Best Management Practices for 

Mosquito Control ___________________________________________________ 

Washington State Department of Ecology 

Water Quality Program 

May 2004 

Publication 03-10-023 revised 

Best Management Practices for 

Mosquito Control ___________________________________________________ 

Washington State Department of Ecology 

Water Quality Program 

May 2004 

Publication 03-10-023 revised 
For additional copies of this document contact: 

Department of Ecology 

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Table of Contents 

Table of Contents...................................................................................................................i 

Tables ...................................................................................................................................ii 

Introduction ...........................................................................................................................1 

Best Management Practices for Mosquito Control ................................................................3 

I. Minimize Mosquito Breeding Sites and Utilize Personal Protective Measures .......... 3 

II. Monitor Mosquito Populations and Disease .............................................................. 6 

III. Establish Targeted Densities for Mosquito Populations........................................... 11 

IV. Mosquito Control Treatments ................................................................................ 12 

Appendix A: State Listed Species Restricted Areas................................................................A-1

Appendix B: Insect Repellant Use and Safety ..........................................................................B-1

Appendix C: Response to Publlic Comments........................................................................... C-1

Best Management Practices for Mosquito Control i 

Washington State Department of Ecology 
Tables 

Table 1. Disease Vector Mosquito Species Associated With Drainage Control Facilities .... 9

Table 2. Potential Disease-Carrying Mosquitoes in Washington State.................................. 10

Table 3. Permitted Insecticides Used For Mosquito Control.................................................... 16

Table 4. Insecticides Used for Adult Mosquito Control............................................................. 19

Page ii Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
Introduction 

On April 10, 2002, the Washington State Department of Ecology (Ecology) issued NPDES General 

Permit No. WAG-992000, covering mosquito control activities that discharge insecticides directly 

into surface waters of the state. Under the permit, the use of insecticides for mosquito control in 

water is allowed when the effects are temporary and confined to a specific location, though 

locations where insecticides are used may be widespread throughout the state. Applications of 

insecticides are subject to compliance with the Federal Insecticide, Fungicide, and Rodenticide Act 

(FIFRA) labels, the Washington Pesticide Control Act (15.58 RCW), the Washington Pesticide 

Application Act (17.21 RCW), the General Pesticide Rules (WAC 16-228), the Worker Protection 

Standard (WAC 16-233), a number of pesticide and/or county specific regulations, 

monitoring/reporting requirements, and approved best management practices (BMPs) that include 

integrated pest management options. When adopted by a mosquito control entity, these BMPs for 

Mosquito Control satisfy that NPDES Permit No. 992000, Condition S4 requirement. 

These Best Management Practices for Mosquito Control were developed through a collaborative 

effort of representatives from Washington and Oregon based mosquito control districts, 

Washington State counties, Washington State University, mosquito control insecticide industry and 

the state departments of Health, Agriculture, Fish and Wildlife, Transportation and Ecology. Our 

appreciation goes out to the many mosquito control experts and others who took the time to review

the draft and offer their expertise and suggestions. 

Mosquito control entities in Washington State that wish to develop their own BMPS may do so, but 

they must be approved by Ecology. An approvable integrated pest management (IPM) program for 

mosquitoes must involve natural resource scientists when planning control measures that could 

harm delicate ecosystems and include all the features of IPM as defined in Washington State law 

RCW 17.15.010 (as adapted to mosquito management): 

1) Minimize mosquito breeding and feeding sites. 

2) Monitor mosquito populations and disease. 

3) Establish the targeted densities of mosquito populations based on community factors of 

health, public safety, economic and aesthetic thresholds. 

4) Treat mosquitoes to reduce populations below the targeted threshold using strategies that 

may include biological, cultural, mechanical, microbial, biochemical and chemical control 

methods and that consider human health, ecological impact, feasibility, and cost 

effectiveness. 

5) Evaluate the effects and efficacy of pest treatments. 

Best Management Practices for Mosquito Control Page 1 

Washington State Department of Ecology 
Page 2 Best Mana

gement Practices for Mosquito Control 

Washington State Department of Ecology 
Best Management Practices for Mosquito Control 

I. Minimize Mosquito Breeding Sites and Utilize Personal Protective Measures 

Risk Assessment: Probability of outbreak in humans: Remote 

Action threshold: The presence or even the suspected presence of mosquitoes (any species) in an 

area identified for control efforts triggers minimization efforts in the early spring and summer. The 

mean development time from egg hatch to pupation takes 5 to 10 days at temperatures near 25o C 

(77o F)(Pratt and Moore, 1993). However, “eggs of certain species can hatch in water as cold as 45o

F” (

Lilja, 2002, p. 24). Minimization actions are most effective in the early spring and continued 

through fall on an as-needed basis. 

Rationale: Minimizing man-made breeding sites in the targeted area of control and personal 

protection, especially for those with compromised immune systems, are the best defenses against

getting bitten by mosquitoes, giving the best protection for the least cost. 

Minimum BMP Response:

1. Provide information to those in the area of control on eliminating artificially created 

mosquito breeding sites, use of biological controls (including bio-larvicides), use of 

repellents, and on protecting animals of concern. Local and state environmental health

departments have a variety of informational brochures. See 

http://www.doh.wa.gov/ehp/ts/Zoo/WNV/WNV.html for links. 

2. Take appropriate minimization actions. 

3. If possible, obtain resources to enable effective responses. 

Minimization Actions 

 

Eliminate Artificial Breeding Sites around Homes and Offices 

3⁄4 Empty or turn over anything that holds standing water—old tires, buckets, wheelbarrows, 

plastic covers, and toys. Do not let water stagnate for more than seven days. 

3⁄4 Change water in birdbaths, fountains, wading pools, and animal troughs weekly. 

3⁄4 When practical, drill holes in the bottoms of containers that are left outdoors. 

3⁄4 Clean and chlorinate swimming pools that are not in use and be aware that mosquitoes can 

breed in the water that collects on swimming pool covers. 

3⁄4 Consider aerating ornamental pools and use landscaping to eliminate standing water; 

mosquitoes can potentially breed in any stagnant puddle that lasts more than four days. 

3⁄4 Recycle unused containers—bottles, cans, and buckets that may collect water. 

3⁄4 Make sure roof gutters drain properly, and clean clogged gutters in the spring and fall. 

3⁄4 Fix leaky outdoor faucets and sprinklers. 

3⁄4 Keep all ornamental shrubs and bushes trimmed and pruned to open them up to light and 

air flow. This will not only give mosquitoes fewer places to hide, but will promote growth 

and vigor in the plants. 

Best Management Practices for Mosquito Control Page 3 

Washington State Department of Ecology 
Use Appropriate Bio-controls 

3⁄4 Stock water gardens that have no surface inlet or outlet with mosquito-eating fish (i.e., goldfish, 

mud minnow, stickleback, and perch). Tadpoles, dragonfly larvae, diving beetles, back 

swimmers, and front swimmers also prey on mosquito larvae. For more information, see 

http://www.wdfw.wa.gov/factshts/westnilevirus.htm. 

3⁄4 Native vegetation and nest boxes can help attract mosquito-eating birds and bats. However, 

property owners should avoid introducing non-native fish or wildlife in an attempt to 

control mosquitoes. While it is permissible to release some fish commonly available in pet 

stores into small, contained backyard pools and ponds, non-native fish should not be 

released into open or partially contained waters that may occasionally flood into natural 

water bodies. Some non-native species, including so-called mosquito fish, Gambusia affinis, 

can be major pests when introduced outside their natural range. Gambusia are aggressive 

and have been known to feed on eggs, larvae and juvenile native fish and amphibians. 

Because of these negative impacts on native species, Gambusia is a regulated species in 

Washington State, and may not be introduced without a fish stocking permit issued by 

WDFW. 

3⁄4 Under WDFW policy, transfer/stocking permits may only be issued to organized mosquito 

control districts, the U.S. Army Corps of Engineers, and local or state health departments; 

permits may not be issued to private individuals. To protect the Olympic mud minnow, a 

state sensitive species, Gambusia stocking is prohibited in Clallam, Jefferson, Kitsap, Grays 

Harbor, Mason, Thurston, and portions of Lewis County that drain into the Chehalis river. 

For information on fish stocking permits contact the WDFW regional office in your area. 

3⁄4 Selective bio-pesticides such as Bacillus thuringiensis israelensis (Bti), B. sphaericus or 

methoprene are very effective preemptive controls when applied in the spring to specific 

sources identified by surveys. Amplifying and bridge vector species should be targeted 

(also see p. 11.). 

Personal Protective Measures 

3⁄4 Make sure window and door screens are "bug tight." Repair or replace if needed. 

3⁄4 Stay indoors at dawn and dusk when mosquitoes are the most active. 

3⁄4 Wear a long sleeve shirt, long pants and a hat when going into mosquito-infested areas such 

as wetlands or woods. 

3⁄4 Use mosquito repellant when necessary, and carefully follow directions on the label. For 

extensive repellent information from the Centers for Disease Control see Appendix B: Insect 

Repellent Use and Safety

3⁄4 Areas frequented by the public, such as parks, zoos, outdoor concert areas, and wildlife 

reserves should consider making repellents available. 

Protect Animals of Concern 

3⁄4 To protect your horses and other equines, talk to your veterinarian about the West Nile 

virus vaccine. The vaccine requires two doses three to six weeks apart, and immunity may 

Page 4 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
not be achieved until up to six weeks after the second dose. An annual booster should be 

given a few weeks to a month prior to the start of the mosquito season in your area. 

3⁄4 Veterinarians should be consulted if you have concerns about your household pets or other 

animals. Repellents may be used in some instances. 

3⁄4 Thoroughly clean livestock watering troughs weekly. 

3⁄4 For more information see: http://www.aphis.usda.gov/lpa/issues/wnv/prv.html and/or 

http://www.cdc.gov/ncidod/dvbid/westnile/birds&mammals.htm

New Construction and Storm Drains 

As new facilities are being designed, consideration should be given to reducing 

mosquito habitat as much as possible. 

When considering a drainage or water treatment facility for mosquito control, the first 

consideration should be whether the problem could be reduced by physical 

modification or repair without compromising the facility’s function. Physical 

modifications should be designed by an engineer and reviewed by the local 

government to insure they meet applicable design requirements. A possible design 

modification may include scarifying the pond bottom where it is no longer infiltrating 

as originally designed, providing slope to the bottom of the drainage facility or 

enhancing infiltration by some other method. Eliminating low spots that collect small 

amounts of standing water and altering excessive overgrown vegetation may also be 

options. Alterations of slopes or repairs to a facility should not involve a reduction in 

the water retention or carrying capacity of the facility. As an example, soil should not 

be added to fill low spots. Instead, low spots should be graded flat such that the 

carrying capacity is not reduced. 

Sprinklers and Irrigation Systems 

Over-watering and poor irrigation practices are common producers of mosquitoes around the 

home, in parks, in irrigated fields, and on golf courses. Report standing water to appropriate 

maintenance personnel.

3⁄4 Irrigated lands are among the highest producers of mosquito breeding sites in Washington 

State. High numbers of mosquitoes can develop in standing water as a result of flood

irrigation. The actions below can help eliminate mosquito breeding sites by using physical 

controls (Colorado, 2002; Pratt and Moore 1993). 

1) Minimize standing water in fields so that it does not lie fallow for more than four 

days by improving drainage channels and grading. 

2) Tail waters should not be allowed to accumulate for more than four days at the end 

of the field.

3) Keep excessive overgrown vegetation out of ditches to promote more rapid 

drainage, but retain ground cover to prevent soil loss. 

Best Management Practices for Mosquito Control Page 5 

Washington State Department of Ecology 
4) Have ditches repaired to reduce seepage to the extent practicable (elevated water 

tables can produce unintended standing water in fields). Modification or repairs to a 

ditch should not reduce the carrying capacity. 

5) Minimize flood and rill irrigation practices to the extent practicable. 

6) Avoid over-watering. 

Foster Healthy Wetlands 

Do NOT drain or fill wetlands. The chance of mosquito "outbreaks" increases in wetland and 

stream ecosystems that have been changed or tampered with. In disturbed systems, the predators 

of mosquito larvae are often excluded while the mosquitoes thrive. Thus, draining wetlands and 

removing greenbelts will not eliminate mosquitoes. In fact, such actions could actually increase the 

mosquito population if their natural predators are destroyed. The draining of wetlands will still 

leave behind many small puddles or wet depressions that are prime habitat for mosquitoes. 

Wetlands perform at least three classes of functions: hydrologic functions (i.e., flood peak reduction, 

shoreline stabilization, or groundwater exchange), water quality improvement (sediment accretion, 

filtration or nutrient uptake), and food-chain support (structural and species diversity components 

of habitat for plants and animals, including threatened endangered and sensitive species). Many 

wetlands recharge ground water critical for local drinking water supplies and prevent streams from 

drying up during the summer. Given the critical functions wetlands perform, Ecology does not 

condone draining wetlands as a method for mosquito control. Since most predation on mosquitoes 

occurs when they are larvae, the best mosquito control is often to target the larvae, either by 

fostering predators native to the area of control (amphibian larvae, aquatic salamanders, small fish) 

or by applying selective larvicides such as Bti. (Tom Hruby, Ecology Wetland Specialist, personal 

communication 2/26/03 and 1/16/04). 

II. Monitor Mosquito Populations and Disease 

Risk Assessment: Probability of outbreak in humans: Remote to low; areas with limited or 

sporadic WNV epizootic activity in birds and/or mosquitoes. 

Action Threshold. The presence of vector or nuisance mosquitoes suspected or confirmed in the 

area. 

Rationale. Base-line data on mosquito populations and mosquito-borne disease will help target 

educational efforts and are essential to control efforts, should they become necessary. 

Minimum BMP Response. Obtain and track avian mortality, human encephalitis/meningitis, and 

equine surveillance in the area of control. Further quantify epizootic activity by inventorying 

mosquito habitats, and trapping and testing for vector mosquitoes. Consider targeted insecticide 

control if surveillance indicates high potential for human risk to increase. 

Monitoring Strategies for Landowners of Private Property and Contracted Licensed Applicators

3⁄4 Contact your local health department for information about birds, horses, and humans 

found to test positive for West Nile virus or other mosquito-borne diseases in your area of 

control. 

Page 6 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
3⁄4 Accurately map and identify rearing areas for mosquitoes, by species if possible. These are 

those sites for mosquito rearing that cannot be eliminated by following preventative 

measures such as container emptying, proper pond maintenance, and eliminating excess 

standing water by using appropriate irrigation BMPs. This is important because 

appropriate treatment measures are contingent on the habitat (species) encountered. 

The following northwest mosquito habitats and control issues have been identified in the 

Mosquito-Borne Response Plan developed by the Department of Health (Lilja, 2002). Vectors in 

specific regions have not all been identified. Contact your local health department for the latest

mosquito vector information. 

Floodwater.: Aedes vexans and Ochlerotatus sticticus develop in large numbers along the borders of 

the Columbia and other rivers and create important mosquito problems in this region. The larvae 

hatch in the spring or early summer when the streams overflow areas such as willow and 

cottonwood swales where the eggs have been laid. The eggs of these species are dormant when 

temperatures remain below 45-50o F. Partial dormancy of the eggs may continue until sometime in 

June so that only some of the eggs are hatched by floods occurring in April or May. In some 

seasons, the larger rivers may rise, recede, and rise again to cover the same egg beds and produce 

an additional hatch. In other seasons, two or three successive rises may occur, each of which is 

higher than the last. Females that emerge in the first hatch may lay eggs that will hatch in the 

second or third rises of the river. Most of the eggs are laid between the 10 and 20 foot levels, and 

some of the eggs that are not flooded during a series of low flood crest years remain viable for as 

long as four years. 

Large Aedes vexans and Ochlerotatus sticticus breeding areas have been managed efficiently by 

controlling water levels above dams such as the Bonneville Dam. Dikes have prevented flooding in 

other areas. Clearing of brush has been of value in some locations. However, control of the major 

section of these types of breeding areas must often be accomplished with insecticide applications.

Irrigation Water: Breeding places for several mosquito species are provided by irrigation water. 

Aedes dorsalis, A. vexans, Ochlerotatus melanimon, and Ochlerotatus nigromaculis are among the most 

important species that may develop when water is applied and stands for a week or ten days. 

Other species such as Culex tarsalis, Culiseta inornata, and Anopheles freeborni may also be produced. 

Tremendous numbers of mosquitoes breed in many areas where uncontrolled irrigation is 

practiced. Applications of insecticides are effective but are not substitutes for proper grading. 

Elimination of standing water is effective in preventing development of mosquitoes. Application of 

insecticides may be necessary for breeding places that cannot be drained. See Sprinklers and 

Irrigation Systems in Section I above. 

Tidal Waters: Aedes dorsalis is the only species that can breed in large numbers in both fresh and 

salt water in the Northwest. The larvae develop in some coastal areas where potholes are filled by 

the higher tides or where water levels fluctuate in permanent or semi-permanent pools. Leveling, 

drainage, or similar practices are effective in preventing breeding, but such areas must be properly 

maintained. Insecticide control may be necessary where these methods are inadequate or 

ineffective. Ochlerotatus togoi has also been found in coastal areas including San Juan, Island,

Best Management Practices for Mosquito Control Page 7 

Washington State Department of Ecology 
Skagit, Kitsap, and Mason counties. Larvae of this species have been found in pools of pure 

seawater along rocky shorelines. 

Snow Water: In many high mountain meadows and also at lower levels, mosquitoes breed in pools 

caused by snow melt. Development may require several weeks at higher elevations. Aedes 

communis,A. cinereus, Ochlerotatus hexodontus, O. fitchii, and O. increpitus are the most common 

species found in these locations. Usually there is only one generation per year, but the large 

numbers that may be produced are a severe annoyance to those who are working or seeking 

recreation in these areas. Elimination of breeding areas by drainage or maintenance of constant

water levels is practical in some situations. Insecticide applications might have to be made by hand 

or by plane because of inaccessibility to heavy ground equipment. 

Permanent Waters, Ponds and Artificial Containers: The mosquitoes that lay their eggs on the 

water are usually found where water is present continuously during the season or at least for 

several days. Such locations include natural permanent ponds, including still waters along the 

borders of lakes and rivers sheltered from wave action and currents with some degree of 

vegetation, log ponds, tree holes, semi-permanent ponds and wetlands of various types, and 

artificial containers. Culex tarsalis, C. pipiens, C. peus, Anopheles freeborni, A. punctipennis, Culiseta 

incidens, and C. inornata are commonly found in such places. C. tarsalis and C. pipiens develop in 

large numbers in log ponds. C. pipiens also develops in large numbers in sewer drains, catch basins, 

and water left in artificial containers. Coquillettidia perturbans are found in permanent water in 

wetlands, swamps, and marshes that have emergent or floating vegetation. Insecticides are often 

used effectively to control most of these species, except those breeding in artificial containers that 

can be emptied. Larvae of C. perturbans are difficult to control because they are attached to the 

roots of plants. Insecticide granules are sometimes applied, but eliminating host plants may be the 

most useful procedure to control this species. Consult with your local WDFW office before 

removing plants on WDFW-managed lands or in ecologically sensitive areas. 

Stormwater: In response to the anticipated arrival of West Nile virus in King County, King County 

Water and Land Resources developed recommendations for dealing with the mosquito control at 

County drainage facilities. The study (Whitworth, 2002) identified the four basic habitats preferred 

by mosquitoes, the types of mosquitoes associated with the habitat type, and the WNV vector 

mosquito species that prefers each habitat type. Table 1 summarizes this information. 

Page 8 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
Table 1. Disease Vector Mosquito Species Associated With Drainage Control Facilities 

Habitat Type Facility type Vector Species

Permanent Water Year round wet ponds

Larger Regional Ponds

Wet Bioswales

Anopheles punctipennis

Marshes & Wetlands Wet Bioswales

Some Regional Facilities

Aedes cinereus

Coquilletidia perturbans

Temporary or Flood Water Temporary Wet Ponds

Dry Bioswales

Retention/Detention Ponds

Open Ditches

Aedes vexans

Culiseta inornata

Artificial Containers / Tree Holes Catch Basins

Underground Tanks/Vaults

Discarded containers & Tires

Ochlerotatus japonicus

Culex pipiens

Culex tarsalis

Culiseta inornata

Table 2 summarizes biological information of vector mosquitoes found in Washington State. 

Best Management Practices for Mosquito Control Page 9 

Washington State Department of Ecology 
Table 2. Potential Disease-Carrying Mosquitoes in Washington State 

Mosquito

Species

Day or

Night

Biter

Range

Generations

per Year

Preferred Habitat Breeding Comments

Aedes 

cinereus

Aggress

ive

during

day

Does not 

travel far

from

habitat

One-eggs 

hatch at

different

times

A woodland species: semipermanent bogs & swamps,

wetlands, wet bioswales &

floodwaters

Hatches in the early

spring. Larvae found 

among dense aquatic 

vegetation.

Aedes vexans

Day &

Night 20+ miles Many

Any temporary water body

like ditches, puddles,

containers, pools &

floodwater.

Eggs may lie domant 3+ 

yrs, hatches in ditches,

still water.

Anopheles

punctipennis Night

Stays near

habitat. One

Springs and creeks

connected to stormwater

ponds, bioswales and

wetlands.

Prefers algae-laden, cool

pools on edges of slow 

flowing rivers and 

streams. Has entirely

dark palpi.

Coquilletidia

perturbans

Night -

often

comes 

to lights

Strong

fliers,

enters

homes 

and lit

areas.

One, but

hatchlings do

not complete

development

until the 

following 

spring.

Permanent marshes,

wetlands, temporary wet

ponds, dry bioswales &

open ditches.

Needs thick growth of

aquatic vegetation. 

Remains below the water

surface attached to roots 

and stems. Hatchlings

emerge in spring. 

Culex pipien s Night

Usually

migrates

only short

distances. Many

Found around water with

high organic content, as in

catch basins & sewer

effuent ponds, tree holes,

artificial containers &

manholes.

Proliferate in in artificial

containers. Lays eggs in

clusters of 50 to 400. 

Larval and pupal stages

take 8 -10 days.

Culex tarsalis Night

Enters

buildings 

after dark. Many

Any fresh water, artificial

containers, & agricultural

and irrigated areas

Larvae develop from

spring to fall in waters w/

high organic material.

Eggs laid in rafts of 100 -

150 & hatch w/in 48 hrs.

Culiseta

inornata

Dawn &

Dusk

Stays near

habitat. Many

Cold water - associated with

glacial runoff and sunlit

waters, does not like hot 

weather. Found at all

elevations.

Breeds throughout spring

and summer in cold

water, females may

appear during warm

winter breaks. Usually

feeds on livestock, not

people.

Ochlerotatus

japonicus

Day &

Night Not known Many

Artificial containers, catch 

basins, undergound tanks 

and vaults & tree holes

Larvae are found in

artifical containers.

*New information has come in on Ochlerotatus canadensis that adults live for several months in woodland pools by

melting snow or rain. They feed on a large range of mammals, birds, and reptiles. 

Page 10 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
Additional Monitoring for Public and Specialty Targeted Areas of Control

3⁄4 Conduct ongoing mosquito larvae surveillance, including studying habitats by air, aerial 

photographs and topographic maps, and evaluating larval populations. 

3⁄4 Monitor and track data from mosquito traps, biting counts, complaints, and reports from the 

public. 

3⁄4 Keep seasonal records concurrent with weather data to predict mosquito larval occurrence 

and adult flights. 

3⁄4 Consider using sentinel chicken flocks for surveillance (See Centers for Disease Control and 

Prevention, Epidemic/Epizootic West Nile Virus in the United States: Guidelines for 

Surveillance, Prevention and Control, page 10, 

http://www.cdc.gov/ncidod/dvbid/westnile/resources/wnv-guidelines-aug-2003.pdf) 

3⁄4 Accurately map and identify rearing areas for mosquitoes. These would be those sites that 

cannot be eliminated by preventative measures such as emptying containers, proper pond 

maintenance, and eliminating excess standing water by using appropriate irrigation BMPs. 

These habitats can be identified by aerial photo assessments, topographic maps, and satellite 

imagery where available. This is important because appropriate treatment measures are 

contingent on the particular species that live in specific habitats. 

3⁄4 Agricultural site maps should include hay, pasture, circle irrigation, orchards, and rill 

irrigated field crops. An important land use that has caused problems to mosquito control 

districts in the past is flood irrigated pastures where the water stays on more than five to 

seven days. 

Note: Detailed information on mosquito surveillance is available from Washington State 

Department of Health, available online at 

www.doh.wa.gov/ehp/ts/Zoo/WNV/WAArboviralRespPlan.pdf and 

http://www.doh.wa.gov/ehp/ts/Zoo/WNV/WestNileVirusSurv.pdf 

 

III. Establish Targeted Densities for Mosquito Populations 

Risk Assessment: Probability of outbreak in humans: Remote to low; areas with confirmation of 

epizootic WNV in birds before August; a horse/human case, or sustained WNV activity in birds 

and/or mosquitoes.

Action threshold: The presence (positive identification) of any vector mosquitoes in the area 

triggers activities to reduce their presence. Since people with compromised immune systems are 

likely to be the most vulnerable to mosquito-borne diseases, the areas of their exposure should be a 

priority. 

Rationale: Once vector mosquitoes have been positively identified in an area, control treatments 

are warranted, especially around high risk populations. If the cost of treatments is prohibitive, 

every effort should be made to educate those at risk of exposure about minimizing habitat and

personal protection measures. 

BMP Minimum Response: Analyze disease activity data, i.e., avian mortality, human 

encephalitis/meningitis, equine encephalitis and mosquito surveillance information in the area of 

control. Set targeted densities with special consideration being given for segments of the 

Best Management Practices for Mosquito Control Page 11

Washington State Department of Ecology 
population most vulnerable to mosquito-borne diseases such as the elderly. If needed, enhance 

human surveillance and activities to further quantify epizootic activity, such as mosquito trapping

and testing. 

Establish Targeted Mosquito Densities for All Areas of Control

To establish the targeted density of mosquito populations review information on incidences of

avian mortality, human encephalitis/meningitis, and equine encephalitis for your area (the 

Department of Health or your local health department can provide this information). Conduct 

entomologic survey (inventory habitats and map mosquito populations). Using surveillance 

information and input from the people in the control area, establish the targeted density of 

mosquito populations based on the level of control desired by those in the area of control, public 

safety, and funding. 

3⁄4 Demarcate no-spray zones on maps. This may include areas such as schools, hospitals, fish 

farms, wildlife refuges, ecologically sensitive areas, the homes of individuals who are on 

chemically sensitive registers, and crops grown under a certified organic program. Other 

crop sites that do not have a tolerance for the mosquito control products used should also be 

listed. If the control entity is not a mosquito control district organized under RCW 17.28, 

then individual residences where the occupants do not want to be treated should be 

identified as no-spray zones. 

3⁄4 Individual homeowners and businesses determine targeted mosquito population densities 

based on the level of control desired and factors of risk and cost. Mosquito control agents 

must consult with their sponsors to determine targeted mosquito densities. 

3⁄4 Once the targeted density has been established, continue larvae surveys to find density 

response to habitat minimization efforts and need for larvicide treatments. 

IV. Mosquito Control Treatments 

Risk Assessment: Probability of outbreak in humans: Low to moderate 

Action Threshold: The positive identification of vector mosquitoes in the area may trigger 

activities to reduce their presence. Once minimization strategies have been taken, larvae surveys 

(i.e. dipping) can indicate the effectiveness of those efforts and the need for further action. General 

Permit Condition S4.2.C states that the targeted density of larvae is 1 per three dips to commence 

larviciding unless vector mosquitoes are in the area and the probable breeding sites are inaccessible. 

This level is a minimum; mosquito control agents may want to set the targeted density at a higher 

level due to cost and risk factors. 

Rationale: Once vector mosquitoes have been positively identified in an area, control treatments 

are warranted. If the cost of treatments is prohibitive, every effort should be made to educate those 

at risk of exposure about minimizing breeding habitat and personal protection measures. 

Minimum BMP Response: Treat mosquitoes to reduce populations below the targeted threshold 

using strategies that consider biological, cultural, mechanical, and microbial, biochemical, chemical 

control methods. Evaluate methods for effectiveness of control, human health and ecological 

impacts, feasibility, and cost effectiveness.

Page 12 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
Use an Integrated Pest Management (IPM) Approach for All Areas of Control

Ideally, an IPM program considers all available control actions, including no action, and evaluates 

the interaction among various control practices, cultural practices, weather, and habitat structure. 

An ecologically-based IPM strategy relies heavily on natural mortality factors and seeks out control 

tactics that are compatible with or disrupts these factors as little as possible. When biological, 

biochemical or chemical treatment is needed, select treatments based on the species of mosquitoes 

found in larva pools, the age of larva, breeding habitat, density of larval populations and 

temperature. 

Pesticide applications shall not commence unless surveillance of a potential application site 

indicates a larva/pupa count of greater than 1 per 3 dips and the need to apply insecticides to 

control mosquito populations, or unless dead birds, infected horses, or adult mosquito surveys 

indicate the presence of vector mosquitoes when larvae counts cannot be made due to their 

inaccessibility. In these cases beginning control methods such as larviciding may be desirable or 

even necessary without the larvae dips. However, just because a dead bird is found which tests

positive for WNV in an area does not mean that the vector mosquitoes are breeding in the nearest 

storm drain. Those in the business of controlling mosquitoes will have to know the breeding sites 

and species of vectors in the area to perform effective mosquito control. 

Natural resources biologists (e.g., WDFW) must be notified of planned control measures whenever

delicate (easily impacted) ecosystems could be harmed by mosquito control practices. Other 

resource management agencies (e.g., National Marine Fisheries Service and U.S. Fish and Wildlife 

Service) should be consulted to determine when and where operations may harm ecosystems 

critical to threatened or endangered species, as well as appropriate treatments in these situations. 

Biological Controls 

Natural Waters: WDFW has several concerns with stocking biological mosquito predators in 

natural waters. Along with the introduction of non-native fish, the transfer of fish diseases from

one location to another, even among native populations, can cause disease outbreaks. That is why 

all movement and stocking of fish requires a permit from WDFW, whether the fish are native or 

not. Due to the inability to test live fish without killing them, the transportation of fish from one 

watershed to another requires disease testing (usually on the adults at spawning, or by sacrificing a 

number of young fish) and verification that the remaining fish are reared on disease-free water. In

addition, any non-native fish stocking currently needs to go through SEPA review prior to 

approval. The laws in Washington State are designed specifically to prevent this type of “Johnny 

Apple-seeding” from occurring. For more information, please contact your nearest Regional Office 

of the Department of Fish and Wildlife. 

Ponds or Impoundments with No Inlets or Outlets: Biological methods may include stocking 

species such as the Three-Spined Stickleback (Gasterosterous aculeatus) which is native to 

Washington State and known to be an effective predator of mosquitoes. Mud minnow, perch 

tadpoles, dragonfly larvae, diving beetles, back swimmers and front swimmers also prey on 

mosquito larvae. Guppies, goldfish, and other fish commonly sold in pet stores are exempt from 

permitting by Washington’s Department of Fish and Wildlife (WDFW) and may be suitable for 

smaller ponds with no inlet or outlet, horse troughs, and ornamental pools. However, before 

Best Management Practices for Mosquito Control Page 13

Washington State Department of Ecology 
planting any of these exempt fish, consult with WDFW. Some of these fish, such as goldfish, may 

have severe ecological impacts on ponds and lakes. 

Mosquito Fish (Gambusia affinis) have been used for mosquito control in virtually every state 

because of the adult’s ability to consume large amounts of mosquito larvae. These warm water fish 

rarely exceed 2.5 inches and prefer shallow water. They tend to flourish in almost any 

environment, including well discharges, cisterns, water tanks, potholes, rain barrels, and open 

septic tanks. Gambusia have been known to dramatically reduce and even eliminate mosquito 

larvae. WDFW suggests that the use of Gambusia be integrated into an overall mosquito control 

plan rather than used as an exclusive solution to mosquito abatement. Permits must be obtained 

from WDFW for use of Gambusia as a mosquito control measure. 

Microbial, Biochemical and Conventional Chemical Controls 

Applications of insecticides to water must be made by individuals licensed by the Washington State 

Department of Agriculture and permitted by the Department of Ecology's Water Quality Program. 

Information on WSDA license requirements is online at: http://pep.wsu.edu or call WSDA toll-free 

at (877) 301-4555. Permitting information is available from Ecology's website at 

www.ecy.wa.gov/programs/wq/pesticides/index.html or call (800) 917-0043. 

Select product controls by comparing the species and targeted life stage of mosquitoes, the breeding 

habitat, density of larval populations, and temperature with the efficacy of the products, nontarget

impacts, resistance management, and costs. For example, while Bacillus products are effective on 

early instars they do not control older larva. Methoprene can be used on older larval stages (i.e., 

pupa), and for situations where it is too late to use either Bacillus thuringiensis israelensis or Bacillus 

sphaericus, a monomolecular film might be used. Some Bacillus products do not have residual 

characteristics when temperatures are high, and larval populations can grow at the rate of an instar 

a day. In this situation the larva may be in the late third to fourth instar stage before an application 

of Bacillus can be made. Always consult product labels for specific information on efficacy and use. 

Product Material Safety Data Sheets (MSDS) provide additional information such as protocols or 

measures to be taken for accidental releases and other pertinent product information. 

The following is the approved list of larvicides that may be considered for mosquito control 

operations. Consult with federal, state and local agencies as needed. 

1. Bacillus thuringiensis israelensis (Bti) 

2. Bacillus sphaericus (H-5a5b) 

3. Methoprene Granular, Liquid, Pellet, or Briquet (Restricted on state listed species sites 

– see Appendix A). 

4. Monomolecular Surface Films (Restricted on state listed species sites – see Appendix 

A). 

5. Paraffinic white mineral oil. Paraffinic white mineral oil is restricted on state listed 

species sites – see Appendix A and shall not be used in waters of the state unless: 

a. The mosquito problem is declared a public health risk; or 

Page 14 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
b. The other control agents would be or are known to be ineffective at a specific 

treatment site; and 

c. The water body is non-fish-bearing (when uncertain, consult Washington State Fish 

and Wildlife concerning fish and wildlife) and has no inlet or outlet. 

6. Temephos may not be used in lakes, streams, wetlands or the littoral zone of water

bodies. The use of temephos shall be allowed only in highly-polluted water (i.e. tire 

piles) or waters with high organic content (i.e. manure holding ponds and pastures 

with no surface water runoff), or under either of the two following conditions: 

a. As a result of consultation between the Departments of Agriculture and of Ecology 

in response to the development of pesticide resistance or ineffectiveness within a 

population of mosquitoes. When temephos is applied to areas draining to surface 

waters monitoring of persistence and residues are a condition of the approval. 

Temephos must be rotated with one or more of the approved alternatives with a

different mode of action to minimize the development of resistance. 

b. As a result of consultation between the Department of Health and Department of 

Ecology in response to the development of a human health emergency as 

determined by the Washington State Department of Health. 

7. Terrestrially applied insecticides are NOT regulated under federal or state water 

pollution control laws and are not subject to NPDES permit conditions or 

requirements. A variety of adulticides are regulated for use by WSDA in Washington 

State. Table 4 lists some of these products. However, in Washington State all 

applications of insecticides over water must be permitted under a Clean Water Act (NPDES) 

permit. 

Best Management Practices for Mosquito Control Page 15

Washington State Department of Ecology 
Table 3. Permitted Insecticides Used For Mosquito Larvae Control 

Typical

Products

Active

Ingredient 

Label Use

Rate and 

2003 cost 

estimates 

Application Method(s)

Persistence and 

Comments 

Human 

Health

Restrictions

Permit 

Restrictions on

Use** 

Target

Pests on

Label 

Aquabac, 

Bactimos, 

Vectobac and 

Teknar 

(Bti) Bacillus 

thuringiensis 

israelensis 

0.25 to 2 

pints/acre 

or up to 10 

lbs/acre @ 

$24/gal. 

Granules 

$1.65/lb 

Hand sprayer, ground 

sprayer or sprinkler 

cans. 

Effective 1 - 30 days 

depending on 

formulation. 

Broad spectrum, except 

Coquilletidia

Not for 

potable 

water. 

Minimal nondietary and 

dermal risk to 

infants and 

children. 

None. Mosquito

larvae 

VectoLex 

WDG 

Bacillus 

sphaericus

(H-5a5b) 

0.5 to 1.5 

lbs/acre 

$4.65/lb

Granules are mixed with

water and sprayed. 

Effective for 1-4 weeks, 

depending on the 

species of mosquito 

larvae, weather, water 

quality and exact form 

of the granules. 

Effective on Culex spp. 

Less effective against 

other species. 

Not for 

potable 

water. 

Essentially 

nontoxic to 

humans. 

None. Larval

control in

water with

high organic 

content. 

Altosid liquid Methoprene: 

Active 

ingredient is 

a growth 

hormone 

mimic that 

does not 

allow the 

mosquito 

larvae to 

mature. 

3-4 oz./acre 

$226/gal 

Altosid pellets Methoprene 2.5-10 

lbs/acre 

$24/lb

Altosid XR Methoprene 1 briquette 

$2.70 

@100-200

sq ft. 

Altosid 

briquet 

Methoprene 1 briquette / 

$.90 @100 

sq ft. 

Altosid XR-G Methoprene 5-20 lbs/ac 

$8.48/lb

Use hand and ground 

sprayers. 

Effective for a few days 

unless specially 

formulated for slow

release. 

It is not persistent 

because it degrades 

rapidly in water. 

The briquettes are used

in areas needed for 

longer term residual 

control such as ponded 

areas of standing water, 

areas where flood 

waters may make it 

impossible to use Bti. 

Rates increase with 

deeper water. 

Altosid XR-G is a sand 

formulation, good for 

pastures or marshes 

with thick vegetation. 

Not for 

potable 

water. Does 

not pose 

risks to 

human

health. 

Restricted on 

state listed specie 

sites – see 

Appendix A. 

mosquito 

larvae.*** 

Page 16 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
Typical

Products

Active

Ingredient 

Label Use

Rate and 

2003 cost 

estimates 

Application Method(s)

Persistence and 

Comments 

Human 

Health

Restrictions

Permit 

Restrictions on

Use** 

Target

Pests on

Label 

Agnique MMF

Arosurf MSF

Monomolecul

ar surface 

film 

Poly(oxy-1,2-

ethanediyl)Al

phaisooctadecylhydroxy

0.2 to 1.0 

gal/acre 

@ $30/gal. 

Sprayed by hand or 

ground equipment. Film 

remains active for 10-14 

days on floodwaters, 

brackish waters and 

ponds. Susceptible to 

wind breaking surface 

tension. Rendered 

ineffective at winds 

above 10 mph and in

very choppy water. 

Adult females are killed 

by entrapping and 

drowning when they

contact the surface to 

lay their eggs. 

Okay for 

potable 

water,

livestock, 

backyard 

ponds, pool 

covers. 

No risk to 

human

health.

Restricted on 

state listed specie 

sites – see 

Appendix A. 

Larval, pupal 

and midge 

control. 

Adult female 

mosquitoes. 

Golden Bear 

Oil 

Bonide Oil 

Petroleum 

distillate oils 

prevent the 

larvae from 

obtaining

oxygen

through the 

surface film 

3 to 5 

gal/acre 

$5 -8/gal

Liquid formulations are 

sprayed by hand or 

ground equipment. 

Persists for 12 – 15 

hours, then evaporates. 

Less expense--kills 

pupae stages 

No risk to 

human

health. 

May not be 

applied to fishbearing waters or 

on state listed 

specie sites – see 

Appendix A. 

Larval and 

pupal 

control. 

Abate temephos 0.5 to 1.5 

oz/acre 

$2.00/oz 

Sprayed liquid. Breaks 

down within a few days 

in standing water, 

shallow ponds, 

swamps, marshes, and 

intertidal zones.

Temephos is applied 

most commonly by

helicopter but can be 

applied by backpack 

sprayers, fixed-wing 

aircraft, and right-of-way

sprayers in either liquid 

or granular form. 

Not for

potable 

water. 

Poses low

risk to human 

health. High 

dosages, like 

other OPs*, 

can overstimulate the 

nervous 

system, 

causing 

nausea, 

dizziness, & 

confusion. 

Highly restricted 

use – see permit 

condition 1A.5. 

 

Mosquito 

larvae, 

midge, 

punkie gnat, 

and sandfly

larvae in 

non-potable 

water. 

Malathion 

8EC 

malathion 8 oz/acre, 

cost NA 

Labeled for use in 

intermittent flooded 

areas, stagnant water 

and temporary rain 

pools. 

Harmful by

swallowing, 

inhalation or 

skin contact.

Can only be used 

under an 

agreement 

between Ecology

and Health in the 

event of a 

disease outbreak. 

Aphids, 

leafhoppers, 

grasshopper

s, spider 

mites, bugs, 

beetles, 

moths, 

worms, flies, 

mosquitoes, 

& larvae. 

*OPs are organophosphates 

** Restrictions can be waived in the event of a threat to human heath as determined by state and local health departments (see p. 16). 

*** EPA’s 2001 Methoprene R.E.D. Fact Sheet states that methoprene “has activity against a variety of insect species, including horn 

flies, mosquitoes, beetles, tobacco moths, sciarid fly, fleas (eggs and larvae), fire ants, pharaoh ants, midge flies and Indian meal

moths.” However, no effectiveness is claimed against these insects at the dose label rate for mosquito larvae control. 

Best Management Practices for Mosquito Control Page 17

Washington State Department of Ecology 
When Adulticides Fit into a Mosquito Control Plan 

Terrestrially applied products are NOT regulated under federal or state water pollution control laws 

and are NOT subject to NPDES permit conditions or requirements when applied to terrestrial sites. 

However, adulticiding is often an integral component of an integrated pest management approach to 

mosquito control. In some instances, adulticiding can reduce or eliminate the need to heavily apply 

larvicides, can be used effectively with less environmental impact to non-targets, and can be costeffective. 

Select triggers for the use of adulticide products: Some mosquito control districts recommend using 

light traps to monitor for mosquitoes. For example, Adams County MD recommends that counts of 

8 to 12 mosquitoes caught in 12 hours or a 3 adult mosquito landing count per minute in a 

residential area triggers the need to adulticide (Thomas Haworth, personal communication, 

November 7, 2003). Some applicators recommend adulticiding residential areas and upland areas 

where mosquitoes are migrating only when there is evidence of mosquito-borne epizootic activity 

at a level suggesting high risk of human infection. The following are examples of this type of 

evidence: high dead bird densities; high mosquito infection rates; multiple positive mosquito 

species including bridge vectors; horse or mammal cases indicating escalating epizootic 

transmission, including bridge vectors, horse or mammal cases, or a human case with evidence of 

epizootic activity. 

Reducing vector densities below transmission threshold usually requires multiple ULV 

applications. Therefore, triggers should take into account this latency effect so that human 

transmission is not proceeding prior to or during operations. This presupposes identifying 

increasing human risk at least 2 weeks before human cases might present. Trigger design and 

implementation should reflect this need for preemptive adulticiding. 

BMPs for Adulticides: 

1) Meteorological conditions: 

• Record wind speed and direction before spraying and be observant of all changes in 

direction and speed during the application. Use appropriate wind indicators. Gauges

are highly recommended for ground applications and smoke for aerial applications. 

• For aerial applications, check temperature at different elevations to decide if there is an 

inversion. 

• Spray only when wind is away from sensitive sites. 

2) Do not spray in winds over 10 mph.

3) Follow label buffers. 

The following table gives a sample of mosquito adulticides that may be used in terrestrial applications 

in Washington State. Labels are available from http://picol.cahe.wsu.edu/. 

Page 18 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
Table 4. Common Insecticides Used for Adult Mosquito Control 

Typical 

Products

Active

Ingredient

Label Use 

Rate Use Cost

Residual 

Life Comments

Biomist & 

Kontrol, 

Permanone 

Aqua Reslin Permethrin

ULV 4 

oz/acre Adult Control $.24/oz 24 hours

Effective, 100 ft setback from water.

Pyrenone 

25-5 Public

Health 

Insecticide Pyrethrin

ULV 1-4 

oz/acre Adult Control $1.20/oz 1 hour

No set-backs to 

water. Approved for

crop and pasture

applications. 

Expensive 

Scourge Resmethin

ULV 4 

oz/acre Adult Control $.58/oz 1-4 hours

Has not performed

well in some areas.

No setbacks to 

water. 

Fyfanon 

ULV Malathion

ULV rates

vary Adult Control $.24/oz 24 hours

Very highly toxic to 

nontargets.

Anvil Sumithrin

0.0012 lb – 

0.0036 lbs ai 

per acre Adult Control $.40/oz 1-4 hours

Tested and used in 

the NW. No water

precautions.

Dibrom, 

Trumpet Naled

Not 

recommend

ed for

ground ULV 

use. Adult control

No set-backs to 

water. Approved for

crop and pasture

applications. 

What Constitutes an Emergency or a Health Threat? 

Health emergencies and health threats are declared by local health departments in consultation 

with Washington State Department of Health. A health officer should consider demographics, 

population densities and species of mosquitoes, proximity of positive identifications of mosquitoborne disease, and local tolerances for pesticide applications and disease outbreaks when assessing 

risk. 

Permitted insecticides may be applied to waters as conditioned by their FIFRA labels (including 

methoprene in restricted areas) once an application has been submitted but before permit coverage 

is granted as a result of consultation between departments of Health and Ecology, in response to a 

human health emergency or threat as determined by the Washington State Department of Health. 

If an emergency is declared or a threat is determined, and mosquito control actions haven’t already 

been taken, the responsible officials should immediately initiate actions to minimize mosquito 

breeding habitat and educate at risk populations about personal protection (see p. 2); they should 

then begin conducting larval surveys and secure the funding, permits and licenses needed for 

applying insecticides. For practical purposes, once an outbreak is underway, larval surveys and 

Best Management Practices for Mosquito Control Page 19

Washington State Department of Ecology 
other control and habitat minimization measures will have little immediate effect. At this point, 

personal protective measures and large-scale adulticiding may provide the only means to reduce 

human/vector contact and further spread of the disease beyond those already infected. 

References Cited

Colorado Mosquito Control, Inc. Online at: 

www.comosquitocontrol.com/flood_irrigation_&_mosquitoes.htm. 

Lilja, Jack. 2002. Mosquito-Borne Disease Response Plan, November 2002 Edition. Washington State 

Department of Health. Online at: www.doh.wa.gov/ehp/ts/zoo/wnv/wnv.html. 

O'Malley, Claudia C. 1995. Seven Ways to a Successful Dipping Career. Wing Beats, vol. 6(4): 23-24. 

American Mosquito Control Association. 

Pratt, Harry D. and Moore, Chester G. 1993. Mosquitoes of Public Health Importance and Their Control. 

U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, Selfstudy Course 3013-G: Vector-Borne Disease Control. 

Von Grey, Ed. 2002. Draft Appropriate Mosquito Management Strategies as a Response to West

Nile Virus. Washington State Department of Agriculture. 

Whitworth, Terry. 2002. Mosquito Management Policy for Residential Stormwater Ponds in King County, 

Washington. King County Department of Natural Recourses and Parks, Water and Land Recourses 

Division. 

Page 20 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
Appendix A 

State Listed Species Restricted Areas 

Best Management Practices for Mosquito Control Page A-1 

Washington State Department of Ecology 
Page A-2 Best Mana

gement Practices for Mosquito Control 

Washington State Department of Ecology 
Appendix A 

State Listed Species Restricted Areas 

Basis of Restrictions 

Ecology’s Aquatic Mosquito Control National Pollutant Discharge Elimination System (NPDES) General Permit 

No. WAG-992000 Condition S4 3 states, “In developing the IPM plan, the permittee shall consult with local 

governments and state and federal agencies as needed.” 

Ecology took the lead developing an IPM plan to assist many local governments and others performing mosquito 

control operations who were suddenly in the business of mosquito control due to the spread of the West Nile 

virus. As the permit required, Ecology consulted with the Department of Fish and Wildlife (WDFW) in spring 

2003 during this process. As a result, the WDFW identified wildlife species that it considered most vulnerable to 

certain mosquito control larvacides, identified the primary areas occupied by these species, and requested that 

pesticide applications be restricted in these areas. 

These areas, along with areas identified as habitat for federal and state listed fish species were compiled into a 

document called Insert A and listed as areas where larvicides containing monomolecular surface films, 

methoprene, petroleum distillates, malathion and temephos were not allowed for use. Larvicides containing 

Bacillus thuringiensis israelensis (Bti) and Bacillus sphaericus (BS) were allowed for use due to their low toxicity to

non-target species. Unfortunately, the recommendations for restrictions were given to Ecology after the mosquito 

spray season had begun and in the interest of having a permit pathway in place for applicators for the 2003 spray 

season Ecology listed the recommendations as Insert A and opened them for public review after the season was 

over. 

The restrictions in Insert A have been considerably revised as a result of a more thorough review initiated the fall 

of 2003. Significantly, the restrictions on the use of methoprene in waters containing federal and state listed 

salmon have been lifted to allow monitoring for effects. The restrictions for state listed species that are still 

recommended by WDFW are listed here and a map of those areas is provided. Restrictions specific to larvicide 

active ingredients are also noted on Table 3. 

Criteria Used for the Restrictions

The criteria WDFW used for denoting species as vulnerable were: 

1) State species of concern (i.e., listed as state endangered, threatened, sensitive, or candidate). 

2) Current range and distribution of the species was highly localized. 

3) The species inhabited freshwater wetlands during most of the mosquito control treatment period. 

Six species initially met those criteria: northern leopard frog (Rana pipiens), Oregon spotted frog (Rana pretiosa), 

western toad (Bufo boreas), western pond turtle (Clemmys marmorata), American white pelican (Pelecanus 

erythrorhynchos), and one butterfly, the Yuma skipper (Ochlodes yuma). 

WDFW identified areas occupied by the two extant populations of northern leopard frog; two extant western 

Washington populations of Oregon spotted frog; western toad breeding ponds (for western Washington only); the 

Best Management Practices for Mosquito Control Page A-3 

Washington State Department of Ecology 
three remaining western pond turtle populations; the single American white pelican breeding colony; and the one 

known Washington Yuma skipper population. 

Based on the review of published literature, expert advice, and the vulnerability of these rare and endangered 

animals and their freshwater invertebrate food resources WDFW requested that if mosquito control is deemed 

necessary in the areas WDFW described in April 2003, that it be restricted to the use of Bacillus products. 

Aerial Applications

An exception to the WDFW request regarding the use of certain larvicides was made for the single American 

white pelican breeding colony. The colony resides on islands and along the shores of the Columbia River, south of 

the confluence of the Snake River, in Walla and Benton counties. While bio-chemicals such as methoprene were 

not of concern for the pelican breeding colony, the method of aerial application was believed to be unacceptably 

disturbing for their successful breeding. It was agreed that aerial applications would not be made on the pelican 

breeding areas, whereas less obtrusive methods of mosquito control were acceptable near these sites. 

Discussions among several WDFW biologists found that aerial applications of larvicides disturbed work being 

done in a few wildlife refuge areas around the state. It was therefore agreed that operators making aerial 

applications over wildlife refuges should notify the appropriate regional WDFW office of their scheduled aerial 

applications at least 24 hours prior to spraying. The notification can be made by phone or fax. 

Area of Impact

Only a few populations of northern leopard frog, Oregon spotted frog, western toad, western pond turtle, and 

Yuma skipper remain in Washington. The total area occupied by these species in rivers, lakes, ponds, and 

wetlands is tiny, comprising of portions of 117 sections (<0.18% of Washington State). The areas identified for 

northern leopard frog (36 Sections) and western pond turtle (13 Sections) are owned or managed by WDFW. A 

prudent, risk-adverse approach is warranted with vulnerable threatened and endangered species. 

The following areas are restricted to the use of Bacillus thuringiensis israelensis (Bti) and Bacillus sphaericus (H-5a5b) 

only. 

Sections added January 22, 2004 are in bold: 

1) Grant County, north of Moses Lake, within the Crab Creek watershed: T21N R27E 

 Sections 1, 12, and 13; T21N R28E Sections 7, 17, 18, 19, 28, 29, 30, 31, 32, and 33. 

2) Grant County, south and west of Moses Lake and south of Interstate-90, the northern 

 portion of the Potholes: T19N R27E Sections 33, 34, 35, and 36; T19N R28E

 Sections 31 and 32, 29, 30; T18N R27E Sections 1, 2, 3, 4, 8, 9, 10, 11, 12, 13, 14, 15, and

 16, 17; T18N R28E Sections 5, 6, 7, 8, 17, and 18. 

3) Grant County, area within and near the Sun Lakes – Dry Falls State Park wetlands:

 T24N R27E and 28E. 

4) Kitsap County: lakes, ponds, and wetlands located in T22N R1W Sections 1, 2, 10, 11, and 12. 

5) Klickitat County, west of the Klickitat River, all waters in T3N R12E Sections 28, 29, 32, and 33. 

6) Mason County, on the Kitsap Peninsula: lakes, ponds, and wetlands located in T23N

 R2W Sections 11, 12, 13, 14, 15, 22, and 23. 

7) Pierce and Kitsap counties, Carney Lake, located in T22N R1W. 

Page A-4 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
8) Pierce, Thurston, and Lewis counties, within the Nisqually River watershed: Alder 

 Lake (or Alder Lake Reservoir), located in townships: T15N R4E and 5E.

9) Pierce County, south of Tacoma, Chambers Creek and associated waters in T20N 

 R2E Sections 26 and 27.

10) Skamania County, east of Carson, all waters in T3N R8E Sections 23, 24, 25, 26, and 36; T3N R9E 

Sections 30 and 31. 

11) Thurston County, west of Yelm: lakes, ponds, and wetlands located in T17N R1E Sections 8, 9,

16, and 21. 

12) Thurston County, south of Olympia and east of Interstate-5, within the Black River 

watershed, the Beaver Creek drainage, located in T16N R2W Sections 9, 10, 11, and 

12; T16N R1W Section 7. 

13) Thurston County, south of Olympia and west of Interstate-5, within the Black River 

watershed: Black River proper from south of Black Lake to the Chehalis Riverconfluence, and 

the following tributaries, Stony Creek, Dempsey Creek, SalmonCreek, and Blooms Ditch. Legal 

description as follows for these sensitive areas: T17N R3W Sections 10, 11, 12, 13, 14, 15, 23, 24, 

25, 35, and 36; T17N R2W Sections 7, 18, 19, and 30; T16N R3W Sections 2, 11, 14, 19, 20, 21, 22,

23, 30, and31; T16N R4W Sections 25, 26, 27, 31, 32, 33, 34, 35, and 36. 

Best Management Practices for Mosquito Control Page A-5 

Washington State Department of Ecology 
Page A-6 Best Mana

gement Practices for Mosquito Control 

Washington State Department of Ecology 
 

Appendix B 

Insect Repellent Use and Safety 

From the Center for Disease Control 

Best Management Practices for Mosquito Control B-1 

Washington State Department of Ecology 
B-2 Best Mana

gement Practices for Mosquito Control 

Washington State Department of Ecology 
 

Appendix B 

Insect Repellent Use and Safety 

From the Center for Disease Control 

Q. Is DEET safe? 

A. Yes, products containing DEET are very safe when used according to the directions. Because 

DEET is so widely used, a great deal of testing has been done. When manufacturers seek 

registration with the U.S. Environmental Protection Agency (EPA) for products such as DEET, 

laboratory testing regarding both short-term and long-term health effects must be carried out. 

Over the long history of DEET use, very few confirmed incidents of toxic reactions to DEET 

have occurred when the product is used properly. (From the National Pesticide Information 

Center [NPIC], EPA re-registration eligibility decision. See 

npic.orst.edu/factsheets/DEETgen.pdf . )

Insect Repellents and Sunscreen 

Q. Can I use an insect repellent containing DEET and sunscreen at the same time?

A. Yes. People can and should use both sunscreen and DEET when they are outdoors to protect 

their health. Follow the instructions on the package for proper application of each product. 

Apply sunscreen first, followed by repellant containing DEET. 

To protect from sun exposure and insect bites, you can also wear long sleeves and long pants. 

You can also apply insect repellent containing DEET or permethrin to your clothing, rather than

directly to your skin. 

Q. Has CDC changed its recommendations for use of DEET and sunscreen? 

A. No. Based on available research, CDC believes it is safe to use both products at the same

time. Follow the instructions on the package for proper application of each product. Apply 

sunscreen first, then insect repellent containing DEET, to be sure that each product works as 

specified. 

Q. Should I use a combination sunscreen/DEET-based insect repellent? 

A. Because the instructions for safe use of DEET and safe use of sunscreen are different, CDC 

does not recommend using products that combine DEET with sunscreen.

In most situations, DEET does not need to be reapplied as frequently as sunscreen. DEET is 

very safe when applied correctly. The rare adverse reactions to DEET have generally occurred 

in situations where people do not follow the product instructions. Sunscreen often requires 

frequent reapplication, so using a combined product is not recommended. You do not need to 

reapply insect repellent every time you reapply sunscreen. Follow the instructions on the 

package for each product to get the best results. 

Best Management Practices for Mosquito Control B-3 

Washington State Department of Ecology 
Q. I heard about a study saying that there may be some type of interaction between 

repellents containing DEET and sunscreen. Is this true? 

A. There has been attention to a study concerning the chemicals in DEET and sunscreen 

presented at a scientific meeting. This is an in vitro study, which means that it is a laboratory 

study that did not include human or animal testing. The goal of the study was to examine 

absorption of these chemicals, and it did not evaluate or make conclusions about health effects 

related to this issue. The study authors stated that further evaluation of the interaction of these 

chemicals should be conducted. The study has not yet been published (as of July 2003). 

Evaluation by the EPA, which regulates products such as DEET, indicates that it is safe to use 

insect repellents containing DEET and sunscreen at the same time. CDC recommends using two 

separate products because sunscreen requires frequent applications while DEET should be used 

sparingly. Follow the directions on the package for each product, and consult your physician or 

pharmacist if you have questions. CDC's recommendations for the safe use of insect repellents 

on children and adults remain unchanged. 

Insect Repellent Use

Q. Why should I use insect repellent? 

A. Insect repellents help people reduce their exposure to mosquito bites that may carry 

potentially serious viruses such as West Nile virus, and allow them to continue to play and 

work outdoors. 

Q. When should I use mosquito repellent? 

A. Apply repellent when you are going to be outdoors and will be at risk for getting bitten by 

mosquitoes. 

Q. What time of day should I wear mosquito repellent? 

A. Many of the mosquitoes that carry the West Nile virus are especially likely to bite around 

dusk and dawn. If you are outdoors around these times of the day, it is important to apply 

repellent. In many parts of the country, there are mosquitoes that also bite during the day, and 

these mosquitoes have also been found to carry the West Nile virus. The safest decision is to 

apply repellent whenever you are outdoors. 

Q. How often should repellent be reapplied? 

A. Follow the directions on the product you are using in order to determine how frequently 

you need to reapply repellent. Sweating, perspiration or getting wet may mean that you need 

to re-apply repellent more frequently. If you are not being bitten, it is not necessary to re-apply 

repellent. Repellents containing a higher concentration of active ingredient (such as DEET) 

provide longer-lasting protection. 

Q. Should I wear repellent while I am indoors? 

A. Probably not. If mosquitoes are biting you while you are indoors, there are probably better 

ways to prevent these bites instead of wearing repellent all the time. Check window and door 

screens for holes that may be allowing mosquitoes inside. If your house or apartment does not 

have screens, a quick solution may be to staple or tack screening (available from a hardware 

B-4 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
 

store) across the windows. In some areas community programs can help older citizens or others

who need assistance. 

Q. How does mosquito repellent work? 

A. Female mosquitoes bite people and animals because they need the protein found in blood to 

help develop their eggs. Mosquitoes are attracted to people by skin odors and carbon dioxide 

from breath. Many repellents contain a chemical, N,N-diethyl-m-toluamide (DEET), which 

repels the mosquito, making the person unattractive for feeding. DEET does not kill 

mosquitoes; it just makes them unable to locate us. Repellents are effective only at short 

distances from the treated surface, so you may still see mosquitoes flying nearby. As long as 

you are not getting bitten, there is no reason to apply more DEET. 

Q. Which mosquito repellent works the best? 

A. The most effective repellents contain DEET (N,N-diethyl-m-toluamide), which is an 

ingredient used to repel pests like mosquitoes and ticks. DEET has been tested against a variety 

of biting insects and has been shown to be very effective. The more DEET a repellent contains 

the longer time it can protect you from mosquito bites. A higher percentage of DEET in a 

repellent does not mean that your protection is better—just that it will last longer. DEET 

concentrations higher than 50 percent do not increase the length of protection. 

Q. How does the percentage of DEET in a product relate to the amount of protection it gives? 

A. Based on a recent study: 

• A product containing 23.8 percent DEET provided an average of five hours of protection 

from mosquito bites. 

• A product containing 20 percent DEET provided almost four hours of protection 

• A product with 6.65 percent DEET provided almost two hours of protection 

• Products with 4.75 percent DEET and 2 percent soybean oil were both able to provide 

roughly one and a half hours of protection. 

Choose a repellent that provides protection for the amount of time that you will be outdoors. A 

higher percentage of DEET should be used if you will be outdoors for several hours while a 

lower percentage of DEET can be used if time outdoors will be limited. You can also re-apply a 

product if you are outdoors for a longer time than expected and start to be bitten by 

mosquitoes. (For more information, see Table 1: Fradin and Day, 2002. See Publications page.) 

Q. Why does CDC recommend using DEET? 

A. DEET is the most effective and best-studied insect repellent available. (Fradin, 1998). 

Studies using humans and mosquitoes report that only products containing DEET offer longlasting protection after a single application.(Fradin and Day, 2002. See Publications page.) 

Q. Are non-DEET repellents effective (e.g. Skin-So-Soft, plant-based repellents)? 

A. Some non-DEET repellent products which are intended to be applied directly to skin also 

provide some protection from mosquito bites. However, studies have suggested that other 

products do not offer the same level of protection, or that protection does not last as long as 

products containing DEET. A soybean-oil-based product has been shown to provide protection 

Best Management Practices for Mosquito Control B-5 

Washington State Department of Ecology 
for a period of time similar to a product with a low concentration of DEET (4.75%) (Fradin and 

Day, 2002. See Publications page.). 

People should choose a repellent that they will be likely to use consistently and that will 

provide sufficient protection for the amount of time that they will be spending outdoors. 

Product labels often indicate the length of time that protection that can be expected from a 

product. Persons who are concerned about using DEET may wish to consult their health care 

provider for advice. The National Pesticide Information Center (NPIC) can also provide 

information through a toll-free number, 1-800-858-7378 or npic.orst.edu. 

Q. I'm confused. Which products contain "DEET"? 

A. Most insect repellents that are available in stores are labeled with the chemical name for 

DEET. Look for N,N-diethyl-m-toluamide or, sometimes, N,N-diethly-3-methylbenamide. 

Choose a repellent that offers appropriate protection for the amount of time you will be 

outdoors. A higher percentage of DEET should be used if you will be outdoors for several 

hours while a lower percentage of DEET can be used if time outdoors will be limited. 

Using Repellents Safely 

Q. What are some general considerations to remember in order to use products containing 

DEET safely? 

A. Always follow the recommendations appearing on the product label.

• Use enough repellent to cover exposed skin or clothing. Don't apply repellent to skin 

that is under clothing. Heavy application is not necessary to achieve protection. 

• Do not apply repellent to cuts, wounds, or irritated skin. 

• After returning indoors, wash treated skin with soap and water. 

• Do not spray aerosol or pump products in enclosed areas. 

• Do not apply aerosol or pump products directly to your face. Spray your hands and 

then rub them carefully over the face, avoiding eyes and mouth. 

Q. How should products containing DEET be used on children? 

A. No definitive studies exist in the scientific literature about what concentration of DEET is 

safe for children. No serious illness has been linked to the use of DEET in children when used 

according the product recommendations. The American Academy of Pediatrics (AAP) 

Committee on Environmental Health has recently updated their recommendation for use of 

DEET products on children, citing: "Insect repellents containing DEET (N,N-diethyl-mtoluamide, also known as N,N-diethyl-3-methylbenzamide) with a concentration of 10 percent 

appear to be as safe as products with a concentration of 30 percent when used according to the 

directions on the product labels." 

The AAP and other experts suggest that it is acceptable to apply repellent with low 

concentrations of DEET to infants over two months old. Other guidelines cite that it is 

acceptable to use repellents containing DEET on children over two years of age. 

B-6 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
 

Repellent products that do not contain DEET are not likely to offer the same degree of

protection from mosquito bites as products containing DEET. Non-DEET repellents have not 

necessarily been as thoroughly studied as DEET, and may not be safer for use on children. 

Parents should choose the type and concentration of repellent to be used by taking into account 

the amount of time that a child will be outdoors, exposure to mosquitoes, and the risk of 

mosquito-transmitted disease in the area. Persons who are concerned about using DEET or 

other products on children may wish to consult their health care provider for advice. The 

National Pesticide Information Center (NPIC) can also provide information through a toll-free 

number, 1-800-858-7378 or npic.orst.edu. 

Always follow the recommendations appearing on the product label when using repellent. 

• When using repellent on a child, apply it to your own hands and then rub them on your 

child. Avoid children's eyes and mouth and use it sparingly around their ears. 

• Do not apply repellent to children's hands. (Children may tend to put their hands in

their mouths.) 

• Do not allow young children to apply insect repellent to themselves; have an adult do it 

for them. Keep repellents out of reach of children. 

• Do not apply repellent to skin under clothing. If repellent is applied to clothing, wash 

treated clothing before wearing again. 

Using repellents on the skin is not the only way to avoid mosquito bites. Children and adults 

can wear clothing with long pants and long sleeves while outdoors. DEET or other repellents 

such as permethrin can also be applied to clothing (do not use permethrin on skin), as

mosquitoes may bite through thin fabric. Mosquito netting can be used over infant carriers. 

Finally, it may be possible to reduce the number of mosquitoes in the area by getting rid of 

containers with standing water that provide breeding places for the mosquitoes. 

Q. Is DEET safe for pregnant or nursing women? 

A. There are no reported adverse events following use of repellents containing DEET in 

pregnant or breastfeeding women. 

Q. Are there any risks due to using repellents containing DEET? 

A. Use of these products may cause skin reactions in rare cases. If you suspect a reaction to this 

product, discontinue use, wash the treated skin, and call your local poison control center. There 

is a new national number to reach a Poison Control Center near you: 1-800-222-1222.

If you go to a doctor, take the product with you. Cases of serious reactions to products 

containing DEET have been related to misuse of the product, such as swallowing, using over 

broken skin, and using for multiple days without washing skin in between use, for example. 

Always follow the instructions on the product label.

Best Management Practices for Mosquito Control B-7 

Washington State Department of Ecology 
More information

Q. Where can I get more information about repellents? 

A. For more information about using repellents safely please consult the EPA Web site: 

http://www.epa.gov/pesticides/citizens/insectrp.htm or consult the National Pesticide 

Information Center (NPIC), which is cooperatively sponsored by Oregon State University and 

the U.S. EPA. NPIC can be reached at: npic.orst.edu or 1-800-858-7378. 

B-8 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
 

Appendix C 

Response to Public Comments 

Best Management Practices for Mosquito Control C-1 

Washington State Department of Ecology 
C-2 Best Mana

gement Practices for Mosquito Control 

Washington State Department of Ecology 
 

AQUATIC MOSQUITO CONTROL 

GENERAL PERMIT 

RESPONSE TO COMMENTS 

RECEIVED DURING THE PUBLIC COMMENT PERIOD FOR 

GENERAL NPDES PERMIT WAG-992000 MODIFICATION 

AND 

THE BEST MANAGEMNT PRACTICES FOR MOSQUITO CONTROL 

This appendix contains Ecology’s response to comments to the Mosquito Control General NPDES 

Permit WAG-992000 Modification and changes to the Best Management Practices for Mosquito Control 

(BMP) document received during a public review period from December 15, 2003, to March 25, 2004. 

Prior to the formal public review period the Departments of Health and Fish & Wildlife, many 

mosquito control districts, local governments, mosquito control product manufacturers and other 

interested parties worked with Ecology to revise the statewide permit and BMPs to make them more 

pertinent and usable for those conducting mosquito control efforts in the field. Meetings were held 

October 14 and 15 in Moses Lake, on October 21 in Lacey, and again on March 5 in Olympia, at the 

Washington State Department of Agriculture. As a result, Ecology received many informal comments 

and suggestions that were incorporated into the proposed modifications. 

Formal comments received during the public comment period pertain to wetlands, the use of 

methoprene and other mosquito control products, thresholds for larviciding (including preemptive 

methods of control), adulticiding, dipping requirements, suggestions for clarification, and other

miscellaneous comments. 

Commenters 

1. Joseph M. Conlon, American Mosquito Control Association 

2. Karl Malamud-Roam, Ph.D., Contra Costa Mosquito & Vector Control District, Concord, CA 

3. Steve Foss and Wendy Sue Wheeler, Washington State Department of Agriculture 

4. Ann Potter and Rocky Beach, Washington State Department of Fish & Wildlife 

5. Tom Haworth, Adams County Mosquito Control District, WA 

6. Kevin Shoemaker, Northwest Mosquito and Vector Control Association 

7. Heather Hansen, Washington Friends of Farms and Forests

8. LaDell Yada, Washington State citizen 

9. Lou Dooley, Environmental Health Director, Clark County Mosquito Control District 

10. Doug Van Gundy, Wellmark International 

11. William Meredith, Delaware Mosquito Control Section, Department of Fish and Wildlife 

12. Wayne Switzer, Eden Advance Pest Technologies 

13. Art G. Losey, Washington State Pest Control Association 

14. William Peacock, City of Spokane 

15. Jim Thompson, Grant County Mosquito District #1 

Best Management Practices for Mosquito Control C-3 

Washington State Department of Ecology 
16. Jim Tabor, WDFW 

17. Mike Young, Snohomish Health District 

18. Gerald Campbell, Grant County Health District 

19. Mark Newberg, Wellmark International 

20. Benjamin Hamilton, Washington State Department of Health 

21. David Ensunsa, Columbia Mosquito Control District 

22. Dan Mathias, City of Everett 

Comments have been summarized and those commenting are referenced by the number given to them 

above. Where comments resulted in a change to either the BMP document or the permit, that change is 

noted. 

Wetlands 

Comment 1. The role of natural predation in the control of mosquitoes in the document and the 

webpage from which it is derived is somewhat overstated. Although there is a wealth of literature 

recording observations and extolling the importance of the Odonata as predators of diurnally active 

adult mosquitoes, this has not been supported by controlled field studies. While predation provides a 

worthy and welcome contribution to our integrated mosquito control efforts, it simply cannot provide 

the level of control needed when human lives are at stake This is certainly not meant to downplay the 

important part proper wetlands management plays in ecology and a fully-integrated mosquito 

management strategy, for the American Mosquito Control Association fully supports and endorses

proper wetlands management. But I would caution against promulgating this as the sole means of 

mosquito control in areas where these wetlands are found. I would advise further caution against 

underestimating the potential magnitude of mosquito production even in natural, healthy wetlands. 

(1, 11, 12) 

Response to Comment 1. Mosquito "outbreaks" most often occur in destabilized wetland and stream 

ecosystems that have been changed or tampered with so that the predators of the larvae such as invertebrates, 

insects, and amphibians are excluded. Ecology does not suggest that predation is the only means of mosquito 

control for wetlands. While draining and/or filling wetlands are not approved methods, taking an integrated 

approach that targets mosquito larvae control, including bio-chemical control, is recommended in the BMPs. 

(Tom Hruby, Ecology Wetland Specialist, Personal Communication, 1/16/04) 

Methoprene 

Comment 2. This comment addresses the issue of restricting the use of methoprene in areas known to 

provide habitat for state threatened and endangered species during the mosquito spray season. There 

were divergent views on the proposed restrictions. One view suggests the restrictions are unnecessary, 

the other suggests precaution due to unknowns. Let it be noted that the WDFW restricted areas are the 

only conditions for methoprene in the Permit that are more stringent than the application conditions 

set by FIFRA labels. 

View 1. The 2001 USEPA document together with the 2003 USEPA research and the World 

Health Organization/FAO review state that methoprene will have minimal adverse effects on 

non-target species. The Fish and Wildlife letter pretty dramatically overstates the risks 

C-4 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
 

associated with this product. Decisions should be made on sound science. Methoprene should 

be allowed in areas identified by WDFW to minimize the potential increase use of adulticides, 

which will likely result due to insufficient control of third and fourth instar larvae using Bacillus

products only. (1, 2, 3, 5, 6, 7, 8, 9, 10, 12, 13, 15, 18, 19, 41) 

View 2. We [WDFW] appreciate Mr. VanGundy's [VanGundy represents Wellmark 

International, a manufacturer of methoprene products] explanation regarding the contents of 

the EPA RED documents. We notice that EPA does denote a level of amphibian toxicity from 

methoprene ("minimally toxic to amphibians"). In our October 13 letter, we state that research 

on methoprene and frog deformities is inconclusive. There are studies that have found 

developmental effects to amphibians when methoprene was applied at mosquito control 

treatment levels, and there are studies that have not observed this. We do not think it would be 

productive to engage in debate/rebuttal over all research on this matter. Because the body of 

research on this topic is not definitive we have chosen to use the precautionary principal when 

conserving state and endangered species. We did not feel that there was sufficient information 

to recommend that methoprene products be restricted other than in very localized areas where 

we have identified T & E species. Given that we are making recommendations for state 

threatened and endangered species, in very limited areas, and that other efficacious mosquito 

control products (Bacillus) are available, we continue to support our original recommendation 

on methoprene restriction. (4, 16) 

 

Response to Comment 2. Aquatic Mosquito Control Permit No. WAG – 992000, Section S4. Best Management 

Practices/Integrated Pest Management requires the preparation and implementation of an Integrated Pest Management 

Plan (IPMP) by the permittee. Among other conditions, the section states, “in developing the IPM plan, the permittee 

shall consult with local governments and state and federal agencies as needed.” The Permit Fact Sheet provides the 

following rationale for this condition: 

...an IPM program considers all available control actions, including no action, and evaluates the 

interaction among various control practices, cultural practices, weather, and habitat structure. This 

approach thus uses a combination of resource management techniques to control mosquito populations 

with decisions based on surveillance. Fish and game specialists and natural resources biologists should be 

involved in planning control measures whenever delicate ecosystems could be impacted by mosquito 

control practices (p. 9). 

Ecology took the lead developing an IPM plan to assist local governments and others performing mosquito control 

operations who were suddenly in the business of mosquito control due to the spread of the West Nile virus. As 

the permit required, Ecology consulted with the Department of Fish and Wildlife (WDFW) in spring 2003 during 

this process. WDFW identified wildlife species that it considered most vulnerable to certain mosquito control 

larvacides, identified the primary areas occupied by these species, and requested that pesticide applications be 

restricted in these areas. Ecology also invited representatives from the industry to comment on the basis of the

WDFW recommendations. An evaluation of the issue yields the following facts: 

1. The criteria WDFW used for denoting species as vulnerable were appropriate. Only those listed as state 

endangered, threatened, sensitive, or candidate species that inhabited freshwater wetlands during most of the 

mosquito control treatment period were considered. Five species met the criteria: northern leopard frog (Rana 

pipiens), Oregon spotted frog (Rana pretiosa), western toad (Bufo boreas), western pond turtle (Clemmys 

marmorata), and one butterfly, the Yuma skipper (Ochlodes yuma). 

Best Management Practices for Mosquito Control C-5 

Washington State Department of Ecology 
2. The total area occupied by these species in rivers, lakes, ponds, and wetlands is tiny, comprising of portions of 

117 sections (<0.18% of Washington State). Many areas identified for northern leopard frog (36 Sections) 

and western pond turtle (13 Sections) are owned or managed by WDFW. 

3. EPA’s Methoprene Registration Eligibility Document (RED) is dated March 1991. This document has not 

been updated. A Fact Sheet for the RED was updated in 2001. The RED document states, “The Agency does 

have data, however, that show that methoprene is highly acutely toxic to estuarine invertebrates” (p. 12). The 

Fact Sheet for the RED document updates this assessment and describes the level of amphibian and fish 

toxicity from methoprene as "minimally toxic" but does not define what that means or explain if that is 

sufficiently protective for federally and state listed species of concern. The World Health Organization 

indicates methoprene is slightly toxic to fish but lists no data on amphibians. 

4. Recent research on methoprene and frog deformities and developmental toxicity is inconclusive. For example, 

La Clair el al. (1998) found that methoprene breaks down quickly in sunlight and very low concentrations of 

the byproducts from degradation interfere with normal amphibian development. The La Clair study 

concluded “the addition of 1μL/L of several of S-methoprene’s degradates to the environment of developing 

[amphibian] embryos resulted in juveniles with deformities similar to that found naturally.” Ankley et al. 

(1998) found that UV light caused amphibian limb malformations whereas methoprene did not. The study 

further reported that concentrations of 500 ppb of methoprene caused mortality in amphibians. Degitz et al. 

(2003) was unable to reproduce the results of the La Clair study, but did determine that methoprene and its 

metabolites did not cause any adverse effects at rates < 1.25 ppm. These data are too disparate to be 

conclusive. Even though risk levels appear to be low, much uncertainty still exists with both the 

concentrations and the role methoprene and its metabolites play with the normal development of amphibians. 

5. Levels of methoprene that may be found in the environment after mosquito control applications are also 

variable. Concentrations have ranged from 4 ppb at seven days post treatment from an Altosid 30-day 

briquette (Ross et al 1994) to 0.01 ppm from sustained-release formulations (Degitz et al. 2003). Henrick, 

et.al. (2002) found 26 ppb s-methoprene in ponds treated with Altosid Liquid Larvicide (ALL) at day one, and 

1 ppb at day seven. However, one of the metabolites, 7-methoxycitronellal acid, was found at 267 ppb at day 

1 and 237 ppb at day 7. Notably, these levels do not represent multiple treatments or potential accumulation

or any number of other factors, such as shade, wind, water flow, temperature, pH, turbidity, etc. that may 

affect concentrations of applications to the natural environment. 

6. Several studies concluded that a dose-exposure connection between frog deformities and methoprene 

applications for mosquito control is unlikely and that correlations between locations of methoprene 

applications for mosquito operations and frog deformities have not been found (Henrick, et.al. 2002, Johnson 

et.al. 2001, Ankley et al. 1998). 

7. Larvicides containing Bacillus thuringiensis israelensis (Bti) and Bacillus sphaericus (BS) are allowed for use 

in these areas due to their extreme low toxicity to non-target species. 

8. The restricted areas identified by WDFW may pose a threat to human health when used as breeding grounds 

by mosquito vectors due to the narrow window of effectiveness of Bacillus products. 

Ecology proposes to allow the use of methoprene in more than 99% of the state as conditioned by the federal FIFRA label 

but will continue to restrict the areas of application for methoprene as recommended by WDFW except in the event of a 

human health threat from mosquito-borne disease as determined by the State and local health departments. Monomolecular films, oils and organophosphates are also restricted in these areas, but the restriction on the use of methoprene 

was questioned due to its low toxicity to non-targets and high selectivity for mosquito larvae. 

To aid future decisions regarding the use of methoprene Ecology will complete a SEPA checklist evaluation of the use of 

methoprene for mosquito control operations and initiate a monitoring strategy to document concentration levels of 

methoprene applications in relation to possible adverse effects to non-target species. Many individuals, including people 

C-6 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
 

representing mosquito districts, local governments, Wellmark and WDFW have offered to review and/or assist with the 

monitoring strategy. We intend on taking advantage of these offers. The objective of the evaluation and monitoring plan 

will be to provide data on methoprene for decisions relevant to permit renewal by November 2006. 

Comment 3. Page 13 of the BMP under permitted pesticides for mosquito control: The chart should be 

corrected to show specifically listed pests for Altosid products. Under the heading “Target Pests on Label,” the 

identified species in the chart for Altosid indicates a variety of pests. There are other methoprene labels that 

reflect these pests but for Altosid, mosquitoes are the only listed species. (10) 

Response to Comment 3. Agreed, the chart has been edited. However, EPA’s 2001 Methoprene R.E.D. Fact Sheet 

states that methoprene “has activity against a variety of insect species, including horn flies, mosquitoes, beetles, tobacco 

moths, sciarid fly, fleas (eggs and larvae), fire ants, pharaoh ants, midge flies and Indian meal moths.” This information

has been noted as a footnote to the chart. 

Comment 4. The statement in Comment 2, that “We do not think it would be productive to engage in 

debate/rebuttal over all research on this matter [methoprene and amphibians],” is frankly stunning. I do not 

believe that I have ever seen a government entity express the thought that they do not want to review the 

scientific facts on a controversial issue. Given that Ecology's proposed alternatives to methoprene are 

frequently less effective in numerous circumstances, that resistance management through pesticide rotation is 

a cornerstone of modern IPM, and that USEPA and numerous other independent reviewers have found 

"minimal toxicity" or equivalent wording, the proposed prohibition should not occur without strong scientific 

evidence supporting it, and this has not been provided. (2) 

Response to Comment 4. The comment referred to in the above statement, made by WDFW, was explained in the 

context: “Because the body of research on this topic is not definitive we have chosen to use the precautionary principal 

when conserving state and endangered species. We did not feel that there was sufficient information to recommend that 

methoprene products be restricted other than in very localized areas where we have identified T & E species. “ They did 

review scientific facts, it was the lack of evidence regarding methoprene’s toxicity that lead them to recommend the 

precautionary principle. 

Comment 5. The WSDA would like to inform Ecology that the Centers for Disease Control and Prevention, 

(CDC), recommends the alternation of biorational larvicides (Bti and Bs) and insect growth regulators 

(methoprene) annually or at longer intervals to prevent the development of insecticide resistance in vector 

populations. The WSDA recommends that the restriction of the use of Bti and Bs only in certain areas 

identified in the BMPs be amended by allowing some use of methoprene in rotation and in combination with

the approved biorational larvicides so as to prevent the development of resistance to Bti and Bs. (3) 

Response to Comment 5. Since the restriction on the use of methoprene applies only in very select sites the rotation 

process recommended would not be precluded in 99% of the state. 

Further, in discussions with mosquito control operators around the state, we found that resistance to Bacillus products 

has not been found. 

Comment 6. The language allowing local jurisdictions to declare a health threat so they have access to 

methoprene is very unclear. This BMP gives no guidance as to how local boards of health are to be proactive 

in protecting the communities or what thresholds should be used to determine the potential for human health 

risks. Inconsistent mosquito control thresholds could result in water quality problems and people taking 

illegal control measures into their own hands. (7, 16, 17) 

Best Management Practices for Mosquito Control C-7 

Washington State Department of Ecology 
Response to Comment 6. Permit condition S1.4. restricts the use of methoprene in areas designated by Washington 

State Department of Fish and Wildlife except when a heath threat exists in those areas as determined by the State and 

local health departments. No health-based thresholds are stated in the BMPs or the permit. The Department of Health 

requested that health-based determinations be stated in general terms because the process and criteria used by local health 

jurisdictions to determine health threats is dependent on local conditions such as demographics, population densities and 

species of mosquitoes, proximity of positive identifications of mosquito-borne disease, tolerances for pesticide applications 

and tolerances for disease outbreaks. However, the language on page 16 of the BMP under the section, “What Constitutes 

an Emergency of Health Threat?” has been clarified. 

Comment 7. Page 3, paragraph 3. Regarding the statement in the BMP that methoprene is an endocrine 

disrupter, this statement is untrue and I would propose that this language be removed from the revised BMP. 

While endocrine disruption is becoming an area of concern, there are still ongoing discussions surrounding 

testing methodologies. Currently there is a lack of validated test systems. Methoprene does not disrupt the 

production of any glandular hormone within insects, other invertebrates, vertebrates or mammals. In insects it 

merely augments naturally occurring insect juvenile hormone (JH) at times in the insect life cycle where 

natural production of JH is at a minimum. An example would be during the molt from the last larval instar to 

the pupa or adult stage. I have included a more detailed commentary as Attachment 1. In mammalian 

systems, methoprene is broken down and excreted primarily through urine. (10)

Response to Comment 7. Ecology agrees, the statement has been removed. 

Comment 8. There has been much discussion of the non-target effects of methoprene. As I outlined in my 

other correspondence, there exists a wide margin of safety to non-targets when methoprene is used according 

to label directions. There is no concern for accumulation of methoprene in the environment as it rapidly 

degrades, further Henrick et al. 2002, report that the degradation products of methoprene rapidly degrade as 

well, without accumulation. Methoprene can be used with confidence against mosquito larva but also 

provides for wide safety margins to non-targets. (10) 

Response to Comment 8. While there is no evidence that the use of methoprene for mosquito control will lead to 

amphibian malformations or other adverse effects to non-targets, the data are inconclusive. Scant monitoring has been 

done of methoprene applications for mosquito control in the natural environment. Ecology is initiating a monitoring 

strategy to document concentration levels of methoprene applications in the environment and will evaluate those levels in

relation to possible adverse effects to non-target species. 

Comment 9. Page 18. There are some errors in the chart that need correction. The Altosid Liquid use rate 

should be changed to 3-4 ounces per acre instead of 2-20 pounds per acre. The 2-20 pound rate is incorrect for 

this formulation. The rate for Altosid XRG should be changed from 8-10 pounds to 5-20 pounds to reflect the 

label rate. In the target pest category there are a variety of pests listed for the Altosid products. While this list 

is inclusive of several product lines, it does not reflect the fact that the Altosid products are labeled only for 

mosquitoes. I would suggest that pricing be removed from the chart. Pricing is subject to change by time and 

location. Since the BMP is a document that will exist for some time, the pricing that is stated now in the BMP 

may not be indicative of a current price for future referrals to the BMP. (10) 

Response to Comment 9. The errors have been corrected. Prices, based on 2002 levels, will be retained only as a 

general guide for cost comparisons, one of the factors for consideration in an IPM plan. 

Comment 10. Methoprene's Impacts To Amphibians? -- A few years ago, the U.S. Fish and Wildlife Service

(USFWS) imposed a condition for methoprene's use on one of our two National Wildlife Refuges that 

C-8 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
 

methoprene not be applied over wetlands where the salinity was less than 5 ppt, done in what appeared (at 

least to us) to be an overly-zealous application of the precautionary principle, because of the service's 

supposed concerns about the impacts of methoprene upon the developmental stages of amphibians (which of 

course are found more typically in freshwater habitats than in salt marshes, hence the Service's 5 ppt 

demarcation). However, this restriction only lasted for one year in relation to our then questioning the 

USFWS's scientific foundations about their position -- after further review of the scientific evidence, it then 

seemingly became apparent to the USFWS that there was no credible scientific evidence to link any amphibian 

developmental abnormalities or deformities seen in the field with exposures to methoprene associated with 

operational mosquito control, and this unnecessary restriction was rescinded. As I probably don't have to tell 

you or others in your state agency (if you're familiar with the scientific literature about these matters), several 

other much more probable causes of amphibian developmental abnormalities have now been scientifically 

identified (e.g., parasitic infections, excessive UV light exposure, etc.) and been linked as the primary culprits 

for what has been observed for amphibian abnormalities, with any lingering connection here to the use of 

methoprene being an unwarranted, poorly-founded leap that serves little purpose (but wherever such claims 

still unfairly persist, then this bias certainly hinders the beneficial use of an important, environmentallycompatible mosquito control tool). (11) 

Response to Comment 10. We would be interested in any written assessment made by the USFWS on this matter. 

Thank you for your comments. 

Comment 11. Methoprene Use In Coastal Wetlands -- As part of our statewide Integrated Pest Management 

(IPM) approach to mosquito control, we use methoprene (a juvenile growth-hormone mimic) as our frontline 

operational larvicide for salt marsh mosquito control, in spraying thousands of acres of Delaware's coastal 

wetlands with Altosid up to several times each summer (with our primarily using the A.L.L. 20% Concentrate 

formulation), including extensively using Altosid on Delaware's two National Wildlife Refuges. We find that

Altosid gives us effective control achieved in practicable manner, and to the best of our knowledge does not 

have any unacceptable non-target impacts or environmental problems. 

Methoprene Use In Freshwater Wetlands -- We also use methoprene for control of freshwater mosquitoes in 

stormwater management basins and constructed wetlands, often using some type of extended release 

formulation for this product in these settings, which similar to our salt marsh use also gives us effective control 

without any unacceptable side effects. (11) 

Response to Comment 11. Thank you for your comments on the use of methoprene. However, lacking any qualification 

on what is meant by “to the best of our knowledge” or “unacceptable side effects” we cannot make decisions on the use of 

methoprene based on these testimonials.

Comment 12. [My] Only comment is on the BMP page eleven last paragraph, with the sentence that begins 

with "Methoprene can be used on older larval stages and for ...to late to use either Bacillus or methoprene ..." 

seems to say methoprene can be used when its too late to use methoprene??????? What gives? (14) 

Response to Comment 12. It was a typo. Thanks for catching it. The text has been corrected to read: Methoprene can

be used on older larval stages (i.e., pupa), and for situations where it is too late to use either Bacillus thuringiensis 

israelensis or Bacillus sphaericus, a monomolecular film might be used. 

Comment 13. Statement in the draft: “Ecology proposes to continue to restrict the areas of application of 

methoprene." But then ecology goes on to say, "they will initiate a monitoring strategy to document 

Best Management Practices for Mosquito Control C-9 

Washington State Department of Ecology 
concentration levels and if necessary, levels of toxicity to non-target species.” The objective of monitoring will 

be to provide data on methoprene for permit renewal in 2006. Response: This monitoring, should it take place, 

SHOULD take place in the areas in question that are healthy frog environments now. Not some place that is 

already stressed for some other reason. And in monitoring the areas in question there will be records of 

methoprene application over the past years that will give some basis on where to start. And not take another 

twenty years in another location establishing a track record. And if monitoring is going to be done then it 

would seem that applications of methoprene should take place. Otherwise, what is going to be monitored? (15) 

Response to Comment 13. Comment noted. We will take your comments into consideration as we develop the 

monitoring plan. 

Comment 14. Calling your attention to the California draft fact sheet, written by the California State DOE, 

page 7, par 4, "USEPA has concluded that, used in mosquito control programs, methoprene does not pose 

unreasonable risks to wildlife or the environment." Now granted, each state can say and do whatever they 

want. And it is certain California has endangered species. But decisions being made in California are made on

sound, current research. (15) 

Response to Comment 14. Decisions made in California are reviewed for relevancy to our program. Their control 

operations and monitoring results will be included in our SEPA evaluation. 

Comment 15. It was stated in the [WDFW] comments, "there are studies that have found developmental 

effects to amphibians when methoprene was applied at mosquito control treatment levels and there are studies 

that have not observed this." Response: Those studies that showed negative effects should be produced for 

review in this decision making process. And the studies must be the most current up to date studies. And not 

studies that are old and have been disproved. At the meeting in Olympia on 5 March '04, industry presented,

once again, the current facts on methoprene. With even newer studies done by OSU. And WSFW said, "we 

still have questions." When asked what are your questions the reply was, "we don't know." However, at least 

after three years WSFW and DOE are listening to the mosquito districts when they say, methoprene cannot be 

found or monitored for; only the effects can be monitored. (15) 

Response to Comment 15. See response to Comment 2. The SEPA evaluation and monitoring plan for methoprene 

have been proposed to clear up some of these ambiguities regarding the developmental toxicity of methoprene to non-target 

species. 

Comment 16. The most recent information is from Oregon, where ponds have been treated with methoprene 

and heavily monitored for the past several years. In over ten prior years of methoprene application and close

inspection of populations, there has been no evidence of malformations. A recent discovery of infected snail 

populations, which harbor the deformity-causing trematodes, was a premonition of a deformity outbreak and 

further confirms that trematodes are to be strongly linked with these deformities, not methoprene. The 

newness of this information is encouraging and is a close geographic example of how deformities in the 

amphibian population are not related to methoprene. This should be taken into consideration as the BMP is 

revised another time. (19) 

Response to Comment 16. Ecology would be interested in reviewing the Oregon studies. Please forward any contact 

information you may have. 

C-10 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
 

Comment 17. If the proposed methoprene restriction is related to an absence of long-term testing and 

environmental impact studies, Wellmark requests to review the documentation that Washington is using for 

the long-term studies on other larvicide and adulticides products. (19) 

Response to Comment 17. Please see response to Comment 2. 

Comment 18. As mentioned in the meeting, many mosquito districts are influenced by decisions from other 

parts of the nation when it comes to restrictions on products. If the best management practices document 

continues to have restrictive language for methoprene, we will want a complete explanation for the decision so 

that other states can have a thorough understanding of Washington’s position - how it is solely related to 

Washington State endangered species issues and not for the use of methoprene in general. (19) 

Response to Comment 18. Ecology’s restriction of methoprene is solely related to Washington state endangered species 

issues and not for the use of methoprene in general. General use is not restricted beyond the FIFRA label. Preemptive use 

of biocides, including methoprene, is recommended in the BMP to minimize mosquito breeding sites and the need to use 

more toxic insecticides in the event of a disease outbreak. Further, Ecology did not assess the harm posed by methoprene

to endangered species sufficient to warrant restricted use when a human health threat exists. Also see response to 

Comment 2. 

Comment 19. To date, the comments that we have reviewed and those that have been previously missing are 

overwhelmingly in methoprene’s favor, submitted by professionals who have used the products for many, 

many years without environmental incident. Why is there no impact, reduction or deletion of the restriction 

language, or are these comments to be ignored? (19) 

Response to Comment 19. Our responses largely address those comments that are based in fact or law. The 

preponderance of users of a pesticide does not constitute its lack of adverse effects because oftentimes the causes of adverse

effects and correlations are difficult and costly to determine. Also see response to Comment 2.

Larvicide Preferences 

Comment 20. After reviewing the BMP I would like to state that I believe that many improvements have been 

made and it is now a more concise and user friendly guide. I commend you for taking out the larvicide 

hierarchy wording that was present before. I believe that there is still a preference indicated, but an effort has 

been made to clarify that a particular larvicide will not work best (or at all) in certain situations. (6, 7, 8, 9, 10, 

13) 

Response to Comment 20. We agree. The language has been further clarified. Permit Condition S1.A.4 now reads: 

4. Authorized pesticides are: 

3⁄4 Bacillus thuringiensis israelensis (Bti) 

3⁄4 Bacillus sphaericus (H-5a5b) 

3⁄4 Methoprene Granular, Liquid, Pellet, or Briquette.* 

3⁄4 Monomolecular Surface Films 

3⁄4 Paraffinic white mineral oil. Paraffinic white mineral oil shall not be used in waters of 

the state unless: 

a. The mosquito problem is declared a public health risk; or 

b. The other control agents would be or are known to be ineffective at a specific 

treatment site; and 

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Washington State Department of Ecology 
c. The water body is non-fish-bearing (consult Washington State Fish and Wildlife 

concerning fish and wildlife). 

* Use of methoprene is not restricted for use beyond the FIFRA label in more than 99% of the State. However, 

methoprene is restricted in areas designated by Washington State Department of Fish and Wildlife (see Appendix A) 

except when a health threat exists in those areas as determined by state and local health departments. 

Comment 21. I have a current label of Malathion 8 Spray produced by Wilbur-Ellis Co. with an EPA Reg. # 

2935-83-ZA. The label states: 

"Mosquito Larvae: Apply 8 fluid ozs. per acre to standing water (intermittently flooded areas, 

stagnant water, temporary rain pools). Broadcast use only over intermittently flooded areas. 

Application may not be made around bodies of water where fish or shellfish are grown and/or 

harvested commercially." 

I am not saying that I want to use this all the time. I am not saying that I would use it at all. I did not 

use it as a larvicide this year, but it is registered and as long as it is, it should be left available to use at 

least in case of emergency without having to go through all the bureaucracy that can occur. The more 

products are available the more effective, including cost effective, we will be. (5, 8, 11, 12, 13) 

Response to Comment 21. Section S1, of the permit has been revised to allow the use of larvicides based on 

effectiveness and situation rather than just toxicity. According to Steve Foss, Pesticide Management section of 

WSDA, larvicides containing the active ingredients of malathion or temephos are not likely to be needed due to 

pesticide resistance or in cases of an emergency. However, larvicide products containing temephos are needed in 

areas with high organic content, such as wet manure fields and lagoons because the other larvicides permitted for 

use are often ineffective in these areas. In addition, manure fields and lagoons typically do not drain to surface 

waters. When surface waters are not affected monitoring will not be required. Malathion may be applied under 

an experimental use permit, so it remains in the tool box. 

Permit Condition S1, proposed modification: 

5. Temephos may not be used in lakes, streams, or the littoral zone of water bodies or on statelisted specie sites listed in Appendix A of the BMPs, (Ecology publication 03-10-023). The 

use of temephos shall be allowed only in highly-polluted water (i.e. tire piles) or waters with 

high organic content (i.e. manure holding ponds and pastures with no surface water 

runoff), or under either of the two following conditions: 

a. As a result of consultation between the Departments of Agriculture and of Ecology 

in response to the development of pesticide resistance or ineffectiveness within a 

population of mosquitoes. When temephos is applied to areas draining to surface 

waters monitoring of persistence and residues are a condition of the approval. 

Temephos must be rotated with one or more of the approved alternatives with a 

different mode of action to minimize the development of resistance.

b. As a result of consultation between the Department of Health and Department of Ecology in 

response to the development of a human health emergency as determined by the Washington 

State Department of Health. 

6. Other pesticides may be applied in the context of a research and development effort 

 under the jurisdiction of the Washington State Department of Agriculture through 

 the issuance of a Washington State Experimental Use Permit. 

C-12 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
 

Comment 22. In the Permit, Condition S4. A. 2. The phase, “in the order of preference in which they should be 

considered” should be changed to, “may be considered.” (7, 8) 

Response to Comment 22. Agreed. The Permit now states: 

S4.A. 2. The IPMP shall consider the approved list of pesticide-based controls found in Section S1. 

Comment 23. Page 9 of the Permit, Condition S4. A. 1. contains the phrase, “except in response to 

documented” should be replaced with “to minimize the”. Resistance is very difficult to document. Resistance 

management is part of an effective IPM plan. (7, 8) 

Response to Comment 23. Ecology met with WSDA over this language because resistance is difficult to document. 

The Permit language now states: 

S4.A.1. In the IPMP, pesticides that are effective in controlling the mosquito population and have the least 

adverse impacts to nontarget species shall be used except in response to documented development of resistance or 

in cases of ineffectiveness or in a declared public health emergency. 

Bio-controls 

Comment 24. Use of Appropriate Bio-controls (p. 2 of the BMP) states, “Stock water gardens that have no 

surface inlet or outlet with mosquito-eating fish (i.e., goldfish, mud minnow, stickleback, and perch). Tadpoles, 

dragonfly larvae, diving beetles, back swimmers, and front swimmers also prey on mosquito larvae. For more 

information, see http://www.wa.gov/wdfw/factshts/westnilevirus.htm” 

*This website has changed: http://www.wdfw.wa.gov/factshts/westnilevirus.htm, is the current site 

address. However, at this website, it only mentions birds and bats as natural predators for mosquitoes. I do 

not question the idea that the animals that you listed will feed upon mosquitoes (larvae or adult), but I think it 

is extremely misleading to imply that efficient control can be achieved with organisms like birds, bats, 

tadpoles, diving beetles, etc. More importantly, the website you are asking people to go does not appear to 

back up the limited claim for bird and bat control with any scientific evidence. (6) 

Response to Comment 24. Thank you for the updated link. Use of bio-controls may or may not yield sufficient control, 

efficacy was not meant to be implied. Of concern is that goldfish and other predacious bio-controls may escape into 

natural water bodies where they could become invasive or problematic. 

Comment 25. In mosquito control plans put together by several other states, pesticides are listed and 

identified as biopesticides or traditional chemical pesticides. The benefits and limitations of each product are 

described. The applicator is free to choose the best fit for the situation. The EPA classifies Bti, Bacillus 

sphaericus, and methoprene all as biopesticides, thus putting them in the same category. Washington is the 

only state that separates out methoprene. When deciding which control agent to use, it is important to consider 

efficacy. “Methoprene has consistently proved to be one of the most effective insect growth regulators against 

mosquitoes and is usually more efficacious than biological control agents” (Glare, 1999). The presence of 

pollutants, salinity, organic and inorganic particles can all reduce the efficacy of Bti. 

According to the Center for Disease Control (CDC), two factors that contribute to the spread of West Nile 

Virus (WNV) include abundance of vectoring species of mosquitoes and wide spread irrigation. Washington 

has both. The proposed BMPs do not follow CDC guidelines for mosquito control. The CDC recommends the 

use of larvicides in targeted locations in risk category one. The proposed BMPs wait until risk category four to

Best Management Practices for Mosquito Control C-13

Washington State Department of Ecology 
recommend larvicide use. By this stage, the CDC has already recommended increased larval control and 

intensifying adult mosquito control. (7, 3, 6, 8, 11, 12, 13, 19)

Response to Comment 25. We agree that bio-chemicals like the bacteria Bti and the growth hormone methoprene should 

be included in this category. The following language has been added to this section (p. 3) of the BMP to encourage 

preemptive chemical bio-controls where predators alone may not be effective. 

Minimization Actions

Use Appropriate Bio-Controls 

3⁄4 Selective bio-pesticides such as Bacillus thuringiensis israelensis (Bti), B. sphaericus or methoprene are very 

effective preemptive controls when applied in the spring to specific sources identified by surveys. Amplifying and 

bridge vector species should be targeted (also see p. 11.). 

Comment 26. Page 11: The title, “Chemical Controls” is misleading. Bti, Bacillus sphaericus, and 

methoprene are all classified by the EPA as biopesticides. The title should reflect that this section includes 

biopesticides, oils and traditional pesticides. (7, 8) 

Response to Comment 26. Agreed, the title has been changed to, “Microbial, Biochemical and Conventional Chemical 

Controls.” 

Comment 27. Preemptive treatments (larviciding known breeding sites early in the season) may reduce the 

need for adulticiding later. (1, 7) 

Response to Comment 27. Agreed, preemptive larviciding with bio-chemicals has been added as an option under the 

breeding site minimization actions. 

Adulticiding 

Comment 28. I believe that the section giving a BMP for adulticiding is misplaced. I am unclear as to your 

implication of adulticiding applications needing a NPDES permit (unless you mean when directly affecting 

water). Also, I feel that if you admit you don’t have jurisdiction in terrestrial applications of adulticides, then

indicated the Best Management Practices for them seems inappropriate. This should be left up to the 

organization that has direct jurisdiction. 

In addition, Ecology’s BMP states that there should not be any adulticiding done unless there is a disease

present. (5, 6, 9) 

Response to Comment 28. A large majority of workshop attendees (October 14 & 15 in Moses Lake and October 21 in 

Lacey) wanted the adulticiding section left intact because it is an integral component of their integrated pest management

approach to mosquito control. Treatment triggers are left to the mosquito control operator or the organizations they work

for to determine. The BMP states: 

“Select triggers for the use of adulticide products: Some mosquito control districts recommend using light traps to 

monitor for mosquitoes. For example, Adams County MD recommends that counts of 8 to 12 mosquitoes caught 

in 12 hours or a 3 adult mosquito landing count per minute in a residential area triggers the need to adulticide 

(Thomas Haworth, personal communication, November 7, 2003). Some applicators recommend adulticiding 

residential areas and upland areas where mosquitoes are migrating only when there is evidence of mosquito-borne 

C-14 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
 

epizootic activity at a level suggesting high risk of human infection. The following are examples of this type of 

evidence: high dead bird densities; high mosquito infection rates; multiple positive mosquito species including 

bridge vectors; horse or mammal cases indicating escalating epizootic transmission, including bridge vectors, 

horse or mammal cases, or a human case with evidence of epizootic activity (p. 21).” 

In some instances, adulticiding can reduce or eliminate the need to heavily apply larvicides, can be used effectively with 

less environmental impact to non-targets, and can be cost-effective. So for this best practices plan, some information on 

commonly used products and methods are appropriately included. However, since there have been objections to the 

inclusion of this section we will more clearly identify Ecology’s regulatory jurisdiction in those sections. To be sure, 

Ecology has no intention of over-stepping its regulatory bounds. 

Comment 29. Page 13 of the BMP states, “Terrestrially applied insecticides are NOT regulated under federal 

or state water pollution control laws and are not subject to NPDES permit conditions or requirements. 

However, in Washington State applications of insecticides used for adult mosquito control, even if they are labeled for 

use over water, i.e., streams, wetlands, rivers, lakes, ditches, etc, must be permitted under a Clean Water Act (NPDES) 

permit.” *Please clarify; I am assuming that the preceding sentence is referring to when adulticides are used

on, in or directly above water. Is this the intent of your sentence or are you stating the regardless of use, an 

adulticide must have a NPDES permit? If the latter is true this seems to be in direct conflict with the first 

sentence of section 7. If the former is true then this should be made clear in the BMP. (6) 

Response to Comment 29. The only time adulticide applications would have to be permitted is when they are applied, 

directly or indirectly, to waters of the state. Ecology’s Water Quality Program does not permit the application of 

pesticides that are applied to terrestrial sites. However, the italicized language has been removed because it was confusing 

and most likely not needed. 

Comment 30. BMP minimum response does not consider DOH and or CDC guidelines, which consider 

targeted adult mosquito control by stating, “...adulticiding based on surveillance is an extremely important 

part of any integrated mosquito management program. (11)

Response to Comment 30. See Response to Comment 5 and 9. Also, the section that discusses adulticiding describes 

the various triggers that may be appropriate. 

Dipping Criteria 

Comment 31. Could you clarify the dipping criteria for larviciding storm water ponds? The 2003 permit said 

larvicides could be applied if greater than 0.3 larvae/pupae per dip are found. The 2004 BMPs says 1 larva per 

3 dips is the larviciding threshold. The 2004 permit says that greater than 1 larva per 3 dips is the larviciding 

threshold. With last year's criteria 1 larvae per 3 dips meant we could larvicide. Do we now need 2 or more 

larvae per 4 dips to larvicide? (22) 

Response to Comment 31. Only 1 larva per 3 dips is needed. 

Comment 32. Could we state in the permit that entities that did control the year before based on their 

surveillance don't necessarily have to wait for mosquitoes to appear again the following season before using 

control measures in that water body. This question came up last year from King County, who identified their 

water bodies that needed control and wanted to continue that control without having to do the surveillance all 

over again. I'm getting questions such as "We found mosquito larvae last year in this pond, can we apply the 

Best Management Practices for Mosquito Control C-15

Washington State Department of Ecology 
mosquito dunks as a preventative measure this year without doing the dipping all over? Does the permit 

allow this?" (20) 

Response to Comment 32. The permit requires mosquito dunks prior to treatment unless the site is inaccessible (see

response to comment 33 below). Mosquito breeding sites may change and there is no need to use larvicides if no larvae are 

present. 

Comment 33. Page C-5 - Other references use 1 per 3 dips rather than 0.3 per dip. (3, 13) 

 

Response to 33. Agreed. Section S4.B of the permit now states: 

“Pesticide applications shall not commence unless surveillance of a potential application site indicates a 

larva/pupa count of greater than 1 per 3 dips, or unless dead birds, infected horses, or adult mosquito surveys 

indicate the presence of vector mosquitoes when larvae counts cannot be made due to their inaccessibility. In 

these cases larviciding may be desirable or even necessary without the larvae dips.” 

Miscellaneous Clarifications 

Comment 34. The following clarifications are suggested:

 

Page 1 - Add - Applications of pesticides are also subject to the Washington Pesticide Control 

Act (15.58 RCW), the Washington Pesticide Application Act (17.21 RCW), the General Pesticide 

Rules (WAC 16-228), the Worker Protection Standard (WAC 16-233), a number of pesticide 

and/or county specific regulations. 

 

Page 3 - The heading "Eliminate Mosquito Feeding Sites" does not correspond to bullets beneath 

the heading. Suggest different heading such as "Personal Protective Measures" 

 

Page 4 - Clarify that the use of larvicides is one of the Minimum BMP Responses to Minimize 

Mosquito Breeding and use of adulticides. 

 

Page 5 - Amend Risk Assessment: Probability of outbreak in humans: Remote to low; areas with

limited or sporadic WNV epizootic activity in birds and/or mosquitoes.

 

Page 10. IV. Mosquito Control Treatments 

Use larvicides at specific locations when WNV epizootic activity is found in birds and/or 

mosquitoes. 

 

Page 11 - Amend heading to Microbial, Biochemical and Conventional Chemical Controls. 

Bacillus thuringiensis israelensis (Bti) and Bacillus sphaericus (H-5a5b) are microbial pesticides. 

Methoprene is a biochemical pesticide. 

Monomolecular surface films, paraffinic white mineral oil, and temephos are conventional 

chemical pesticides. 

Page 12 - Amend web page reference to guide of larvicide products. 

For a guide to larvicides see the WSDA website: 

http://www.kellysolutions.com/WA/showproductsbypest2.asp?Pest_ID=IOAMAAC04. 

C-16 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
 

Page 13 - Clarify insecticides listed in table 3 are for larvae control and not adulticides. (3, 13) 

Response to Comment 34: All the comments were accepted and changes made to the BMP document except the WSDA 

web site address. The WSDA web site was not included because larvicides not permitted for use under this permit may be 

listed there and it would be confusing to list products not permitted for use. 

Comment 35. Page 12 of the BMP states, “Fish and game specialists and natural resources biologists (WDFW) 

must be notified of planned control measures whenever delicate ecosystems could be harmed by mosquito 

control practices.... Could you please clarify how to determine a “delicate ecosystem?” Is this a legal 

description? Is this only WDFW land? Clarification would be helpful. (6) 

Response to Comment 35. “Delicate ecosystems” are those that can only survive under a narrow range of 

environmental conditions including light, salinity, temperature, water quality, and nutrients, and are extremely 

vulnerable to anthropogenic activities. An airplane flying over nesting grounds demonstrates this sensitivity. The phrase 

was not used as legal terminology, but as plain English. 

Comment 36. The fact that it takes 38 days to obtain a license and permit to undertake mosquito control 

activities would likely inhibit a community to address an emergency health issue... (11) 

Response to Comment 36. The vast majority of permitted entities in Washington come under the Department of 

Health’s coverage by contracting with them. It takes about a seven-day turn around to do that, largely because of mailing 

time constrains. It’s conceivable to have the turnaround time reduced to a day or two in a real emergency. 

Comment 37. Education of the public in the areas of mosquito habitat reduction and personal protection in 

and of itself is not protecting the public from disease as is implied in the draft document. (11) 

Response to Comment 37. Personal protection is the best precaution anyone can take to ensure minimal exposure to 

mosquito borne diseases. If a person tries to kill every mosquito in a two-mile square area they will probably not succeed 

and when they leave that area they are exposed to increased risk again. The best insurance anyone has against mosquito

borne disease is to eliminate the chance of exposure in their direct personal space. This is why Appendix B, Insect 

Repellent Use and Safety from the Center for Disease Control has been added to the document. It is an excellent 

informational source. 

Comment 38. An important component of the CDC guidelines is to include monitoring for the disease as well 

as monitoring for mosquito populations. (11) 

Response to Comment 38. The Washington State Department of Health is the lead on monitoring for the disease. 

Section II of the BMP directs mosquito control agents to their local health departments for questions and issues related to

monitoring for the disease.

Comment 39. What is going to constitute monitoring? Record keeping or actual testing? Please clarify. (5) 

Response to Comment 39. When the permit was issued, monitoring was intended to examine persistence of the 

insecticides used in Washington State. Since persistence data for the larvicides allowed for use already exists, possible 

adverse affects to non-target organisms, rather than persistence, will be monitored. Additionally, the arrival of the West 

Nile virus in Washington State, has resulted in many local governments and others new to mosquito control practices 

applying larvicides during the spring and summer months. These new control operations will result in a sharp increase 

in the amount of larvicide being applied to waters. For these reasons, the monitoring requirement will be modified to 

Best Management Practices for Mosquito Control C-17

Washington State Department of Ecology 
only require reporting the type, location, and quantities of larvicides used. Ecology will use this information to prepare a 

plan to monitor for possible adverse affects in areas targeted by the reporting results. 

Comment 40. We treat hundreds if not thousands of small ponds through out the season, many of which are 

less than one acre. If we report those small ponds as <1 acre our rate per acre will all off. My district tries to 

record each treatment whether 10 acres or .03 acre. (5) 

Response to Comment 40. Reporting in tenths or hundredths of an acre is more accurate and can be done on the 

current reporting form. We allow permittees to also ‘clump’ the acreage they treat in an area so if they treat ten small 

ponds that are .10 acre each, they can report it as one acre. This works so long as the ponds all drain to the same receiving 

water. 

Comment 41. Comment to draft: By adding the WSDA RCWs and WACs it would seem that now mosquito

control operations are under FIFRA and federal label requirements and NOT second level government rule 

making. There are so many levels of government that ALL requirements will be difficult for the home owner 

and layperson to sort out. (15) 

Response to Comment 41. Ecology must be responsive to Court decisions. However, we agree and have petitioned 

EPA several times to consider streamlining these requirements for the benefit of operators/applicators. 

Comment 42. How will the comments be handled in this process? It seems they are as much questions as 

comments. Because all that happens in this process is a response is given to comments with no action taken.

(15) 

Response to Comment 42. The comments are reviewed for basis in fact or law. Where inaccuracies or deficiencies are 

found, they are corrected. 

Comment 43. This is the second modification in less than one year on the permit, how often will changes be 

made in the future? (15) 

Response to Comment 43. This is the last modification that will be made prior to permit renewal. 

Comment 44. I suggest adding “ecologically sensitive areas” to demarcated no-spray zones on maps (p. 11 of 

the BMP). (16) 

Response to Comment 44. Agreed. The language has been added to the section: 

“Demarcate no-spray zones on maps. This may include areas such as schools, hospitals, fish farms, 

wildlife refuges, ecologically sensitive areas, the homes of individuals who are on chemically sensitive 

registers, and crops grown under a certified organic program.” 

Comment 45. C1, Number 3, may cover this, but it would be nice to have this language in more detail 

somewhere. Could we put what water bodies should have permit coverage? The "waters of the state" catchall 

works to a certain point, but there seems to be confusion over the water bodies (i.e. storm drains with an 

outlet) that could reach "waters of the state." I have instructed folks that if their water body has the potential to

reach waters of the state, they need permit coverage. If "waters of the state" includes water bodies that have a 

potential to reach waters of the state, we should try to make this very clear. I'm still obviously a little 

confused... (20) 

C-18 Best Management Practices for Mosquito Control 

Washington State Department of Ecology 
 

Response to Comment 45. The Department of Ecology’s Aquatic Mosquito Control permit covers mosquito control 

activities that discharge insecticides directly into surface waters of the state of Washington. All who conduct mosquito 

control activities in water for communities, districts and private landowners are required to obtain coverage. "Waters of

the state" includes water bodies that have a potential to reach waters of the state.

References cited: 

Ankley, G.T., J.E. Tietge, D.L. DeFoe, K.M. Jensen, G.W. Holcombe, E.J. Durham, and S.A. Diamond. 1998. 

Effects of ultraviolet light and methoprene on survival and development of Rana Pipiens. Environmental 

Toxicology and Chemistry 17: 2530-2542. 

Degitz, S.J., E.J. Durham, J.E. Tietge, P.A. Kosian, G.W. Holcombe, and G.T. Ankley. 2003. Developmental 

toxicity of methoprene and several degradation products in Xenopus laevis. Aquatic Toxicology 64: 97-105. 

Henrick, C.A., J.K. Ko, J. Burleson, G. Lindahl. D. VanGundy and J. Edge. 2002. Investigation of the 

relationship between s-methoprene and deformities in anurans. J. of Am. Mosq. Control Assoc. 18(3):214-221. 

Johnson, C. M., L.B. Johnson, J. Murphy and V. Beasly, Evaluation of the potential effects of methoprene and 

Bti on anuran malformations in Wright County, MN. National Resources Research Institute, University of Mn. 

NRRI technical report no. NRRI/TR-2001/01 

La Clair, J.J., J.A. Bantle and J. Dumont. 1998. Photoproducts and metabolites of a common insect growth 

regulator produce developmental deformities in Xenopus. Environmental Science and Technology 32: 1453-

1461. 

Ross, D.H, D. Judy, B. Jacobson, R. Howell. 1994 Methoprene concentrations in freshwater microcosms treated 

with sustained-release Altosid formulations. J. of Am. Mosq. Assc. 10(2): 202-210. 

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Washington State Department of Ecology