typically it takes as long as two to three days depending upon dosage and ambient temperature. Adequate recycling of Bs for sustained control is dependent on the presence of dead mosquito larvae.
International Toxic Units (Bs ITU) values are determined by a standardized laboratory bioassay similar to that developed for Bti H-14. The bioassay uses 3 and 4 instar Cx. quinquefasciatus. The signal word “CAUTION” appears on Bs product labels.
The term organophosphate (OP) refers to all pesticides containing phosphorus. OPs were discovered in Germany during a search for a substitute for nicotine, which was heavily used as an insecticide but was in short supply. The insecticidal qualities were first observed there during World War II (http://ipmworld.umn.edu/chapters/ware.htm). OPs have been used for mosquito control since the early 1950s. OPs work after entry into and distribution through the body of a target organism by modifying the normal functions of some nerve cells by inhibiting the activity of cholinesterase enzymes at the neuromuscular junction. This action results in the accumulation of acetylcholine, thereby interfering with neuromuscular transmission. In insects, OPs produce a loss of coordination leading to paralysis and ultimately death.
Temephos (O,O’-(thiodi-4, 1-phenylene) O,O,O’,O’-tetramethyl phosphorothiolate) is an OP compound. During the 1960s, temephos was studied extensively as a replacement for the persistent organochlorine DDT in malaria control programs. It was registered as a mosquito larvicide in 1965. A review of Florida pesticide use records indicates that temephos has been utilized in the state since 1969.
Temephos is currently the only OP registered for use as a larvicide in Florida. It is labeled for use in many habitats including tidal marshes, woodland pools, polluted water, tires, and as a pre-hatch treatment. Temephos is often recommended as a rotation larvicide where it is used in place of the microbial or IGR larvicide in an IPM program. Temephos is a GUP with a low toxicity when used according to the label with little or no detrimental effects on non-target organisms. Temephos is one of the least toxic OPs to mammals (http://extoxnet.orst.edu/pips/temephos.htm). Product labels bear either the signal word “WARNING” or the signal word “CAUTION.”
Surface oils and films used as larvicides include oils and ethoxylated isostearyl alcohols.
As previously noted, surface oils, such as waste motor oil and diesel, were the first larvicides used for mosquito control in Florida. Howard (1931) considered low grade kerosene or fuel oil more satisfactory than other larvicide methods. The State of Florida developed its own “Florida Mosquito Larvicide” oil, also called the “Florida Formula”, in the 1960s, but by the 1980s, crude formulations such as these were losing status in Florida. Studies had begun on potential replacement products such as Arosurf, a thin layer alcohol-based surface film (Mulrennan 1982), and highly refined petroleum oils (Mulrennan 1983). New oil formulations replaced the “Florida Formula” by the mid 1980s (Mulrennan 1986). The new thin layer surface films and highly refined oils are virtually colorless and odorless (Floore et al. 1998), and they exhibit the same larval and pupal control properties as the waste oils they replaced.
The larviciding oils are probably the least studied of the mosquito larvicides, despite their long period of use for mosquito control. Specific control mechanisms are difficult to pinpoint but likely include poisoning of the larvae (pers. comm., E. J. Beidler, Indian River MCD). Oils also can suffocate – but only at the very highest dosage rates. Inert ingredients include emulsifiers which help them spread over the water’s surface and kill larvae and pupae when inhaled into the tracheae along with air. With low dosages (e.g., 1 gallon per acre), oils can work very slowly, taking four to seven days to provide control. Higher dosage rates (3-5 gallons per acre) are usually used to decrease the control time. Surface oils also are considered one of the most effective tools for pupal control and can control newly emerged adults that are resting on the water surface when drying their wings.
Larviciding oils are GUPs that are non-selective, and mosquito control efficacy is limited to those species which breathe air at the water surface. They have a low toxicity when used according to the label with minimal detrimental effects on non-target organisms. An "oil slick" can be viewed on the water surface. Both their odor and appearance may be objectionable, precluding widespread use in some areas. Larviciding oil labels bear the “CAUTION” signal word.
Monomolecular films (MMFs) are biodegradable, ethoxylated alcohol surfactants, made from renewable plant oils. MMFs are lighter than water and do not mix particularly well with it. As their name implies, MMFs produce an extremely thin film on the water’s surface. They were originally developed by the U.S. Navy during World War II to help remove oil slicks. MMFs have been widely used in the cosmetics industry for over 30 years as a component of skin care products. Monomolecular films were investigated as mosquito larvicides and pupicides beginning in the early 1980s. Nayar and Ali (2003) have reviewed MMFs and their mosquito control uses.
Monomolecular surface films do not kill by toxic action but exert a physico-chemical impact on mosquito populations (pers. comm., Richard Levy 2007). When applied, they spontaneously and rapidly spread over the surface of the water to form an ultra-thin film that is about one molecule in thickness. They act by significantly reducing the surface tension of the water and wetting mosquito structures, which leads to drowning. Mosquito adults, eggs, larvae, and pupae utilize the surface tension of water in various aspects of their life cycle. With the surface tension reduction, mosquito larvae, pupae, and emerging adults cannot properly orient at the air-water interface and will eventually drown. Adults of both sexes that utilize the water surface for normal resting, and adult females who use the surface for oviposition also may drown. Eggs and egg rafts of certain species may not float normally or may sink and become unviable. Monomolecular surface films can affect species that depend on the air-water interface. They may be used safely in potable waters, waters bearing fish and other aquatic organisms, and in runoff waters that enter fish-bearing waters. Monomolecular film labels bear the “CAUTION” signal word.
Mixing materials “on-site” to formulate products has historically been popular with mosquito control operations in Florida. Applying liquid larvicides to granular carriers has been the most widely used type of home-made formulation. One early product involved applying Paris-green liquid to light-weight silica particles (pers. comm., E. J. Beidler 1996). Another notable practice involves combining two mosquito larvicides into a single-end product in order to take advantage of the properties of each component ingredient. The most widely used of these on-site formulations and larvicide combinations are discussed below.
Methoprene Sand Granules are on-site granular formulations that are produced by combining liquid methoprene with washed sand. Thirty years ago, this process was developed at the Indian River Mosquito Control District in Vero Beach, and the formulation was named “Altosand” because Altosid Liquid Larvicide was used as the active material (pers. comm., E. J. Beidler 1996). Altosand was developed primarily to control mosquitoes in densely canopied mangrove swamps and coastal salt marshes where it is often necessary to penetrate dense canopies. Methoprene sand granules, prepared on-site, are used in Florida.
Bti Sand Granules were not available as commercial formulations until the latter part of 1996. However, technical Bti powder and labeling has been available since the mid 1980s to allow end-users to make their own "on-site” Bti sand granules. Sand formulations require coating the particles with oil (GB-1111 or BVA 2) and then applying dry Bti powder which will stick to the oil. Although Bti technical powder is currently produced by both Becker Microbials and Valent Biosciences, it is not commonly used in on-site formulations in Florida. Bti sand granules are no longer produced commercially; they are, however, still produced “on-site” by mosquito control operations in western states.
Duplex is the name that has been attached to the end-user formulation which is made by combining Bti liquid and liquid methoprene. This mixture was developed principally to control larvae such as Culex spp. where many different instars may be present. The rationale for this mixture is that lethal Bti doses are somewhat proportional to a mosquito larva's body size and therefore less Bti is required for control of early instars. The opposite is true for methoprene which is most effective after 4th instars have stopped eating and the amount of methoprene required for control is the least. Combining Bti with methoprene theoretically allows operations to use less of each product than if using only one product. The Pasco County Mosquito Control District (PCMCD) occasionally duplexes both products at maximum dosages for control at sites with a large synchronous brood of both 3 and 4 instars. In a variation on this process, PCMCD also has combined liquid methoprene with Bti granules to produce an on site “Granular Duplex" formulation. A product combining Bti and Bs is commercially available.
Monomolecular Films used with other larvicides have been investigated. Levy et al. (1982, 1984) reported significantly improved efficacy of several larvicides when formulated with ethoxylated alcohol surfactants. The authors indicated that “the use of mixtures of Agnique MMF or Agnique MMF mosquito larvicides and pupacides with other mosquito biolarvicides, IGRs, and/or central nervous system inhibitors has been shown to enhance the translocation of the bioactive agents over the surface of the water and provide improved joint-action mosquito-controlling efficacy.” The dual-action larvicide formulations also are expected to be a good tool for use in resistance management programs. The use of a variety of ethoxylated alcohol surfactants that are approved by the EPA for use as inert materials in pesticide formulations is being evaluated on an operational basis as adjuvants for a variety of conventional mosquito larvicides. Lee County Mosquito Control District (LCMCD) has been using a mixture of a MMF and temephos for many years as a joint-action larvicide that rapidly spreads over the water surface (pers. comm., W. Gale and R. Levy 2006).
PCMCD employed a MMF-temephos combination for several years with low doses of the monomolecular film used as a spreader and temephos as the intended active ingredient. However, a laboratory study with this mix introduced into long gutters populated with live larvae showed that temephos by itself spread nearly as well as when mixed with a MMF and that there was a slight tendency for reduced mortality when the two were combined (Wassmer, unpublished data). Consequently, the mixture was abandoned in favor of a temephos and water only mix. The results suggest a need for further study.
Pesticide usage reports (in PDF format) dating back to FY 97-98 are available for downloading at http://www.flaes.org/aes-ent/mosquito/reports.html. A number of special taxing districts, municipalities, developments, golf courses, and individuals throughout Florida also conduct mosquito control operations but do not report activities to FDACS.
During the fiscal year beginning October 1, 2004 and ending September 30, 2005 (FY 04-05), 58 mosquito control agencies in Florida reported monthly pesticide usage to FDACS. For FY 04-05, the 58 agencies reported larvicide applications on 385,900 acres. Corresponding FY 94-95 totals reported by 50 agencies in the first printing of this document showed that 458,937 acres were treated with larvicide. Many of additional agencies reporting for FY 04-05 were started in response to the Florida WNV outbreak which began in 2002. They did not have the time or the budgets to fully develop IPM programs, and adulticiding was the predominant control method employed. Ground larviciding was reported by 49 of 58 agencies (85%), and 18 of them also reported aerial larviciding. In contrast, only 80% (40 of 50) of reporting agencies for FY 94-95 made larvicide applications. The absolute number of agencies that larvicide (49 versus 40) increased by 23%, and the number that larvicide aerially (18 versus 15) increased by 20%. Ground larviciding totaled 172, 816 acres (average 3,527 acres), while aerial larviciding totaled 213,024 acres (average 11,835 acres).
Florida mosquito control operations employ a variety of larviciding equipment for both aerial and ground applications, as necessitated by the wide range of larval mosquito habitats, target species, and budgetary constraints. Each operation typically will use more than one type of application equipment. There are advantages and disadvantages to each application system used and to the aerial and ground treatments themselves.
Almost all Florida mosquito control agencies use some type of four-wheel drive equipment as a primary larvicide vehicle. In most cases an open-bed pickup is equipped with a chemical-container tank, a high-pressure, low-volume electric or gas pump, and a spray nozzle. A switch and an extension hose allow the driver to operate the equipment and apply the larvicide from inside the truck's cab. Some agencies have the sprayer mounted on the front bumper of the truck and install a mechanical control that allows the driver to direct the spray while remaining in the cab. Roadside ditches, swales, retention ponds, treatment ponds, and other similar bodies of water can be treated with this setup.
Increasingly, mosquito control agencies are moving towards the use of all-terrainvehicles (ATVs), which allow operators to reach larval habitats that are inaccessible by truck. These units can carry a reasonable payload allowing operators to treat a number of remote sites consecutively without having to return to replenish pesticides. As with a truck, a chemical container is mounted on the ATV, a 12-volt electric pump supplies a high-pressure low-volume flow, and a hose and spray tip allow for manual application by an unaccompanied operator while steering the ATV with the other hand. ATVs are ideal for treating areas such as agricultural fields, pastures, salt marsh areas, and other off-road sites. Training in ATV safety and handling should be provided to employees operating these machines.
Ultra Low Volume (ULV) machines also can be mounted in the bed of the truck or on the back of an ATV to apply larvicides. These setups require the installation of a gas engine and compressor plus a metering system to accurately control output (see Chapter 6 for a detailed description of ULV systems). ULV applications of liquid larvicides from the ground were introduced in the late 1980s and early 1990s. Current applications are limited primarily to the use of hand-held ULV machines. ULV larviciding allows the product to drift into inaccessible areas. A more common use of ULV equipment involves diverting air from the compressor to propel granules and briquets into the target habitat via special granule nozzles or pneumatic guns.
Additional equipment used in ground applications includes dippers, horn seeders, handheld sprayers, and backpack blowers and sprayers. Dippers and horn-seeders may be used to broadcast small amounts of granular or pelletized larvicides in spots that require minimal treatment. Hand-held sprayers are standard one- or two-gallon garden style pump-up sprayers used to treat small isolated areas with liquid larvicide formulations. Backpack sprayers usually have a gas-powered blower with a chemical tank and calibrated proportioning slot. Generally, pellet or small granular material is applied with a gas-powered backpack sprayer. They are extremely useful for treating tire piles. Pump-up backpack sprayers are sometimes used for dispensing liquid larvicides.
There are several advantages to using ground application equipment when on foot or from vehicles. Ground larviciding allows more accurate pesticide applications to the intended treatment area and consequently to only those micro-habitats where larvae are actually present. Ground larviciding applications are less affected by weather conditions than are aerial applications and are less susceptible to drift and product deposition outside the intended treatment area. This feature reduces the likelihood of unnecessary pesticide load on the environment and the financial cost of wasted pesticide. Also, initial and maintenance costs of ground equipment are generally less than those for aerial equipment.
With ground application, there is greater risk of chemical exposure to applicators than aerial larviciding. Ground applications rely on human estimates of both the size of treatment areas and of equipment output during pesticide applications. Calibration of the applicators to the equipment can be difficult since an applicator’s pace can vary, especially in areas with uneven terrain. It is difficult to provide even coverage with manually-operated ground equipment, and the possibility of under-applying or over-applying a larvicide is problematic. Ground larviciding is impractical for large, inaccessible, or densely wooded areas.
Many of Florida’s organized mosquito control operations have adopted aerial larviciding as a control strategy on otherwise large, unmanageable larval mosquito habitats. Agencies may not actually own the aerial equipment, as agricultural flying services can be contracted to apply larvicides as needed. Outsourcing the usually seasonal activity of aerial larviciding eliminates the need for and expense of an aircraft purchase, aircraft maintenance costs, and the expenses associated with having a pilot and perhaps an aircraft mechanic on staff.
Aerial larviciding is accomplished via fixed wing or rotary aircraft. Both types of aircraft can apply both solid and liquid larvicide formulations. A variety of hoppers, nozzles, and metering systems can be adapted to the aircraft, depending upon the desired equipment configuration and its size. The decision on whether to use liquid or granular applications depends on the target habitat and prevailing meteorological conditions.
Granular formulations provided by manufacturers incorporate a paper product, sand, gelatinous material, or corncob particles as the carrier for the active ingredient. Granules also may be prilled (pelletized) and contain little if any carrier. One prilling process is similar to that of making large snowballs, where the active ingredients are continuously packed onto a small seeded core as the ball of material is slowly rolled in a rotating tray. The tilt of the tray and the rotational speed help determine the resulting product size, as larger balls of material roll off the edge. In some instances, agencies can formulate their own granular materials (e.g., sand mixes). Most granular formulations are applied at rates ranging from 6 to 20 pounds of product per acre.
Deciding which larvicide formulation to apply is critical for successful control efforts. There is considerable debate about which formulations are best for each mosquito control program. Debates often focus on habitat differences and which product type (liquid or granule) will best reach the target habitat or combination of habitats to be treated. The relative efficacy of pesticide types, their initial cost, the costs of any mixing, and the costs of loading and ferrying the pesticides to the application sites also needs to be considered.
With liquid applications, there is debate over the ideal droplet size and carrier. Wind, temperature, evaporation, and droplet movement have major impacts on the success or failure of ULV applications. Using large droplets eliminates some of the drift problems of ULV. Low volume or ULV applications of undiluted liquid products (no water added) maximize acreage per load, thereby reducing overall costs. Diluting liquid products increases the costs of loading and ferrying and greatly reduces the payload. However, dilution may allow the application of more droplets within an application site, which in some circumstances may lead to a better presentation of the toxicant to the mosquito larvae and thus better control.
Liquid larvicides can deposit and stick on foliage, reducing the amount available for larval control. Using small droplets or ULV may reduce the loss due to canopy impaction, but the amount of material actually reaching the target under these conditions is not well documented. Some organizations attempt to minimize losses by using “raindrop” nozzles which produce extremely large droplets. These large droplets are thought to “punch” their way through the canopy, but this concept needs evaluation; this type of application may render overall efficacy unacceptable for some target areas with specific canopy types and density. Despite these shortcomings, ease of product handling and relatively lower product costs combine to make liquid larviciding a viable operational option.
Dry pesticides formulations such as powders utilize bulky and/or heavy carriers to prevent them from drifting away from target application sites. New formulations such as prilled granules may eliminate some of the weight and bulk, but they are essentially unknown to Florida mosquito control operations. Granular products, in contrast to liquid formulations, usually have less drift and are less apt to stick to foliage, allowing somewhat better penetration. Granulars are not as easy to handle as their liquid counterparts because of their bulk (e.g., corncob formulations) or their weight (e.g., sand formulations). Initial costs (especially the costs of premixed formulations) tend to be higher than the initial costs of closely related liquid formulations. Aircraft load weight restrictions limit the amount of granules per load and thus the number of acres that may be treated as compared to diluted liquid formulations. In addition, pilots and their mechanics are extremely cautious about applying formulations containing sand or other hard carriers with turbine driven aircraft.
Over the past decade since the first printing of this document, the Florida Mosquito Control Association (FMCA) has held annual aerial application workshops, called the Aerial Short Courses, at LCMCD in Ft. Myers, Florida. The courses have included expert presentations on relevant pesticide application topics, field demonstrations, and actual on-site application research. In addition, during the period of 2001 through 2004, extensive aerial field trials were conducted in Pasco County, Florida, to evaluate canopy penetration for typical over-stories and to determine the potential for improved penetration as a function of both emitted liquid droplet size and corncob granule size in six common habitats. Pasco County is located on the west coast of Florida and contains southern coastal vegetative communities dominated by mangroves and northern coastal vegetative communities dominated by rushes. Field trials (Mickle 2002b, 2004, 2005) and knowledge gained at the aerial short courses (Mickle 2002a) are discussed below and in the next section, 22.214.171.124 Measuring and Perfecting the Application of Aerial Larvicides. They may be downloaded from the PCMCD website at http://www.pascomosquito.org/oldsite/Research_Development.htm or obtained by contacting the District office.
Results of the studies shown in Figure 5-1 indicate a significant difference between what is applied and what reaches the target surface for all formulations tested. Although Valent Biosciences products were used in the study, the results should apply to all Bti brands. For Bti liquid (Vectobac 12AS, Teknar HP-D) applications, penetration analyses revealed relatively low deposits under all canopy types. In addition, difficulty in removing dried deposits from droplet samplers suggested that foliage deposits would most likely not be washed off by subsequent rainfall events.
Penetration was highest through the saltwort (Batis maritima) site and the black needle rush (Juncus romerianus) site (which normally does not produce mosquitoes), yet only 35% and 45%, respectively, of the liquid mix reached the ground, regardless of droplet size. At other test sites, results were even less impressive. Application over 10-inch high knotgrass (Distichilis spicata) resulted in over 97% of the 80 micron drops and all of the 300 micron drops depositing on the vegetation. Generally, sprays above the taller 10 foot -30 foot canopies of Brazilian pepper (Schinus terebinthifolius), black mangrove (Avicennia germinans), and cabbage palm (Sabal palmetto) with live oak, (Quercus virginiana), longleaf pine (Pinus palustris), and red maple (Acer rubrum) resulted in less than 30% of Bti liquid mix penetrating to the ground. Smaller droplets tended to penetrate canopies slightly better than larger droplets. These results are generally in agreement with a study (Pierce et al. 1989) by Mote Marine Laboratory in Lee County, Florida, where only about 20-70% of a temephos-water mixture reached the ground when applied over dense black mangrove habitats at five gallons per acre with raindrop nozzles.
In contrast to the liquid applications, more than 50% of either size of the granules penetrated the canopies at all sites. At the knotgrass site, 10/14 grit granules performed better than the larger 5/8 granules with 82% versus 50% penetration. In the black mangrove habitat, 100% of both the 5/8 and the 10/14 grit granules penetrated the canopy to the substrate. Because only 40% or less of the liquid pesticide mix reached the ground, and more than 50% of the Bti granules reached the ground in the same habitats, the local mosquito control agency decided to use 10/14 grit granular formulations wherever it was practical for aerial larval control. This practice is an example of selecting the best formulation through a rigorous examination of the pertinent factors.
When attempting to control larvae and/or pupae of many Florida mosquito species, complete coverage of the larval mosquito habitat is critical. Missing just a tiny fraction of the target area can result in the emergence of huge numbers of biting adults. A pilot must be completely familiar with the application equipment and know what kind of swath width to apply for each product under different environmental conditions. A pilot must know the mosquito-producing habitats and know when to apply “heavy” in order for enough pesticide to reach the water’s surface to establish control. While many pilots claim that they can fly accurate swaths based on their skill alone, some type of guidance and offset system is necessary when performing aerial larviciding over large areas.
Spray system calibration is necessary to ensure that pesticides are being applied according to label requirements. For liquid formulations, a spray calibration confirms that the droplet size distribution is appropriate. For both liquid and granular larvicides, swath characterizations and trial applications highlight the need for modifications that should provide the best chance for uniform deposit at labeled rates. In an effort to assist in spray calibration efforts, two free companion software programs – Grainalysis and Stainalysis – have generously been made available by REMSpC Spray Consulting at Grainalysis: REMSpC Granular Deposit and Larval Mortality Analysis Tool
The Grainalysis program can be used to calculate the deposit characteristics of granular swath-characterization trials from input data including product weight, number of granules, and measured larval mortality at each sampler. Output, available in tabular or graphic form, is displayed relative to the aircraft flight line and includes deposit (kilograms per hectare or pounds per acre), cumulative deposit fraction, number of granules per unit area (square feet or square meters), number of granules per gram weight of product, and mortality. Swath analyses of deposit and granular uniformity also are available.
The Stainalysis program can be used to analyze drop characteristics on Kromekote cards that have been scanned using any flatbed scanner (256 color depth). BMP, GIF and TIFF file formats are supported. Notch filtering allows for stain discrimination by color. Correcting for spread factor, an output file for each card includes: spray documentation, digitization documentation, drop density, deposit volume (ounces per acres or liters per hectare), volume median diameter, and the contribution of individual drop sizes to volume fraction, number fraction, and cumulative volume fraction.
At the January 2002 FMCA Aerial Short Course, aerial swath calibration and application efficiency were demonstrated (Mickle 2002a). A Bell 47 Soloy helicopter equipped with an Isolair granular spreader was used to apply 10/14 grit corncob blanks at an operational application speed of about 50 miles per hour and an altitude of 50 feet. Analysis of deposit patterns revealed that aerial applications were somewhat similar when flying into the wind and when flying with the wind. In both of these cases, there was a tendency to deposit more (about twice as much) on the outer edge of the swath and less under the flight line, with heaviest deposits on the left edge of the swath. The data showed that spreader adjustments were needed to smooth out the distribution of granules across the swath.
These observations highlighted the importance of both calibrating equipment prior to an application and maintaining a constant speed for which the application equipment was calibrated when applying pesticide. The latter can be facilitated using an onboard Global Positioning Systems (GPS) device capable of measuring ground speed as the pesticide is being applied. The second phase of the demonstration revealed that areas of high deposit paralleled the flight lines and that significant deposit variation occurred along the flight path, i.e., the contour lines did not align with, but crossed the flight lines. The author (Mickle 2002a) concluded that further modification to the delivery system could have provided a more uniform deposit across the swath, which in turn should have resulted in less deposit variability. The trials also pointed out the desirability of an onboard GPS ground-speed readout, which would allow the pilot to compensate for wind experienced during applications.
Deposit variability can be minimized only through rigorous calibration programs and optimum flight-path positioning. Using flaggers is a simple alternative to the use of GPS guidance if the influence of wind has been considered in advance. One or two flaggers use a flag or other signaling device on each end of the treatment area and pace off a measured distance for each swath. The pilot is guided by the flaggers, who then pace off the next swath, and so on. While not practical for all areas, when used it greatly increases the accuracy of the treatment coverage.
With today’s electronic environment, a ground-to-air radio also may be employed where a field technician on the ground guides the pilot by pointing out landmarks that are easily seen from the air. This arrangement works especially well at small sites and where there is dense canopy since it is often impractical to flag these areas. Another method is to show the pilot a recent aerial photo of the target site during the period when the aircraft is being loaded and explain which spots are to be treated. In contrast, contour flying of large areas while applying pesticides in multiple swaths requires special GPS equipment because no one is capable of recalling all of the necessary landmarks to maintain proper lane spacing.
With improved GPS equipment, new computer-guidance programs for aircraft are now available. These new systems can accurately track the mission parameters (e.g., treatment area, coordinates of treatment area, swath width, etc.) and provide the pilot with almost instant necessary course corrections. In addition to improving treatment accuracy, these systems log flight information which may be downloaded and used to produce a map or a visual display, providing mosquito control operations with accurate records of treatments.
The number of programs utilizing aerial larviciding has been increasing in recent years suggesting that there are advantages to larviciding by air. Aerial larviciding poses a lower risk of chemical exposure to applicators than ground larviciding. Aerial applications can be more economical for large sites, especially when larvae are distributed throughout the area. Utilizing aircraft is often the only way to treat remote sites and those sites inaccessible by ground equipment. Calibration is simplified by the fact that target areas are often mapped, and the larvicide to be applied is usually measured or weighed when loading.
If the costs of the aircraft and aircraft maintenance are included, it is generally more expensive to aerially larvicide than to perform ground applications. To ensure accuracy in hitting the target, either additional labor for flagging or an expensive electronic guidance system is needed. As with all aerial applications, treatment windows can be narrow due to adverse weather conditions. Aerial applications also require special licenses, staff training, and additional liability insurance.
Historically, mosquito control agencies have adopted the general view that larviciding is typically not as effective or as economical as permanent source reduction but is usually more effective than adulticiding. However, this view was derived long ago when wetlands were not considered to be as important as they are today. Many of the compounds used were different as were costs in terms of money, manpower, and equipment. It was easy to assume that it was "cheaper in the long run" to move dirt and change the hydrology of an area than to apply pesticides. With federal, state, and local government agencies strongly advocating that wetlands not be drained, the engineers who ran control operations had only to decide if it was "cheaper" to chemically control larvae or adults, and larval control through water manipulation won out.
The enlightened view of modern mosquito control professionals includes a strong commitment to minimizing environmental impacts. They recognize that undisturbed wetlands should remain pristine and that any disturbance will have long-term effects on non-target species of plants and animals. Source reduction in these areas should be avoided. One debate is over how to simultaneously manage mosquitoes in wetlands and at the same time maximize the wetlands’ value to ecosystems. Our modern approach to mosquito control is reflected by the FMCA’s commitment, along with the American Mosquito Control Association (AMCA), as a Partner in the EPA’s Pesticide Environmental Stewardship Program (PESP) since the late 1990s (http://www.epa.gov/pesp/).
Many mosquito control professionals once believed that it was often illogical to attempt larviciding. However, advances in application technology, product formulations, and the ability to predict larval development have led to larviciding success in areas considered unmanageable even ten years ago. While larviciding is not always the preferred control alternative in all situations, it is a key component of an effective IPM program. There is no single answer to mosquito control that can be applied to all circumstances.
A successful IPM program relies on a variety of control methods and often on a combination of management techniques. As a practical matter, a director will view an agency’s entire area of responsibility before making an informed decision on whether or not to employ source reduction techniques, larvicides, or adulticides to control mosquito populations. The director must carefully weigh potential risks and benefits associated with each method in an integrated program and then utilize the method that is most appropriate.
Selecting the proper class of larvicide and the formulation are both important in larval resistance management. See Chapter 10 for detailed explanations of how pesticide resistance occurs and for resistance management techniques. The FDACS, Bureau of Entomology and Pest Control discourages control agencies from using the same (or any) OP compound to larvicide when it or another OP is used to adulticide because this practice may lead to resistance.
Resistance also may arise by applying sublethal dosages. Many people feel that the EPA erred when it began allowing the market (cost) to dictate what the low dosage would be, despite the recommendations on the product label. Insects with inherent tolerances for weakly applied pesticides may survive to produce tolerant offspring. Soon, an entire population of tolerant mosquitoes may arise. Beyond recommended use periods, slow-release formulations may cause resistance if larvae are exposed to sublethal doses of the active ingredients. Agencies that use slow-release formulations should be aware of this possibility and monitor treatment sites.
Dame et al. (1998) reported resistance to methoprene in an island population of Ae. taeniorhynchus in Lee County after control problems were noted in areas treated with extended life (briquet) formulations. However, the issue appeared to be local. The Florida Keys had been using methoprene briquets since the early 1980s. Floore et al. (2002) reported no methoprene resistance in Florida Keys’ Ae. taeniorhynchus populations at sites also controlled with slow-release formulations when control levels were compared to those for a susceptible colony at PHEREC.<
The loss of any mosquito larvicide because of resistance would have a tremendous impact on Florida mosquito control operations. Proper product rotation – along with susceptibility monitoring – are the keys to ensuring that the pesticides currently available to mosquito control professionals remain effective for continued use.
Currently used mosquito larvicides, when applied properly, are efficacious and environmentally safe. Typically, there is less concern for the drift of mosquito larvicides than for the drift of adulticides, primarily due to the droplet size. Larvicides are typically dispensed aerially through spray systems producing larger droplets (300 - 400 microns) for canopy penetration, while adulticides are applied as smaller droplets (15 - 60 microns) for space spraying. Mosquito larvicides usually are applied directly into natural and artificial aquatic habitats as liquid or solid formulations, and aerial drift is negligible. Drift into water can result from tidal flushing or rainwater runoff. Under these conditions, dilution greatly reduces post-application pesticide concentration and consequently reduces exposure to non-target organisms.
It is possible to reduce non-target exposure to larvicides by using novel application techniques and new product formulations. Larviciding with machines that produce fine airborne particles, such as Bti applied with rotary atomizers or turbines, spreads the larvicides so that the concentration of active ingredients at any one point is minimized. In addition, these techniques may have the added benefit of allowing control agents to drift to inaccessible containers and remote aquatic habitats. Larviciding with fine particles is not widely practiced in Florida or elsewhere in the U.S. The LCMCD is currently developing slow-release technology for larvicides. Using different granular carriers, these new formulations provide better canopy penetration and larval control, while reducing the acute exposure rate for non-target organisms.
A variety of aquatic habitats and communities, ranging from small domestic containers to larger agricultural and marshland areas, are treated with larvicides. Natural fauna inhabiting these sites may include amphibians, fish, and invertebrates, particularly insects and crustaceans. Frequently, the aquatic habitats targeted for larviciding are temporary or semi-permanent. Permanent aquatic sources usually contain natural mosquito predators such as fish and do not require further treatment, unless littoral vegetation is so dense that it prevents natural predation. Temporary sites such as tidal marshes, flooded agricultural areas, and woodland depressions produce prolific numbers of floodwater mosquitoes. These sites are generally very low in species diversity due to the time needed for most species to locate and colonize them (Ward and Busch 1976, Pierce et al. 1991). While floodwater mosquitoes develop during the first week post-inundation, it may take several weeks for the first macro invertebrate predators to become established. Finally, many non-target species exploiting temporary aquatic habitats are capable of recovering from localized population declines via recolonization from proximal areas. Currently used larvicides, applied properly, have no known phytotoxic effects.
The use of any pesticide always involves a tradeoff between desired effects (effective control) and undesired side effects. No known larvicides are exempt from this conundrum. Even the seemingly innocuous use of predatory fish may result in an unwanted or unknown impact on an aquatic community, however temporary. More effective methodologies are needed to apply larvicides that will minimize undesirable impacts. As a group, mosquito control agencies constantly seek new and better application techniques. Mosquito control professionals are committed to the development and evaluation of new materials, as shown by the activities of numerous university and mosquito control scientists around the state.
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