ADULTICIDES AND ADULTICIDING

The following is taken wholly from Chapter 6 (Adulticides and Adulticiding) 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

Summary

Chemical treatment for adult mosquitoes – adulticiding – is the most visible form of mosquito control. In Florida, ground and aerial applications for one or more of the state’s more than 80 mosquito species are common year-round. These applications may be for pestiferous mosquitoes or mosquitoes that vector disease. The spray treatments typically are Ultra Low Volume.

Adulticides used in Florida include malathion, naled, chlorpyrifos, permethrin, resmethrin, sumithrin, and other products. The decision about which material to use is based on several factors including the efficacy as determined by scientifically conducted field trials, mosquito species susceptibility, safety, and cost. The insecticide choice is made by each mosquito control agency and varies throughout the state due to differing mosquito species and application requirements. Applications are made to coincide with mosquito flight activity so that the insecticide droplets contact the target insects and to avoid the flight activity of non-target insects such as bees and butterflies.

Training and certification are an integral part of adulticiding operations. The Florida Department of Agriculture and Consumer Services oversees the certification of public health pesticide applicators and routinely inspects mosquito control operations. This inspection checks surveillance records to verify the need for chemical applications and reviews application methods and amounts.

6.1 INTRODUCTION

Pest management techniques are many and varied: mechanical, cultural, biological, and chemical. Treatment of adult mosquitoes – adulticiding – is achieved entirely via pesticide applications targeted to adult mosquitoes. The process of adulticiding is a step wise process that often is considered the method of last resort in an Integrated Pest Management (IPM) approach to mosquito control. Information on the biology of the pest organism is required, and thresholds must be determined before treatments begin. Once the thresholds have been met, the target is defined as flying insects, a barrier (vegetation), and/or a solid surface. Then, the appropriate equipment and chemical must be chosen, and the application must be made in a timely fashion. The chemical dose and type has a significant effect on the outcome of an application. The chemical must reach the adult mosquito through the most appropriate use of available methods.

Space sprays typically use Ultra Low Volume (ULV) technology, sometimes referred to as cold fogging. Space sprays are applied with specialized spray equipment mounted in aircraft, on the back of trucks, or even carried by hand. With space sprays, aerosols are released to drift through a target zone. Chemical concentrates most often are used and, even if diluted, volumes of material used remain low. The aerosol persists in the air column for an appreciable length of time at suitable droplet densities to contact the flying mosquito and is only effective while the droplets remain airborne. Hence, a space spray is short-lived and is not expected to have any residual effect.

Where a more long-term effect is required, residual spraying is employed. In this case, the mosquito is required to land on a surface deposit of the insecticide to pick up a toxic dose. Residual sprays often are referred to as barrier or surface treatments. A barrier treatment is applied to prevent adult mosquitoes from moving into an area such as a stadium, park, or resident’s yard and often is applied with a modified vehicle mounted hydraulic sprayer. Interest in this technique is continuing to develop in Florida. A surface treatment is used to kill and/or exclude adults from a harborage area or resting site often around the home. Because the areas treated are generally small, handheld devices such as a backpack mist blower or a compression sprayer are employed. In Florida, surface sprays are used primarily in urban pest management scenarios and are rarely used by mosquito control agencies.

Adulticides are broad-spectrum pesticides and that have the potential to impact non-target organisms. Space spraying relies on the prevailing meteorology to carry the pesticide as small droplets (aerosols) to and through the target area, which increases the probability of off-target drift. To minimize the potential for environmental impact, the applicator needs to understand the methods and equipment used and the potential risks involved.

This chapter discusses current and historical adulticiding and best management practices used by mosquito control programs in Florida. For a discussion on the risks and benefits of adulticiding, see Chapter 9, Mosquito Control Benefits and Risks.

6.1.1 Surveillance and Thresholds

Accurate detection and assessment of the current mosquito population is essential and is achieved through regular monitoring and surveillance programs. Mosquito numbers and distribution patterns are assessed, and this surveillance data is used to determine the area(s) to be treated. Surveillance methods to gather this data vary among mosquito control programs and are discussed in the Chapter 3, Mosquito Surveillance and Environmental Monitoring. Adulticiding should be considered to be the last resort and conducted only when larviciding and cultural control methods are not practical due to concerns about sensitive habitats, or when these methods have failed, and adult thresholds have been exceeded.

When chemicals must be used, IPM strategies aim to maximize on-target deposition and minimize off-target deposition. Adult mosquito control via aerosol application is extremely complex, because it attempts to control numerous species over vast areas and changing habitats in a three-dimensional space. Thresholds, which are the keystone of most agricultural IPM programs, are difficult to establish in mosquito control. To treat or not to treat is typically a response to a nuisance level or an individual perception of the problem, rather than a quantifiable presence or absence of mosquitoes. Thresholds can change with time and location as the human population’s tolerance to biting changes. When there are issues of public health, typical thresholds can be superseded by criteria described in approved emergency response plans.

Setting a realistic trigger or action threshold for management decisions is specific to each mosquito control program and must be in compliance with Section 5E-13.036 of the Florida Administrative Code. Once all the criteria have been met to treat an area, the appropriate application may be initiated.

6.1.2 Timing

Timing is essential for space sprays to target actively flying mosquitoes. The timing needs to be precise because different species are active at different times. In general, most mosquito species targeted by space sprays fly in the crepuscular hours, and, hence, most adulticide applications occur in the crepuscular hours. Problems may arise with timing applications because:

Some targeted species are not active during the crepuscular hours. The Anopheline malaria vector, Anopheles quadrimaculatus in Florida, exhibit a nocturnal activity pattern. They are most active in the middle of the night when their blood hosts (humans and other mammals) are sleeping and usually exhibit no daytime activity. Florida’s common domestic mosquitoes, Aedes aegypti and Aedes albopictus, are day-biters or diurnal. They tend to have peaks of activity during the hours after sunrise and the hours before sunset with less activity during the heat of the day and little to no activity at night. Their activity coincides with the times of highest vehicular traffic in their urban environments and poor meteorology (unstable atmospheric conditions) making aerial or truck adulticiding both impractical and ineffective. Localized spot treatments with handheld equipment are the only effective adulticiding method for these species at this time.

Meteorological parameters also influence mosquito activity and timing of the application. Some general trends are:

The activity of some mosquito species is more affected by meteorological parameters than the activity of other species. The principal vector of St. Louis encephalitis in Florida, Culex nigripalpus, is a prime example; it is very sensitive to meteorological changes..

Timing of residual spraying is not nearly as critical as the timing of space spraying. Residual spraying targets the mosquito in harborage at rest on vegetation or other surfaces. An effective residual spray uniformly coats a target surface with an insecticide that will last an appreciable length of time. Applications should be conducted when conditions are conducive to provide the best coverage. Timing is not critical in relation to mosquito behavior; instead, applications must be made to achieve the best deposit. Winds should be low or favorable to the direction of the target related to the sprayer. Conditions should be dry since while most compounds are considered rain-fast, they need time to dry..

6.1.3 Choosing the Chemical

Once the application type has been determined, the chemical to be applied and the dose rate must be selected. This decision is dictated in part by the size of the application area. For example, large area spraying with some compounds can be cost prohibitive. The habitat can have some influence. For example, the use of some chemicals may have to be restricted around waterways. The species that is being targeted also may affect the choice of compound. The comparative efficacy of one compound over another is disputable, but one thing that is known is the effect that mosquito species, habitat preference, and behavior has on ease of control. For example, Psorophora columbiae in open field is a species that is generally considered easy to knock down, so reduced doses may be applied. On the other hand, Cx. nigripalpus is a cryptic species that often is not active unless meteorological conditions are just right. Maximum label rates and perfect timing may be required to get enough of the spray cloud into the wooded areas to achieve significant control of Cx. nigripalpus.

6.2 ADULTICIDES USED IN FLORIDA

Pesticides kill or alter an organism by disrupting some vital physiological function. The method by which this occurs is called the pesticide’s mode of action. The most typical mode of action involves disruption of the insect’s nervous system. One variation is insect growth regulators which mimic insect hormones and disrupt the insect’s development. Also, soaps and oils affect the exoskeleton of the insect, causing the insect to suffocate or desiccate. The mode of action of mosquito adulticides, however, is only through disruption of neuronal activity. General descriptions of the pesticide classes are provided here along with specific information on the individual compounds used in Florida.

Descriptions of individual compounds include the mode of action, general uses, and non-target toxicity data. The toxicity data is presented as an LC, the lethal concentration that will kill 50 percent of the target population. The LC50 is the most universal measure and allows for comparisons on relative non-target mortality between chemicals. Where possible, toxicity data was accumulated using the Re-registration Eligibility Decisions (RED) of the U.S. Environmental Protection Agency (EPA). RED data, however, are not available for all the compounds used in mosquito control. When data was not available from the RED, the Pesticide Manual (2000), a world compendium of pesticide data, was consulted, and information from this source is marked with an asterisk (*). Further comment on risk assessment, pesticide fate, and the re-registration process is outside the scope of this document..

The Florida Department of Agriculture and Consumer Service (FDACS) tracks and oversees pesticide usage by mosquito control agencies in Florida. Pesticide usage reports (in PDF format) dating back to FY 1997-98 are available for download at http://www.flaes.org/aes-ent/mosquito/reports.html..

6.2.1 Organophosphates - General Description

Organophosphates (OP) generally are acutely toxic and work by inhibiting important enzymes of the nervous system that play a vital role in the transmission of nerve impulses. Nerve impulses usually travel along neurons (nerve cells) by way of electrical signals. However, at the junction between two neurons (a synapse) and between a neuron and a muscle (neuromuscular junction), the impulse is transmitted in the form of a chemical substance (neurotransmitter). The neurotransmitter operating in the autonomic nervous system, neuromuscular junctions, and parts of the central nervous system is acetylcholine. In basic terms, acetylcholine fires the nerve impulse. Acetylcholine is broken down and inactivated in milliseconds by the enzyme cholinesterase. With exposure to OPs, cholinesterase is inhibited, and a build-up of acetylcholine occurs. If acetylcholine is not broken down, the nerve impulse does not stop, ultimately causing paralysis of the insect and eventually death. The organophosphates used in Florida include malathion, naled, and rarely, chlorpyrifos..

6.2.1.1 Malathion

Malathion is used for both ground and aerial adulticide applications. In FY 2004-05, malathion treatments constituted 17.6% of all the acreage sprayed by ground adulticiding but only 1.3% of the acreage treated by aerial application.

Mode of Action: Malathion is a non-systemic contact stomach poison with respiratory action. Malathion is used to control Coleoptera, Diptera, Hemiptera, Hymenoptera, and Lepidoptera in a wide range of crops. It also is used extensively to control major arthropod disease vectors (Culicidae) in public health programs, ecto-parasites of animals, household insects, and for the protection of stored grain products. Like all of the OPs, malathion has one of the lowest mammalian toxicities. One disadvantage is that it has been used for a long time resulting in many cases of localized resistance. Malathion Toxicology: Class III Mammals 50Acute oral LD for rats 390 mg/kg Birds 50LC (8d) for a ring necked pheasant 2369 mg/kg Fish 50LC (69h) for a Bluegill sunfish 30 ppb Invertebrates Daphnia magna (48hr) 1.0 ppb *Bees 50LD (topical 0.71 μg/bee)

Malathion Toxicology: Class III
Mammals Acute oral LD50 for rats 390 mg/kg
Birds LC50 (8d) for a ring necked pheasant 2369 mg/kg
Fish LC50 (69h) for a Bluegill sunfish 30 ppb
Invertebrates Daphnia magna (48hr) 1.0 ppb
*Bees LD50 (topical 0.71 μg/bee)

6.2.1.2 Naled

Naled is the primary chemical used in aerial adulticiding in Florida. In FY 2004-05 naled applications constituted 96.6% of the total area sprayed by aircraft. Although labeled for ground adulticiding, no naled formulations were used for this purpose during this period.

Mode of Action: Naled is a non-systemic contact and stomach poison with some respiratory action. Naled is used to control spider mites, aphids, and other insects on many crops. It also is used in animal houses and in public health for control of insects such as flies, ants, fleas, cockroaches, and extensively for the control of mosquitoes. Naled breaks down rapidly in the environment. This product, however, is highly corrosive and therefore requires special consideration in handling and equipment design.

Naled Toxicology: Class I
Mammals Acute oral LD50 for rats 92 - 371 mg/kg
Birds Canada goose LC50 36.9 mg/kg
Fish LC (24hr) for: Bluegill sunfish 2.2 ppb
Lake trout 87 ppb
Fathead minnow 3.3 ppb
Invertebrates Daphnia magna 0.3 ppb
*Bees 0.48 μg ai/bee

6.2.1.3 Chlorpyrifos

Only two counties used chlorpyrifos in FY 2005-06. One county applied a 13.25% formulation to 620 acres, while another county applied a 5% formulation to 71,000 acres. This quantity was not significant enough to be counted as a percentage of adulticiding reported to FDACS.

Mode of action: Chlorpyrifos is a non-systemic contact and stomach poison with respiratory action. Chlorpyrifos is used to control Coleoptera, Diptera, Homoptera, and Lepidoptera in soil and on foliage. It also is used in the control of household pests, for public health mosquito control, and in animal houses. It is rarely used in mosquito control in Florida.

Chlorpyrifos Toxicology: Class II
Mammals Acute LD50 for rats 97 mg/kg
Birds Mallard duck LC 136 ppb
Fish LC (24hr) for: Bluegill sunfish 1.8 ppb
Atlantic silverside 0.28 ppb
Fathead minnow 0.57 ppb
Invertebrates Daphnia 0.1 ppb
*Bees Toxic to bees 70 ng/bee

6.2.2 Pyrethroids – General Description

Pyrethroids are synthetic chemicals whose structures mimic the natural insecticide pyrethrum. Pyrethrins are found in the flower heads of some plants belonging to the family Asteracae (e.g., chrysanthemums). These insecticides have the ability to knockdown insects quickly. Pyrethrums can be degraded very easily by ultraviolet light which oxidizes the compounds. In general, this phenomenon leads to lower environmental risk. Pyrethroids can pose significant hazards to aquatic organisms, and the potential for build up within sediment is a concern. Pyrethroids are highly toxic to insect pests at very low rates (often one order of magnitude less than OPs). Synthetic pyrethroids have been chemically altered to make them more stable and safer to mammals. Pyrethroids are axonic poisons; they poison the nerve fiber by binding to a protein in nerves called the voltage-gated sodium channel. Normally, this protein opens causing stimulation of the nerve and closes to terminate the nerve signal. Pyrethroids bind to this gate and prevent it from closing normally which results in continuous nerve stimulation. Control of the nervous system is lost, producing uncoordinated movement and ultimately mortality.

6.2.2.1 Pyrethrum

Pyrethrum is used in Florida as an aerial adulticide but accounted for only 0.12% of the total acreage sprayed in FY 2004-05.

Mode of Action: Pyrethrum binds to sodium channels prolonging their opening and thereby causes paralysis with death occurring later. It has a non-systemic contact action and some acaricidal activity. Pyrethrum is used to control a wide range of insects and mites in public health and agriculture. It normally is combined with synergists that inhibit detoxification by the insect. A benefit to its use is that it is considered to be a naturally occurring compound and therefore more environmentally acceptable. It also breaks down rapidly in sunlight, so it has few negative residual effects.

Pyrethrum Toxicology: Class III
Mammals Acute oral LD50 for rats 700 mg/kg for 57% ai
Birds Oral LD50 Mallard duck 5,620 mg/kg
Fish Toxic to fish
LC50 (96h) for: Rainbow trout 5.1 μg/l
Sheepshead minnow 16 μg/l
Invertebrates Daphnia magna LC50 11.6 μg/l
Mysid shrimp LC50 1.4 μg/l
*Bees Toxic to bees but exhibits repellant effect
LD50 (oral) 22 ng/bee (contact) 130-290 ng/bee

6.2.2.2 Permethrin

Permethrin is labeled for ground adulticiding in Florida and is the primary chemical used for this type of application. Permethrin can be used for aerial adulticiding in Florida with specific FDACS permission. It was applied to 76.7% of the total acreage treated by ground adulticiding in FY 2004-05.

Mode of Action: Permethrin is a non-systemic insecticide with contact and stomach action. Permethrin is effective on a broad range of pests. Benefits include good residual activity on treated plants, lack of phytotoxicity when used as directed, and low mammalian toxicity. Additionally, it is one of the least expensive compounds available for adulticiding. A disadvantage is that it is highly toxic to aquatic organisms.

Permethrin Toxicology: Class II
Mammals Acute oral LD50 for rats 8,900 mg/kg
Birds LD50 Mallard duck >10,000 ppm
Fish LC50 (96h) for: Atlantic silverside 2.2 ppb
Bluegill sunfish 0.79 μg/l
Invertebrates *Daphnia 0.6 μg/l
Mysid shrimp 0.019 ppb
*Bees Toxic to bees LD (24h) 0.024 μg
Bee topical 0.13 μg/bee

6.2.2.3 Resmethrin

Resmethrin is used for both ground and aerial adulticiding. In FY 2004-05, resmethrin was applied to 4.5% of the total acreage for ground adulticiding and 1.5% of the acreage for aerial adulticiding.

Mode of Action: Resmethrin is a non-systemic insecticide with contact action and is a potent contact insecticide effective against a wide range of insects. It often used in combination with more persistent insecticides. Benefits include rapid mosquito knockdown properties and a low mammalian toxicity. It is photo-labile so does not persist. The disadvantages are that it is highly toxic to aquatic organisms and relatively expensive.

Permethrin Toxicology: Class II
Mammals Acute oral LD for rats >4639 mg/kg
Birds LD for bobwhite quail >5000 ppb
Fish LC50 (96h) for: Rainbow trout LC 0.28 ppb
Sheepshead minnow 11 ppb
Invertebrates Daphnia magna 3.10 ppb
Pink shrimp 1.3 ppb
*Bees Toxic to bees LD 0.063 μg/bee (contact)