Mosquito Management
Integrated Mosquito Management
AdulticidingHome
Mosquito Management
Integrated Mosquito Management
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. 1998. Florida Mosquito Control: The state of the mission as defined by mosquito controllers, regulators, and environmental managers. University of Florida; Vero Beach, FL, USA
Chemical treatment for adult mosquitoes, adulticiding, is the most visible form of mosquito control. In Florida, both ground and aerial applications are common year round, directed at one or more of the 74 species occurring in the state. These applications may be for pestiferous or disease vectoring mosquitoes. Typically, the treatments are either ultra low volume (ULV) or thermal fog spraying.
Adulticides used include, malathion, fenthion, naled, chlorpyrifos, permethrin and resmethrin. The decision of which material to use is based on several factors that include: Efficiency 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 because of differing mosquito species and application requirements.
Applications are made to coincide with mosquito flight activity. This is either predawn or after sunset. Not only is this time selected for mosquito activity it, by intention, avoids flight activity of non-target insects such as bees and butterflies.
Training and certifications are an integral part of the adulticiding operation. The Florida Department of Agriculture and Consumer Services oversees the certification of those involved in the application of adulticides and routinely inspects the operation. The inspection includes, the actual need for chemical applications as determined by mosquito surveillance and a review of the amount and application methods used for insecticide treatments and that they meet established requirements.
Treatment of adult mosquitoes (adulticiding) is the most visible practice exercised by mosquito control operations. Every mosquito control district (MCD) in Florida adulticides to some extent, using aerial and/or ground application. The most common forms of adulticiding are ultra low volume (ULV) and thermal fogging. Ground adulticiding is almost exclusively conducted with ULV equipment. Aerial applications usually use ULV treatments while some use thermal fogging. The efficiency of adulticiding is dependent upon a number of integrated factors. First, the mosquito species to be treated must be susceptible to the insecticide applied. Some Florida mosquitoes are resistant or more tolerant to some adulticides, thus affecting the selection of chemical. All insecticide applications must be made during periods of adult mosquito activity. This factor is variable with species. For example, Wyeomyia vanduzeei, the bromeliad-inhabiting mosquito, is diurnal (daytime biting), the salt-marsh mosquito Aedes taeniorhynchus is crepuscular.
Treatments directed at Ae. taeniorhynchus activity would completely miss the flight activity of Wy. vanduzeei. Adulticiding should be timed when the mosquitoes are flying and exposed to the aerosol mist.
The chemical application has its own set of conditions that determine success or failure. The application must be at a dosage rate that is lethal to the target insect and applied with the correct droplet size. Some mosquito control districts may be applying adulticides based on economical attractiveness, rather than entomological effectiveness.
Whether the treatment is ground or aerially applied, it must distribute sufficient insecticide to cover the prescribed area with an effective dose. Typically with ground applications, vegetated habitats may require up to three times the dosage rates that open areas require. This is purely a function of wind movement and its ability to sufficiently carry droplets to penetrate foliage.
Environmental conditions also may affect the results of adulticiding. Wind determines how the ULV droplets will be moved from the output into the treatment area. Conditions of no wind will result in the material not moving from the application point. High wind, a condition that inhibits mosquito activity will quickly disperse the insecticide too widely to be effective. Light wind conditions (< 10 mph) are the most desirable because they move the material through the treatment area and are less inhibiting to mosquito activity. Thermal fogs perform best under very light wind conditions.
ULV application should be avoided during hot daylight hours. Thermal conditions will cause the small droplets to quickly rise, moving them away from mosquito habitats. Generally, applications are made between sunset and sunrise, depending upon mosquito species activity. Some mosquitoes (Culex and Anopheles) are most active several hours after sunset, while others (Ae. aegypti and Ae. albopictus) are more active during the daytime, and if these species are the targets, application should be made during the period of highest activity for the target species, provided that meteorological conditions are suitable for application (seldom during daylight hours). One notable exception to treatments made when mosquitoes are up and flying is a residual barrier treatment application. Barrier treatments are based on the natural history and behavioral characteristics of the mosquito species causing the problem. Barrier applications use a residual material and are generally applied with a powered backpack sprayer to preferred resting areas and migratory stops in order to intercept adult mosquitoes hunting for blood meals. Barrier treatments are often applied during daylight hours as a large-droplet liquid application and are designed to prevent a rapid re-infestation of specific areas, such as recreational areas, parks, special-event areas, and private residences. Barrier applications can help provide control of nuisance mosquitoes for up to one week or longer.
One of the following signal words will occur on each adulticide label (see Applying Pesticides Correctly, USDA/EPA, 1992):
CAUTION -- This word signals that the product is slightly toxic. An ounce to more than a pint taken by mouth could kill the average-size adult. Any product which is slightly toxic orally, dermally, or through inhalation or causes slight eye and skin irritation will be labeled "CAUTION."
WARNING -- This word signals that the product is moderately toxic. As little as a teaspoonful to a tablespoonful by mouth could kill the average-size adult. Any product which is moderately toxic orally, dermally, or through inhalation or causes moderate eye and skin irritation will be labeled "WARNING."
DANGER --This word signals that the pesticide is highly toxic. A taste to a teaspoonful taken by mouth could kill an average-size adult. Any product which is highly toxic orally, dermally, or through inhalation or causes severe eye and skin burning will be labeled "DANGER."
(See Appendix IV for a material cost comparison.)
The organophosphates, which have been used for mosquito control since the early 1950's, comprise the insecticides most commonly used for adult mosquito control in Florida. They generally are least expensive to use. Organophosphate toxicity is due to their inhibition of cholinesterase. This inhibition interferes with the neuromuscular junction that ultimately causes paralysis of the insect. The organophosphates used in Florida include: Malathion (Fyfanon®), fenthion (Baytex®), naled (Dibrom®), and chlorpyrifos (Dursban®). Concerning safety when handling organophosphates, it should be kept in mind that they are readily absorbed through the skin.
Introduction: Malathion (=Fyfanon®), a general-use pesticide, is the most widely used groundapplied adulticide in Florida, primarily because of its lower cost compared with other approved adulticides. It is also used for aerial treatments. Malathion's label contains a "CAUTION" warning indicating that it is only a slightly toxic material. While malathion is used against all targeted mosquito species in Florida, it is least effective against Ae. taeniorhynchus. Several locations in Florida that have conducted susceptibility studies have demonstrated some natural tolerance to malathion by Ae. taeniorhynchus and several other mosquito species. Additionally, several years of adulticiding with malathion may have furthered this species' resistance, forcing an alternative chemical selection.
Formulations and Dosages: Malathion is most commonly applied as a ground ULV spray; thus no mixing or dilution is needed. Fyfanon® (9.79 lb. AI/gal.) dosages range from 18.0 (0.0255 lb. Active Ingredient (AI)/acre)-25.8 fl. oz./mi. (0.0548 lb. AI/ac.). Less common are, the thermal fog applications where malathion is diluted 6-8 oz./gal. with a suitable oil (usually diesel oil) carrier and then applied at up to 40 gal./hr. at a vehicle speed of 5 mi./hr., or multiple thereof. The aerial application rate is 2.6 (0.20 lb. AI/ac.)-3.0 fl. oz./acre (0.23 lb. AI/ac.).
Target Species: In Florida, mosquitoes produced from freshwater habitats are the primary target species for malathion.
Benefits: The greatest benefits of malathion are its relatively low cost in comparison to other adulticides, its safety, and ease of use. It exhibits low mammalian toxicity, is only moderately toxic to birds and is variably toxic to aquatic organisms. It breaks down rapidly in the environment. Risks/Disadvantages: Malathion's greatest disadvantage is its inefficiency in controlling some adult mosquitoes at labeled rates. Malathion also has a strong odor, a propensity to cause paint damage, and a slow knockdown.
Introduction: Fenthion (=Baytex®) is used by many mosquito control districts in Florida. This product is applied with both ground and aerial equipment. Fenthion's label contains a "WARNING" for this restricted-use pesticide.
Formulations and Dosages: Baytex Liquid Concentrate (9.67 lb. AI/gal.) is applied as ULV from the ground with a maximum dosage rate of 14.4 fl. oz. (=0.03 lb./ac.). Aerial applications may either be ULV at a rate ranging from 0.66-1.3 fl. oz. (0.05-0.10 lb. AI/ac., or thermal at a rate of 0.4 fl. oz. (0.03 lb. AI/ac. mixed with 0.2-0.8 quarts of oil. Fenthion's biggest disadvantage is its relatively high cost in comparison to malathion or Dibrom®, and it also is a relatively slow-acting insecticide.
Target Species: Baytex® is an effective adulticide for most Florida mosquito species. Benefits: Experience has demonstrated that Baytex® is an effective, safe adulticide that provides good control benefits.
Risks/Disadvantages: It is more expensive than some of the other available adulticides, has moderate mammalian and fish toxicity, and if applied at rates 5 to 10 times higher than that needed for mosquito control, can be very toxic to birds. It has a relatively slow knockdown. Culex nigripalpus resistance to fenthion has been demonstrated in some parts of the state.
Introduction: Another organophosphate, naled (Dibrom®) is applied both from the ground and air. The Dibrom® label contains the signal word "DANGER."
Formulations and Dosages: Both ground and aerial ULV are common application techniques for Dibrom®, with ground thermal treatments being the least common. From the ground, Dibrom® is applied up to a rate of 1.2 fl. oz./ac. (=0.02 lb. AI/ac.), and from the air it is applied up to a rate of 0.5-1.5 fl. oz./acre (1.0 fl. oz. = 0.1 lb. AI/acre).
Target Species: This adulticide is effectively used on all mosquito species in Florida. Benefits: Dibrom® is a fast-acting insecticide that degrades rapidly under typical environmental conditions and exhibits little residual activity. Dibrom® hydrolyzes in water at 70F within 16 hours. Dibrom® additionally decomposes by biological degradation quite rapidly. Residuals of this insecticide are virtually undetectable 24 hours post-treatment. Typically, during an outbreak of St. Louis encephalitis (SLE), aerial application of Dibrom® is the mosquito control method of choice. Risks/Disadvantages: Difficulties associated with Dibrom® include its corrosive nature and its irritation to humans. Special application, storage, and handling equipment must be used because of its highly corrosive characteristics. Dibrom®, under certain environmental conditions, may be very irritating to humans, either from inhaling the droplet mist at close range from the output of ground ULV equipment or from getting the ULV droplets in the eyes. The probability of irritation can be lessened by using an elevated output point or by mechanical introduction of air by turbine or fan to dilute the ULV output. Aerial applications of Dibrom® diminish the irritability of this material considerably.
Introduction: Chlorpyrifos (=Dursban®) is a general-use pesticide distributed by Clarke Mosquito Control under the brand name of MosquitoMist® and is used in some ground ULV programs in Florida. MosquitoMist's® label contains the signal word "CAUTION," indicating that the formulation is slightly toxic.
Targeted Species: This adulticide is believed to be effective for most of the species in Florida. Formulations and Dosages: MosquitoMist® is most commonly applied by ground ULV equipment and is available in 1.0 and 1.5 lb. AI/gal. formulations, is formulated with a high- grade mineral oil, and has USDA approval for incidental contact with food. This product is labeled for both ULV and thermal fog applications. In these low formulations, Dursban® has a low toxicity thus the signal word "CAUTION." Research at the John A. Mulrennan, Sr. Arthropod Research Laboratory (JAMSARL) has indicated that ground ULV labeled rates elicit very poor mosquito mortality. Dosage rates equivalent to that label for aerial application in other states are required to attain efficacy similar to that obtained with malathion.
Benefits: MosquitoMist® is a low-cost, ready-to-use product that is low-odor, non-corrosive and has a relatively quick knockdown. MosquitoMist® is biodegradable, will not bio-accumulate, does not leach through soil, and in aquatic environments binds/adheres readily to particulate matter. Risks/Disadvantages: The Florida Department of Agriculture and Consumer Services (FDACS) has not approved this product for aerial applications in Florida because of its larvicidal, as well as adulticidal, characteristics. Unintentional larval habitat treatment with Dursban® could potentially lead to future resistance of mosquitoes to that product or group of products. If applied above mosquito control label rates, it can be toxic to fish and other aquatic organisms. 6.2.2 Pyrethrins and Pyrethroids: General Description
Natural pyrethrins (pyrethrum) are extracted from chrysanthemum flower heads, mainly Chrysanthemum cinerarnaefolium, grown commercially in parts of Africa and Asia. The six pyrethrins are esters of three cyclopentenolone alcohols: Pyrethrolone, cinerolone, and jasmolone with either chrysanthemic acid or pyrethric acid. In spite of the different isomers possible, the six natural pyrethrins are invariably dextrorotatory isomers of the trans form of the carboxylic acids. Synthetic analogues of the natural pyrethrins reached commercial success in the 1950's. The first commercial product, allethrin, represented the ester of racemic allethrolone with racemic cis/transchrysanthemic acid. Bioallethrin is the same ester formed from the natural dextrorotatory trans form of chrysanthemic acid. Other 'first generation' synthetic pyrethroids such as phenothrin and tetramethrin, like the natural pyrethrins, are relatively unstable in light. During the 1960's and 1970's, great progress was made in synthetic light-stable pyrethroids, mainly by Japanese workers studying phenylacetic acid esters (which led to fenvalerate) and by the Elliot team at Rothamsted with esters of the dichlorovinyl analogues of chrysanthemic acid (which led to permethrin and cypermethrin). These photostable pyrethroids represent the 'second generation' of these compounds.
Pyrethroids exhibit rapid knockdown and kill of adult mosquitoes, characteristics that are considered a major benefit of their use. The mode of action of these compounds relates to their ability to affect sodium-channel function in the neuronal membranes.
Synthetic pyrethroids are not cholinesterase inhibitors, are non-corrosive, and will not damage painted surfaces. They are less irritating than other mosquito adulticides and have a less offensive odor. In comparison to other adulticides, pyrethroids may be effectively applied at much lower rates of AI per acre. Disadvantages of the pyrethroids are that they may be less effective in controlling Aedes taeniorhynchus than in controlling other mosquito species and that their cost is relatively high. The synthetic pyrethroids are mimics of natural pyrethrum, a botanical insecticide. Natural pyrethrum, sold under several trade names, is registered in Florida but is used sparingly due to higher cost.
Introduction: Natural pyrethrins are compounds that are not photostable. Pyrocide 7067® is a labeled natural pyrethrin, whose label contains a "CAUTION" statement. It contains 5% pyrethrin with piperonyl butoxide (PBO) at a 1:5 ratio.
Formulations and Dosages: Pyrocide 7067® is applied as a ULV spray with a dosage of 0.0025 lb./ac. (PBO at 0.0125 lb./ac.).
Benefits: Pyrocide treatments result in the rapid kill of adults.
Risks/Disadvantages: It is an expensive material to use and is very toxic to fish.
Introduction: Sumithrin® (=d phenothrin), is a first-generation synthetic pyrethroid used in Florida. Like permethrin and resmethrin discussed below, it is commonly mixed with PBO as a synergist. Sumithrin® is a general-use insecticide with a 30-year history, biodegrades rapidly, and has a label that contains the signal word "CAUTION."
Formulations and Dosages: Sumithrin's® maximum recommended dosage rate is 0.007 lb. AI/acre and is applied with ground ULV equipment.
Target Species: Sumithrin® is used against all Florida mosquitoes; however, it may be less effective on Ae. taeniorhynchus than on other species.
Benefits: Sumithrin® exhibits the favorable characteristics of rapid knockdown, odorlessness, and non-corrosiveness. It biodegrades rapidly and does not bio-accumulate. Sumithrin® is a generaluse insecticide with very low mammalian toxicity and is practically non-toxic to birds.
Risks/Disadvantages: Like other synthetic pyrethroids, Sumithrin® can be an expensive adulticide to use.
Introduction: Resmethrin is another of the first-generation synthetic pyrethroids used in Florida. Resmethrin, like permethrin, is a photolabile pyrethroid compound formulated as the active ingredient in products such as Scourge®. Resmethrin is similar to the other pyrethroids in providing rapid knockdown and quick kill of adult mosquitoes. Resmethrin exhibits very low mammalian toxicity, degrades very rapidly in sunlight, and provides little or no residual activity. As a result, resmethrin was granted a food-additive tolerance.
Formulations and Dosages: Resmethrin products are available in several concentrations that range from 1.5-40.0% and may or may not contain PBO. Scourge® products, containing resmethrin PBO, have a maximum rate of application of 0.007 lb. AI/ac. of the active ingredient. Currently, Scourge® is a restricted-use insecticide with labels that contain the signal word "CAUTION."
Target Species: Resmethrin is used against all Florida mosquitoes; however, it may be less effective on Ae. taeniorhynchus than on other species.
Benefits: Resmethrin provides rapid knockdown of adult mosquitoes and very low mammalian toxicity, is practically nontoxic to birds, and degrades very rapidly in the environment. Scourge® exhibits little toxicity to bees under field-application conditions. When aerially applied to caged bees, Scourge® produces minimal mortality. Based on photolysis studies, the half-life of Scourge® in water varies from 19 to 37 minutes (depending on pH and salinity) with full decomposition in 205 minutes. Additionally, the solubility of resmethrin in water is extremely low.
Risks/Disadvantages: Laboratory studies indicate that resmethrin is potentially toxic to fish. However, with rapid photo degradation in water and low-use rates for mosquito control, the risk to fish welfare is minimal. The high cost of resmethrin is also a disadvantage of this adulticide.
Introduction: Permethrin, a second-generation pyrethroid, is a photostable pyrethroid compound formulated as the active ingredient in products such as Punt®, Permanone®, and Biomist®. Permethrin is similar to other pyrethroids in providing rapid knockdown and quick kill of adult mosquitoes. However, permethrin also provides some residual activity when applied directly to surfaces. Permethrin is a general-use pesticide with labels that may contain either the signal word "WARNING" or "CAUTION," depending on the particular product.
Formulations and Dosages: Permethrin products are available in various concentrations, from 1.5% to 57.0%, and may or may not be synergized with PBO. Synergized permethrin products may contain PBO in various ratios by weight, but the maximum rate of application is 0.007 lb. AI/ac. Permethrin products, if labeled for this use, may be applied at a maximum of 0.1 lb. AI/ac. for a "barrier" effect, whereas rates up to 0.007 lb. AI/ac. may be used for vehicle-mounted ULV applications.
Target Species: Permethrin is used against all Florida mosquitoes; however, it may be less effective on Aedes taeniorhynchus than on other species.
Benefits: Benefits of using permethrin include a rapid adult mosquito knockdown, odorlessness, non-corrosiveness, and very low mammalian toxicity. It is practically nontoxic to birds. Permethrin is rapidly adsorbed onto organic matter in watercourses, resulting in very low aqueous concentrations. Permethrin is available in ready-to-use formulations.
Risks/Disadvantages: With ULV applications, the residual activity of permethrin could be considered a disadvantage although deposition rates are actually too low to cause problems. Permethrin, like many pyrethroids, is toxic to fish, but with low use rates, the risk to fish welfare is minimal. The relatively high cost of some permethrin formulations compared to some adulticides can be a disadvantage of this material. A risk of this synthetic material is the potential of some insects to more quickly develop resistance to it compared to organophosphates.
Introduction: Carbamates, like organophosphates, inhibit acetyl cholinesterase resulting in the over-stimulation of the central nervous system. Baygon MOS® was labeled for use as a mosquito adulticide and contains 1.02 lb. AI/gal. Baygon® is no longer being manufactured. Over the past several years, it has only been used by one Florida mosquito control district (MCD), Lee County MCD (LCMCD) for ground ULV applications. Ficam® (bendiocarb) is another carbamate insecticide now being evaluated for adulticide use.
Target Species: When used by LCMCD, Baygon® was targeted against Aedes taeniorhynchus. Formulations and Dosages: Baygon MOS® can be applied as a ground ULV spray by using 1.25 to 4 fl. oz./ac.
Benefits: Although not formally documented, Baygon® was believed to be a preferable coldtemperature product, very effective against Aedes taeniorhynchus.
Risks/Disadvantages: Baygon® is toxic to birds, shrimp, and crabs. Baygon® is registered for adult mosquito control in Florida, but is not recommended for use because of its potential larvicidal effects. Unintentional larval habitat treatment could potentially lead to future resistance of mosquitoes to that product or group of products.
Ground adulticiding is the most commonly used method of controlling mosquitoes in Florida today and in some counties is often perceived by the public as the only method used. In 1995, mosquito control districts reported adulticiding 19,409,257 acres by ground. This is 78% of all of the adulticiding acreage reported (aerial applications accounted for 5,535,708 acres).
Ground adulticiding consists of dispersing an insecticide as a space spray into the air column which then drifts through the habitat where adult mosquitoes are flying. Much of the language on insecticide labels does not address the requirement for drift. This type of application is contradictory to everything the agricultural applicators strive for when trying to stick material to plants. There are two techniques of mosquito control insecticidal space spraying in use at this time: Thermal aerosol and ultra low volume (ULV) cold aerosol.
The only other form of ground treatment for adult mosquitoes in use today is a residual barrier application to foliage.
5E-13.036 (Florida Administrative Code) Demonstrable Increase or Other Indicator of Arthropod Population Level states:
Mosquito and other arthropod control programs will insure that the application of pesticides are made only when necessary by determining a need in accordance with specific criteria that demonstrate a potential for a mosquito-borne disease outbreak, or numbers of disease vector mosquitoes sufficient for disease transmission or defined levels of, or a quantifiable increase in numbers of pestiferous mosquitoes or other arthropods as defined by Section 388.011(4) FS To determine the need for the aerial application of adulticides, at least one of the following criteria will be met and documented by records:
1. When a large population of adult mosquitoes is demonstrated by either a quantifiable increase in, or a sustained elevated mosquito population level as detected by standard surveillance methods.
2. Where adult mosquito populations build to levels exceeding 25 mosquitoes per trap night or 5 mosquitoes per trap hour during crepuscular periods.
3. When service requests for arthropod control from the public have been confirmed by one or more recognized surveillance methods.
Thermal foggers were developed largely from smoke generators built principally for concealing military maneuvers. In fact, the first units were built by a Navy contractor, Todd Shipyards Corporation (TIFA). The insecticide is mixed into a fog oil, usually #2 diesel or a light petroleum distillate, which is injected into a heated, often double-walled nozzle. The mixture is vaporized by the heat, which may be in excess of 1000o F. A source of forced air drives this vapor out of the nozzle where the outside cooler air condenses it into a visible fog with droplets ranging from 0.5- 1.5 microns.
If the insecticide flow does not overwhelm the vaporization capacity (sufficient BTUs/gal. /hr.) of the machinery, all the droplets will be in this near submicron range often referred to as a dry fog. If the insecticide flow is increased or the heat reduced some of the material will not be completely vaporized and larger droplets will be produced. The insecticide's contact time with the high temperature is so short that little if any degradation takes place.
Cold aerosol generators or cold foggers were developed to eliminate the need for great quantities of petroleum oil diluents necessary for thermal fogging. These units originally were constructed by mounting a modified vortical nozzle on a thermal fogger's forced air blower. Most of the nozzles are based on a design patented by the U.S. Army. The insecticide is applied as a technical material or in moderately high concentrations (as is common with the pyrethroids). This translates into very small quantities per acre and is therefore referred to as ULV. In agriculture, this rate is assumed less than 36 oz./acre, but mosquito control ground adulticiding operations would rarely exceed 1 oz./acre. The optimum size droplet for mosquito control with cold aerosols applied at ground level has been determined to be in the range of 5-15 microns.
The sprayers in use today use several techniques to meet these requirements. Air blast sprayers are almost universal. They use either high volume/low pressure vortical nozzles or high pressure airshear nozzles to break the liquid into very small droplets. Rotary atomizers, ultrasonic, and electrostatic nozzles are other forms of atomization equipment. Centrifugal energy nozzles (rotary atomizers) form droplets when the liquid is thrown from the surface of a high-speed spinning porous sleeve or disc. Ultrasonic equipment vibrates and throws the droplets off. Electrostatic systems repel the droplets.
Any mosquito adulticiding activity that does not follow reasonable guidelines, including timing of applications, avoidance of sensitive areas, and strict adherence to the pesticide label, risks affecting non-target insect species. Ground adulticiding, however, can be a very effective technique for controlling most mosquito species in residential areas economically and with negligible non-target effects. It is the methodology normally recommended for fundamental startup programs. Initially a district is not able or prepared to invest in a larviciding program where most of the mosquito production sites within flight range of the residents must be treated to produce a discernible improvement.
A benefit of thermal fogging is its ability to atomize more insecticide with much less energy (BTUs) input than air blast ULV-delivery techniques. The technique produces a very uniform droplet spectrum of very small droplets if a dry fog is maintained. The small droplets do not settle quickly and may penetrate foliage better than the larger cold aerosol droplets.
The risk associated with a dense enveloping fog is that it creates a traffic hazard. Additional concerns include the amount of non-insecticidal petroleum distillates that function only as a carrier and their possible damaging side effects on the environment. Thermal aerosols are often used in third-world countries because of their efficiency. They have the additional advantage that the public can easily see that something is being done. Although the total volume of thermal fogs is greater than the ULV technique, the amount of insecticide is often one half to one third of the ULV rate. This is due to the efficiency of the small droplets in penetrating and covering a targeted area. A benefit of ULV aerosols is that they do not require large amounts of diluents for application and are therefore much cheaper and may be environmentally safer. The spray plume is nearly invisible, does not create a traffic problem, and may not be perceived as an undesirable function. The machinery to generate cold aerosols can be much simpler in design and operation than that of thermal foggers, but requires sophisticated nozzles and, with pneumatic equipment, a great deal of energy input (horse power) to atomize even a small flow of insecticide.
Risks associated with ULV aerosols include the problems related to applying any undiluted technical pesticide. The material is being handled and transported in a concentrated form. The droplet spectrum is rather wide (submicron-<40 micron), can be difficult to change and may settle into non-target areas more readily than a dry thermal aerosol.
Any discussion of risk versus benefits needs to note that this form of control has been in extensive use for more than 40 years. There have not been any glaring adverse impacts attributed to ground adulticiding when it was done properly. The simple observance of population growth along the coastal areas of Florida and the state's high standing in tourism destinations speaks loudly of the benefits of this technique and mosquito control in general.
Ground adulticiding equipment is normally mounted on some type of vehicle, but smaller units are available that can be carried by hand or on a person's back. Pickup trucks are the most common motorized vehicle for conveyance. All terrain vehicles (ATV), golf carts, and even boats are occasionally used for ground adulticiding with various configurations of equipment. In 1996, organized MCDs in Florida listed the following property as ground adulticiding equipment carriers:
79 compact two-wheel-drive pickups
36 compact 4-wheel drive pickups
121 full-size ½-ton two-wheel-drive pickups
27 full-size ½-ton four-wheel-drive pickups
15 full-size 3/4-ton two-wheel-drive pickups
9 full-size 3/4-ton four-wheel-drive pickups
1 ATV
1 Airboat
The variety of large thermal aerosol equipment is somewhat limited, as its use is gradually being phased out of major programs. Clarke Engineering Technologies produces the Model 120D, an engine-driven blower unit with a liquid propane gas (LPG) or gasoline fueled burner with a 100 gal./hr. capacity. Curtis DYNA-FOG, Ltd.'s Silver Cloud is a twin-resonant pulsejet system capable of atomizing up to 40 gal./hr. The London Fog, Inc. Model F 500-E is a piston-engine-powered fogger that uses friction to preheat the oil mix, and then vaporizes it in the exhaust. It is capable of atomizing up to 30-35 gal./hr.
Clarke Engineering Technologies, Curtis DYNA-FOG, and London Fog make hand-carried, or push-around units with various power sources that are capable of atomizing more than enough material for a walking-rate application. These machines are excellent for spot treatment or touchups that would be cumbersome with a vehicle mounted-unit.
In 1996, organized MCDs in Florida listed the following as ground adulticiding equipment:
108 heavy-duty manufactured ULVs
33 medium-duty manufactured ULVs
5 large thermal foggers
17 hand-carried ULVs
3 hand-carried thermal foggers
This leaves 143 vehicle-mounted ULV systems not listed as to size or type. However, most districts that construct their own equipment build large, heavy-duty ULV units.
Cold aerosol generators are available in a broad range of sizes and configurations. Beecomist, Clarke, Curtis, London Fog, and Microgen all produce so called heavy-duty units for community/county-size operations. The term "heavy-duty" is more tied to larger flow capabilities than to durability. Large-area operations once used the largest equipment available because their choice of insecticide often included malathion (over 45% of 1995's usage), which is the most difficult commonly used mosquito adulticide to atomize to label specifications.
The Leco Model 1600 London Fog 18-20, Clarke's Grizzly and Curtis' Maxi-Pro 4 all use a large twin-cylinder gasoline engine driving a rotary lobed blower. The nozzles on these machines differ, but they all resemble the old Army-patent vortical nozzle. The Beecomist Pro-Mist 25 HD is an electric-driven rotary atomizer operating off the vehicle's electrical system.
The insecticide metering equipment available on these machines ranges from a simple glass flowmeter and a pressurized tank or electric pump on fixed-flow machines, to computer controlled, speed correlated, event recording and programmable flow management systems. The fixed flow units are designed to be operated with the vehicle traveling at a constant speed. Most of these use 12-volt laboratory type pumps that are quite accurate.
Variable flow metering systems regulate insecticide flow relative to the distance the vehicle travels and therefore are forgiving of speed irregularities. About one third (82 are listed) of the vehicle mounted ULVs in Florida are equipped with the cable driven variable flow system (SCAMPs), and many others are equipped with one of the electronic units. Vehicle monitoring systems record vehicle speed and insecticide pump operations over time. These may be incorporated with the flow-control systems to provide complete spray operations management systems.
Many programs construct their own equipment from off-the-shelf components. Some of these are built up from new pieces; others are fabricated from scavenged equipment. They may be locally made for economic reasons or to customize a certain function for a particular operational need. Unique features, added durability, and additional controls are common reasons that programs build their own equipment. Most of this equipment uses nozzle assemblies manufactured by one of the previously mentioned manufacturers. Some use the truck engine for a power source, and some were designed for multiple roles. However, most are unique.
Every manufacturer now produces a midrange machine in the 8-12 horsepower (or equivalent) class and a few even smaller (<6 HP) machines. These units are more compact and light, and typically are more fuel efficient than are their larger relatives. The atomization capabilities of the larger machines in this class are normally sufficient for many of the pesticides now being used, particularly at the 10-mph rates. All the flow systems available for the larger units may be fitted to this class machine as well.
Several hand-held, 2-cycle-engine ULV sprayers available are useful for small-area treatments. There are several units configured as backpacks, with the engine/blower mounted on a pack frame connected to a remote nozzle with a hose. These units use an orifice to control flow and either aspirating or gravity feed to supply the insecticide.
Operators of adulticiding equipment must be trained not only in the proper use and maintenance of the equipment but also in the proper application of the insecticide that they are using. The pesticide labels specify details of the application, including acceptable droplet spectra, flow rates, application rates, areas to avoid, and target insects. The law requires that any operator be certified in the Public Health category or be supervised by a licensed person (a certified person may supervise up to 15 operators). Some programs have all their personnel certified, including office staffs.
The Florida Mosquito Control Association's (FMCA) Dodd Short Courses regularly sponsor programs designed to educate operators, mechanics, and supervisors in the proper techniques of calibration, maintenance, operation, and scheduling of spray activities. FDACS encourages districts to budget course fees and travel monies for a third of their staff to attend courses each year.
There are 18 operational mosquito control programs in Florida that use aerial adulticiding. They have chosen it as a very effective means of controlling adult mosquitoes, particularly in inaccessible areas. Some of the districts base almost all their operations on this form of application. Permits to construct new source reduction works are essentially unobtainable, and larviciding is only effective when a high percentage of the mosquito production sites are regularly treated, which may be difficult and expensive. If the areas are bordered by extensive mosquito production sites or are small, narrow, or inaccessible, and lack a network of roads, aerial applications may be the only reliable means of obtaining effective control. When large residential areas are accessible to ground-based equipment and have evenly spaced streets, ground adulticiding may perform nearly equal to aerial operations.
Aerial adulticiding may be the only means of covering a very large area quickly in case of severe mosquito outbreaks or vector borne disease epidemics. One of the privileges of aerial application is that the pesticide labels permit as much as five times the amount of toxicant to be applied by air as by ground. An example is the Dibrom® label with a ground maximum rate of 0.198 oz./acre but an aerial maximum of 1.0 oz./ac. (over five times the rate). This opportunity for aerial operations biases it heavily toward better levels of control.
Aerial applications are expensive considering the pesticide costs per acre, the high cost of owning and maintaining or leasing aircraft, and the inherent increased salary demands. Low-level flying in a sea of obstructions, including towers, high-rise buildings and recently placed crane booms is dangerous at best. Flying also is very dependent on good weather conditions. However, due to the commitments for any spray mission, decisions are given much thought and are commonly scheduled when adult population levels have peaked.
Three aerial adulticiding techniques have been used in Florida: Low volume spraying, thermal fogging, and aerosols. Low volume (about a quart/acre) sprays were commonly applied with the pesticide diluted in light petroleum oils and applied as a rather wet spray. Their effectiveness was negated by problems of spotting cars or anything else left outside. The size of the droplets reduced drift, thus limiting swath widths, and was not ideal for impinging on mosquitoes. The technique has not been used for some time but is compatible with equipment commonly used for aerial liquid larviciding.
Thermal aerosol applications normally use the exhaust heat of the aircraft's engines (including the helicopter's turbine) to atomize a very dilute mixture of petroleum oil and insecticide. These applications are popular with pilots who can easily see where the spray plume is drifting. It is also an efficient means of producing a very small droplet and tight spectrum. The small droplets will remain airborne much longer than larger ones. The large quantities of fog oil requires larger heavy-lift aircraft and limits the area that can be covered economically to about one tenth that of ULV applications. The insecticide mix needs to be completely atomized because large oil droplets will put a sheen on water beneath the flight path.
The most common aerial adulticiding technique is ULV. Lighter aircraft, including helicopters, can be used because the insecticide load is a fraction of the other techniques. If the aircraft are capable of >120 knots (the Fyfanon® ULV label requires >150 MPH) the fine droplets can be created by the high-speed airstream impacting the flow from hydraulic nozzles. Slower aircraft and most helicopters typically use some variety of rotary atomizer to create the required droplet spectrum. ULV applications can be difficult to accurately place with any regularity. Without the visual cues, drift and settling characteristics can be difficult to access.
The flight parameters differ by program and technique. Some operations fly during daylight hours so their applications begin either at morning's first light or before sunset and work into twilight. At these times, the pilots should be able to see towers and other obstructions as well as keep track of the spray plume. The aircraft can be flown at less than 200 feet altitude, which may make it easier to hit the target area.
Other operations fly at night, typically after twilight or early in the morning before dawn. The aircraft typically are flown between 200 - 300 feet altitude. Swath widths vary from 400-1,200 feet. Most mosquito flight activity is crepuscular so these flights catch the adults at their peak activity. Bees are not active prior to full daylight so should not be at risk of serious impact from the insecticide.
Swaths are flown as close to perpendicular with the wind as is possible, working into the wind and commonly forming a long, tight "S" pattern. A number of factors affect the spray drift offset and settling, such as wind speed, droplet size, aircraft wake turbulence, altitude, and even characteristics of the individual aircraft. Pilots rely to a degree on experience for determining this offset, and some use smoke markers in the exhaust.
Pilots operating aircraft spraying for mosquitoes must hold an Aerial Applicators certification issued by the Florida Department of Agriculture and Consumer Services (FDACS). There is an Aerial Training committee within the Education Coordination Committee of the FMCA. The group functions to keep those involved with aerial operations abreast of the latest developments, demonstrate calibration procedures, and bring experts from related fields to special work sessions.
All leased or contracted aircraft are operated under the FAA's Part 137-Agricultural Aircraft Operations. These aircraft have a certificate of airworthiness and are maintained, modified, and flown in strict conformance to the FAA's regulations governing civil aircraft. Many of the aircraft owned by mosquito control programs are operated under the FAA's public classification (versus civil) and thus do not require airworthiness certificates. Basically, these are surplus military units which had no identical civilian counterpart; therefore, it can be difficult to obtain an airworthiness certificate. However, maintenance standards are very high on these aircraft, particularly since they are flying at low altitudes, often over residential areas.
The aircraft used for aerial adulticiding are as varied as the programs where they are located. Both rotorcraft and fixed wing are used. Most are owned by the local MCDs. Some are contracted, and one program contracts for pilots while owning the aircraft. Also, the State's Dog Fly Control Program contracts with mosquito control programs for aerial ULV adulticiding with their DC-3.
Part 137.51(4)(iii) states:
No person may operate an aircraft over a congested area during the actual dispensing operation, including the approaches and departures for that operation, unless it is operated in a pattern and at such an altitude that the aircraft can land, in an emergency, without endangering person or property on the surface. This implies that any aircraft (other than helicopters) must be of multiengine configuration (in case one fails) if it is spraying over congested areas.
Fixed-wing multi-engine aircraft account for most of the acreage adulticided in Florida. They have a reasonable payload, are moderately fast, and are economical to operate and practical to maintain. Older military cargo planes (C-45s and C-47s) carry large payloads and have impeccable flight/safety histories. They are also easy to maintain because of their relative simplicity for their size. Due to the horsepower of their engines, fuel consumption may be as high as 100 gal./hr. for a C-47, but they are capable of carrying over 6,000 pounds. For thermal fog applications, no aircraft smaller than a DC-3/C-47 was very practical. One district is operating a Beech King Air turboprop equipped for ULV adulticiding (King Airs and C-45s have useful payloads around 3,000 lb.+ fuel and pilots). All these larger, and thus heavier, aircraft create much more wake turbulence, which is useful in dispersing ULV insecticides into proper-size droplets and spreading the spray plume uniformly.
Light general aircraft (including Cessna 336s and Piper Aztecs) may be smaller but have payloads suitable for ULV spraying. They can be economical to purchase and operate, simple to maintain, nimble to fly, and somewhat less conspicuous when spraying. The fuel consumption of a smaller light twin may only be 30 gal./hr., but it will be limited to a useful payload of about 1000 pounds.
Rotor craft are seeing wider use for adulticiding . Many programs which operate them for larviciding duties will change the spray equipment and also adulticide with them. Additionally, programs will use them for adulticiding smaller areas that have difficult obstructions or meandering shapes. They are more maneuverable than fixed-wing aircraft and can be serviced at field sites thus reducing ferry times. Air speeds are somewhere between 70 knots for piston-engined ships and 110 knots for the faster light turbines.
In 1996, organized MCDs in Florida listed the following aircraft used for adulticiding:
Fixed-Wing Aircraft
12 Douglas DC-3/C-47
3 Beech 18/C-45
1 Beech King Air C-90
1 Beech Queen Air
1 Beech Twin Bonanza
2 Piper Aztec
4 Cessna 337
Helicopters
2 BellUH-1B
9 Hughes/MD 500 C, D & E
2 Bell 206
3 Bell 47
4 Hughes 269 A, B & C
Guidance systems such as Loran and now Global Positioning Satellite (GPS) are being used to keep the aircraft on the selected target flying parallel and on even swaths. Even when selective availability (the Department of Defense's "dithering" the atomic clock's output which they encode) distorts the GPS position, repeatability (absolute accuracy isn't necessary for parallel swaths) is good enough for adulticiding where drift should smooth the swath irregularities. Differential GPS that corrects the military's corrupted signal is available throughout Florida and avionics grade DGPS receivers became available in 1996.
GPS units are being used with moving map displays that can be programmed to display a spray area complete with accurately placed swaths. The electronics permit a real-time snail trail showing where the aircraft is flying and where it has been for the flight crew to monitor their progress. Recording systems are available to log the flight path on data cartridges that can be read into a Global Information System (GIS) on a desktop PC. This recording can include parameters such as spray on/off to add an attribute to the flight path recording.
Although the FDACS reporting system requires only the total acres air sprayed and total chemical used for any formulation, most operations keep detailed records of their aerial missions. These records, as per 5E-13.036(5), should include adult mosquito sampling information or at least a reference to where this information is retained. Rule 5E-13.06(5) requires that: "Aircraft applications of mosquito adulticides along beaches and bayshores shall be justified only when there is a demonstrable three-fold increase over a base population."
When aerial adulticiding operations on private lands pose the possibility of "...deposition of airborne substances on public lands determined to be environmentally sensitive and biologically highly productive…," a set of criteria must be met. The rule (5E-13.037) requires that records of the following be maintained for a minimum of three years:
• The area treated
• The application rate and the material used
• The equipment and technique used
• The name of the pilot in command
• The date, time, temperature, and general wind speed and direction
• Pre- and post-treatment records of mosquito and other arthropod presence, including: Number and type of trapping and surveillance methods used Trap and surveillance site location
Pre- and post-treatment trap catches, landing rates or surveillance levels by mosquito species involved
• Apparent non-target effects
The only aerial thermal fogging operation that remains is using three C-47 fixed wing-aircraft and two UH-1B helicopters. The spray apparatus consists of a series of large nozzles arranged in a radial pattern that direct the insecticide/oil mix into the engine's hot exhaust. The tanks are quite large, 800 gallons in the C-47s and 300 gallons in the Hueys.
ULV systems are as diverse as the aircraft on which they are mounted. Many fixed-wing twins and helicopters are equipped with external-belly tanks suspended under the fuselage or cabin. Other programs install their insecticide tanks within the aircraft's passenger compartment. Some of the tanks are commercial fiberglass units, but most tanks are custom fabrications of stainless steel, polyethylene or fiberglass. One operation uses the aircraft's own auxiliary fuel tanks that were separated from the fuel system and now carry an oil/pyrethroid mix. Some units are equipped with in-flight (or return flight) tank-flushing capability. Tank capacity ranges from a C-47s 350 gal. to 20+ gal. for a Hughes 269A.
Pumping systems are typically by-pass controlled positive displacement electric units. Others use variable ratio gearboxes or electronic speed controllers on electric gear pumps. Some aircraft are equipped with flow monitors that incorporate in-line turbine flow transducers.
The nozzles found on ULV systems and their placement add to the individuality between programs. Beecomist rotary atomizers are common on helicopters, but a few of the faster Hughes 500s are equipped with short booms with flat-fan hydraulic nozzles. Micronair rotary atomizers are used on at least one fixed wing. Hydraulic nozzles (mostly 80o flat fans) on booms account for the bulk of fixed-wing installations, but their locations vary significantly. On the several helicopters equipped with hydraulic nozzles for adulticiding, the nozzles are mounted close to the outboard end of a moderate length amidships boom. Some tail booms are mounted above or below the elevator off the horizontal stabilizer, others off the tail cone. Still others mount the nozzles off booms below the wing's trailing edge, everywhere from midwing through to the wing tips.
Because adulticiding is primarily carried out within human-populated areas, such as urban zones, suburban zones, and parks, humans, domestic animals, and wildlife are exposed to drifting and deposited insecticide droplets. Since adulticides are usually applied during the night or twilight hours, nocturnal and crepuscular animals may have a greater chance for direct exposure. Fortunately, due to dispersal characteristics of ULV sprays, only a small mass of insecticide deposits per unit area. It must be realized that droplet size is of greatest consequence to deposition rates and ultimately the degree of exposure to non-target organisms. Most modern adulticides are short-lived in the environment, degrading rapidly from the original compound when exposed to sunlight, water, or soil microbes. Biomagnification has not been documented to occur for currently used adulticides.
A major problem inherent to mosquito adulticiding is controlling/predicting the distribution of the pesticide spray during and after applications. This is because adulticides are applied by atomizing technical solution into micron size droplets that are intended to drift across target areas such as a city block or larger area. Deposition of these droplets occurs within and beyond the target area as dictated by droplet mass (i.e., terminal fall velocity), and prevailing meteorological conditions. Adequate drift of small (<10 mm VMD [volume meter diameter]) droplets through the target area is essential for efficacious mosquito control, yet 10 micron droplets may be calculated to drift 500 meters (0.3 miles) when applied by ground equipment and a wind speed of 4 km/h (Tietze et al. 1994). Under the same conditions, a one micron droplet is expected to drift about 2000 m (1.2 miles). These drift characteristics distribute the insecticide across a wide area, but may lead to incidental deposition into wetlands and other sensitive habitats.
The influence of meteorological conditions on spray drift cannot be understated (Armstrong, 1979). Air temperature at ground level relative to that above it dictates air stability and, consequently, patterns of drift and deposition. Higher temperatures on the ground will cause the spray cloud to become entrained in rising thermal currents, thus interfering with the intended horizontal drift pattern. Wind speed and directionality are important for obvious reasons. In addition to the size of the droplet and meteorological conditions, the amount deposited depends upon application strategy (i.e., ground or aerial treatments), type of equipment used, method of calibration, and flow rate (Dukes et al. 1990). For example, aerial applications using flat-fan nozzles produce larger droplets than do truck-based applications using a cold fogger. LECO sprayers produce a different droplet spectrum from Beecomist sprayers, and in the case of the LECO, higher pressures produce greater numbers of smaller droplets. Drift and deposition is also influenced by the presence of structures such as trees and buildings that create complex airflow patterns and vortices that may promote channeling of the spray cloud and deposition.
The Environmental Hazards section of mosquito adulticide labels instruct applicators to avoid direct application over water or drift into sensitive areas (i.e., wetlands) due to a potentially high toxicity of these compounds to fish and invertebrates. Although habitats to be avoided differ, they usually include lakes, streams, and tidal marshes. Thus, any deposition of adulticides into these areas should be indirect and unintentional. A fundamental conflict is evident when considering that aerial applications of adulticides target active adult mosquitoes that may have emerged from the same aquatic resources that are to be protected from exposure to these compounds. The problem is further compounded by the complex interweaving of aquatic habitats and urban areas typically found along Florida's coasts. The proximity of residential communities to salt-marsh and freshwater habitats increases the potential for direct overspray or unintentional drift into these sensitive areas.
The toxicity of organophosphate and pyrethroid adulticides to non-target organisms has been studied in a variety of laboratory and field experiments (now accessible on the Internet at http://www.famu.edu/jamsarl/index.html). Laboratory bioassays have yielded lethal concentrations ranging from 10% to 50% of the organisms exposed to help predict impacts in the field. Field studies, however, are more difficult to conduct, and therefore greater data gaps exist there. Of field studies conducted, almost all have focused on aquatic habitats, and little is known about impacts in terrestrial systems. The greatest data gap may be understanding population level impacts to non-targets, considering migration/dispersal, recolonization and confounding factors such as habitat destruction, other sources of environmental contamination, natural variation in populations, etc. Predicting impacts on temporary and semi-permanent communities has been extremely problematic because these non-target populations are r-strategists where they have become rapid colonizers with shorter life cycles and frequently undergo localized extinctions. Overall, direct deleterious effects have not been documented for non-targets in aquatic habitats as a result of deposition of currently employed adulticides, probably due to a small mass depositing per unit area and dilution factors such as tidal flushing and water depth.
Occasionally, cottage industry operations such as beekeeping, cricket rearing, aquaculture, and butterfly rearing report losses associated with adulticide treatments. These operations may be located in backyards of urban, suburban, and rural areas where routine adulticiding is conducted. Such operations are encouraged to notify mosquito control managers to avoid exposing their colonies to actions taken by the resident/manager or by mosquito control applicators. By far, the problem most frequently reported is that of bee kills associated with aerial mosquito adulticide operations (Stevenson, 1980). Avoiding bee kills has been problematic due to the practice of seasonal relocation of hives to optimize honey production, which often cross county and state lines, as well as secrecy in the placement of hives. Associating bee kills with mosquito control applications may be unclear since their colonies are susceptible to a variety of diseases and other causes for loss in colony strength and production. For this reason, further research is warranted to delineate impacts of aerial mosquito control on bee colonies.
Chemical trespass has been a concern to some residents in areas receiving routine aerial or ground-based applications of adulticides. Some people are potentially more susceptible to toxic exposure: Children (due to small body size), persons taking certain medication, persons with a preexisting disease, and persons who are heavily exposed to other insecticides. Two other groups need to be mentioned: Individuals with a predisposition to allergies and those who are reportedly sensitive to multiple environmental chemicals. The most frequent symptom in humans exposed to adulticides is acetylcholinesterase depression. Clinically detectable symptoms generally do not occur until there is a 50% reduction in acetylcholinesterase (AChE). Mammals possess a liver enzyme that detoxifies organophosphates, making it far less toxic to us than it is to fish and invertebrates. A health-risk assessment of aerially applied malathion against medflies in California found that AChE depression below 20% was found to be theoretically possible in high-exposure scenarios involving multiple exposures due to repeated spraying (California Department of Health Services, 1991). In the latter review, malathion was not found to cause carcinogenic, teratogenic, or negative reproductive effects on test animals. Epidemiological studies in 1981-82 in Santa Clara County showed that no overall association existed between the aerial application of malathion and the occurrence of congenital anomalies and low birth weight among live-born infants. One major difference between aerial spraying of malathion for medfly abatement and that of mosquito control may be the route of exposure; the former program produces larger droplets that do not drift and exposure due to inhalation is negligible.
Applying pesticides correctly: a guide for private and commercial applicators. 1992. SP-1 USDA/EPA.
Armstrong, J.A. 1979. Effect of meteorological conditions on the deposit pattern of insecticides. Mosq. News 39: 10-13.
California Department of Health Services. 1991. Health risk assessment of aerial application of malathion-bait. Summary Report. Berkeley, CA: California Department of HealthServices. Pesticide and Environmental Toxicology Section.
Dukes, J.C., C.F. Hallmon, K.R. Shaffer, and P.G. Hester. 1990. Effects of pressure and flow rate on Cythion droplet size produced by three different ground ULV aerosol generators. J. Am. Mosq. Control Assoc. 6: 279-282.
Stevenson, H.R. 1980. A review on the effects of ultra low volume insecticide treatments to honey bees, Apis mellifera (L.). Proc. Florida Anti-Mosquito Assoc. 51:11-14.
Tietze, N.S., P.G. Hester, and K.R. Shaffer. 1994. Mass recovery of malathion in simulated open field mosquito adulticide tests. Archives Environ. Contam. Toxicol. 6: 473-477.