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Primary WNV Mosquito Vector Control

During 2002, an outbreak of mosquito-borne West Nile virus in Illinois resulted in more than 800 cases and more than 60 deaths. The appearance of West Nile virus (WNV) cases in Illinois required local health departments to expend many staff hours responding to inquiries by concerned citizens. Additionally, there was intense interest by the news media. In response to the 2002 outbreak, many communities conducted or enhanced mosquito control operations. Anecdotally, areas in Cook County that had existing mosquito control programs with especially intensive mosquito LARVICIDING efforts had fewer human cases than those areas that had less intensive efforts or no larviciding at all.
The 2002 WNV outbreak in Illinois occurred during a summer with above normal temperatures. In contrast, the summers of 2003 and 2004 were much cooler than normal and there were many fewer cases in Illinois (54 and 60 respectively). Public health officials believe hot summer temperatures increase Culex mosquito production, which in turn increases the proportion of birds infected with WNV and the risk of disease to humans. Each year, WNV activity occurs throughout Illinois. Furthermore, mosquito-borne West Nile Virus activity and the number of human West Nile disease cases increased during the hot summers of 2005 and 2006 compared to the cool summers of 2003 and 2004.
In preparation for the upcoming season, local municipalities and mosquito abatement districts need to plan and budget for adequate mosquito control measures.
Because of concerns about possible WNV activity, it is likely that some communities will maintain or increase mosquito control programs. Consequently, Illinois Department of Public Health staff believes local governments that conduct mosquito control programs should emphasize the methods most effective at controlling the primary mosquito vectors (carriers) of WNV and a related disease, St. Louis encephalitis. Additional information about WNV and mosquito control may be found in the Resources on this Web site.
Please be aware that all personnel who do pesticide applications for mosquito control must be licensed by the Illinois Department of Agriculture (IDA). The only exceptions are homeowners who treat their own residential property for mosquitoes and tire recycling companies that use certain pesticides to treat water-filled used tires. Please contact IDA at 800-641-3934 or 217-785-2427 for more information about mosquito control licensing.

Control of the Primary West Nile Virus Mosquito Vector

Control of Culex Larvae Is a Priority.  Although WNV has been detected in several species of mosquitoes, about 70 percent of the positive samples have been from Culex pipiens (the house mosquito) and related species of Culex. The house mosquito breeds most prolifically in stagnant water that has a heavy organic content. In urban areas, catch basins, poorly draining ditches and water-filled artificial containers (such as used tires) are common production sites for Culex mosquitoes. Additionally, most of the house mosquitoes present in a community are produced locally because the house mosquito does not fly more than one or two miles from its production site. In contrast, inland floodwater mosquitoes (Aedes vexans) can fly 10 or more miles from where they hatch, particularly along prevailing winds. Although floodwater mosquitoes, when abundant, can be a nuisance to the public, they have not been significant disease carriers in Illinois and are currently believed to be minor carriers of WNV.
The most effective method of mosquito control is larviciding or the treatment of locations where mosquito larvae are present, such as water impounded in the bottom of catch basins (storm drains). Catch basins are found along streets, in parking lots and sometimes in backyards. Because catch basins are a major source of the house mosquito in urban areas, the Department recommends they be treated at least twice during the summer to control Culex mosquitoes. Ideally, municipalities should treat catch basins and other locations that produce Culex mosquitoes as often as determined necessary by inspection and according to insecticide label directions (Table 1 for a list of mosquito larvicides). However, a minimum response by a municipality should be to treat catch basins and other Culex production sites twice during the summer (June and July). This would reduce numbers of vector mosquitoes during late summer, the period of greatest risk for humans. Local officials should review references about mosquito control (Resources), particularly the Joint Statement on Mosquito Control and Integrated Methods of Mosquito Control, issued by the U.S. Centers for Disease Control and Prevention (CDC) and the U.S. Environmental Protection Agency (USEPA).
Spraying for Adult Mosquitoes: Adult mosquito control (also called fogging, spraying or adulticiding) is the mosquito control method most familiar to the public. However, aerosol fog kills only mosquitoes that contact insecticide droplets, and the fog soon dissipates. Although the local mosquito population is reduced, fogging does not prevent mosquitoes from re-entering the area. Because only a part of the local adult mosquito population is reduced for a few days by fogging, municipalities should give priority to larval mosquito control of Culex mosquitoes. Nonetheless, when the risk of human disease is present, the only method that will reduce the population of WNV-infected mosquitoes throughout a community is adulticiding. Treatment for control of WNV-infected adult mosquitoes is a valid and legal option for local officials to employ as a supplement to larviciding.
To be effective, spraying for adult mosquitoes MUST be conducted when suitable environmental conditions are present. Fogging for adult mosquitoes should ONLY be conducted at the proper time (evening or early morning) and under appropriate environmental conditions (temperatures from 60 degrees to 85 degrees Fahrenheit and low wind speeds). For ultra-low volume (ULV) spraying units to control mosquitoes, they MUST be serviced so they produce a proper droplet size spectrum. Consult the pesticide label and manufacturer's recommendations for further information.
Every effort should be made to inform the public when treatment for adult mosquitoes is planned. An informed public will better understand the measures being taken and will be able to take precautions to limit exposure to pesticides.
Non-chemical Mosquito Control:  Mosquito larvae or "wrigglers" must live in still water for five or more days to complete their growth before changing into adult biting mosquitoes capable of transmitting disease. Often, the number of mosquitoes in an area can be reduced by removing sources of standing water around residences. For example, hundreds of mosquitoes can come from water in a single discarded tire. Local agencies should inform the public how to prevent mosquito production around residences and how to prevent mosquito bites. Individuals can reduce their risk by taking these precautions:

  • Get rid of old tires, tin cans, buckets, drums, bottles or any water-holding containers.
  • Fill in or drain low places (puddles, ruts, etc.) in the yard.
  • Keep drains, ditches and culverts free of weeds and trash so water will drain properly.
  • Keep roof gutters free of leaves and other debris.
  • Cover trash containers to keep out rainwater.
  • Repair leaky pipes and outside faucets.
  • Empty plastic wading pools at least once a week and store indoors when not in use.
  • Unused swimming pools should be drained and kept dry during the mosquito season.
  • Fill in tree rot holes and hollow stumps that hold water.
  • Change the water in birdbaths and plant pots or drip trays at least once each week.
  • Store boats covered or upside down, or remove rainwater weekly.
  • Keep grass cut short and shrubbery well-trimmed around the house so adult mosquitoes will not hide there.
  • Make sure ornamental ponds have fish that eat mosquito larvae.
  • Repair window screens.
  • When outdoors in the evening or when mosquitoes are biting, use personal protection measures to prevent mosquito bites (proper use of insect repellent and appropriate clothing). See the Department’s Web site for specific personal protection recommendations.

In summary, local agencies that conduct mosquito control should give highest priority to eliminating breeding sites and to larviciding. Elimination and treatment of Culex mosquito production sites will help municipalities protect Illinois citizens from mosquito-borne West Nile virus. For additional technical information about mosquito control, contact the Illinois Department of Public Health, Division of Environmental Health. Questions concerning licensing for mosquito control or inquiries about the legal aspects of pesticide use should be directed to IDA.
Table 1. Mosquito Larvicides Commonly Used in Illinois1
Note: Mosquito larvicides with methoprene or Bacillus sphaericus as the active ingredient have been found to be particularly effective for control of Culex mosquito larvae in catch basins.

Larvicide Type Action Primary Use
Abate® (Temephos) Organophosphate Directly toxic Tires, containers, floodwater sites
Altosid® (Methoprene) Growth regulator Prevents larvae from developing to adults Catch basins, containers, floodwater sites
Bacillus thuringiensis israelensis (Bti) Bacterial Gut toxin Sites with “clean” floodwater, catch basins
Bacillus sphaericus (Bs) Bacterial Gut toxin Catch basins, septic waters (for Culex)
Oils (Golden Bear® and BVA® ) 2,3 Surface treatment Suffocation: film covers air tubes of larvae Floodwater sites, catch basins, septic waters

1 Always read and follow all current pesticide label instructions. If any information in these recommendations disagrees with the pesticide label, the label instructions must be followed. Use of a product name does not constitute product endorsement. All pesticides must be registered with USEPA and the IDA.
2 Monomolecular film larvicide (Agnique®, an alcohol-derived product) that acts like an oil larvicide has become available recently.
3 All oils used as larvicides must be registered with USEPA and the IDA.