Department of Entomology | Purdue Extension
MOSQUITO MANAGEMENT BY TRAINED PERSONNEL
Ralph E. Williams, Extension Entomologist, Michael J. Sinsko, Public Health Entomologist, Indiana State Department of Health, and Gary W. Bennett, Extension Entomologist
More than 50 species of mosquitoes are present in Indiana. The biting of most species is simply annoying. However, certain species (especially in the genera Culex and Aedes) can threaten public health because of their ability to transmit viruses that cause human encephalitis. Several such viruses have caused disease outbreaks in various parts of the U.S. over the last few years. In 1975, a strain of virus produced an epidemic of St. Louis Encephalitis in Indiana causing illness and death in several counties. This virus is transmitted from birds to humans by mosquitoes. A limited number of mosquitoes can transmit the virus, and prime concern is centered on species of Culex mosquitoes.
Other mosquito-borne viruses that have been of concern in Indiana include those that are responsible for causing such diseases as La Crosse fever, Eastern equine encephalitis, and Western equine encephalitis. Recent concern has focused on the spread of West Nile virus. With this virus, like many of the other mosquito-borne encephalitis viruses, wild birds serve as the reservoir. Mosquitoes feed on infected birds and transmit the virus to other birds. Infected birds may become ill and recover or may exhibit no noticeable symptoms. Wild birds may also die of the infection, however. Crow mortality has been high.
The virus becomes widespread in the wild bird population by midsummer, when mosquitoes are abundant. The likelihood that mosquitoes will become infected and transmit the virus to dead-end hosts such as people and horses is highest between July and late October. A few Culex species are the probable vectors of West Nile virus. Aedes albopictus (Asian tiger mosquito) is also of concern.
Steps that can be taken to protect individuals and their homes are described in the Purdue Extension publication E-26, “Mosquito Control in and Around the Home” http://extension.entm.purdue.edu/publications/E-26.pdf
. But steps taken by individual homeowners will not completely eradicate the problem. To reduce annoyance and public health concerns, mosquito management should be undertaken on an area-wide basis by trained personnel.
Mosquito feeding on an arm.
(Photo Credit: John Obermeyer)
MOSQUITO BREEDING AND DEVELOPMENT
Mosquitoes always develop in water, but the type of breeding place varies with the species. Common breeding places are flood waters, woodland pools, and slowly moving streams and ditches, particularly if these moving waters are polluted with organic waste. They also develop in any container that holds water, such as tree cavities, rain barrels, fish ponds, house gutters, down-spouts, bird baths, old tires, tin cans, and catch basins.
Mosquitoes lay eggs on the surface of water or in low places where water is likely to accumulate. The eggs may hatch in fewer than three days or when flooding occurs. The larvae, commonly called “wiggle-tails,” mature in 7 to 10 days and change into a pupa or “tumbler” stage. Two or three days later, adult mosquitoes emerge. After taking a blood meal, each female lays 100 to 200 or more eggs. The entire life cycle, depending upon temperature, may be completed in as few as 7 to 10 days.
Mosquito adult, larva and pupa
Mosquito management on an area-wide basis is a complex problem that should be attempted only by professionals. The administration of community programs must be flexible. This flexibility should, however, be based upon the established principles of good mosquito management. A number of techniques are available, depending upon the target species involved and the priorities that have been established. For example, the control of species involved as disease vectors can be quite a different problem from that of species that are strictly nuisance biters.
All good public health programs must include education of the public for understanding and support. This is especially important with mosquitoes, because homeowners can help greatly by managing their own property to eliminate mosquito breeding sources. In areas where extensive breeding occurs in containers on private property, the effectiveness of any community-wide effort directed at public property alone will be greatly reduced. It is, therefore, of utmost importance to inform the citizens of the ways in which they can help.
SURVEY FOR BREEDING PLACES
An effective community-wide mosquito management program cannot be planned or conducted until a survey is made to locate the major breeding places of problem mosquitoes. This takes a great deal of time and work but is well worth the effort. Though mosquitoes usually require standing water for breeding, it is not true that mosquitoes will be produced in every body of standing water. A survey will identify those breeding sites that must be eliminated or treated. This will avoid unnecessary environmental and monetary costs. Because the most efficient management programs concentrate on the control of mosquito larvae rather than adult mosquitoes, the survey is an essential prerequisite.
Any site that accumulates standing water should be examined for possible mosquito breeding. Visual inspection of sites should be made, and mosquito larvae should be sampled with “mosquito dippers” for species identification. Sites identified as actively breeding mosquitoes should be noted for follow-up control efforts.
Adult mosquito surveillance measures mosquitoes populations that have successfully developed and emerged from aquatic habitats. Use of light traps (e.g., New Jersey light trap, CDC light trap) are standard tools for adult sampling. Landing counts can also be used.
For training opportunities in mosquito surveillance techniques and species identification, contact the
Indiana State Department of Health firstname.lastname@example.org
for available sessions.
SOURCE REDUCTION AND HABITAT ALTERATION
Many mosquito problems can be permanently reduced by either eliminating breeding places or altering the habitat in such a way as to reduce the numbers of larvae that can be supported. This might mean cleaning a shoreline of vegetation that provides natural harborage for larvae. Eliminating a source of organic pollution will alter a breeding place to not only deprive larvae of nutrients, but also to provide an environment in which mosquito predators can survive and become established. Under no circumstances should a body of water be drained or an area filled until permission has been obtained from the local drainage board and until it has definitely been established that problem species breed in it in sufficient numbers to cause problems.
The following practices may be used to reduce mosquito breeding sites.
1. Ditch and clean stagnant streams to insure a continuous flow of water to eliminate border vegetation that produces habitat for mosquito larvae to develop.
2. Drain or fill back-water pools and swamps where stagnant water accumulates. Sanitary landfills can often be used in such locations, resulting in the elimination of mosquito breeding sites and improving the value of the land. Check with the Indiana State Department of Health, however, before establishing such landfills.
3. Because all mosquitoes breed in shallow quiet water, remove vegetation and debris from along the shores of lakes and ponds to discourage mosquito breeding. Such bodies of water should have a steep clean shoreline with as little vegetation as possible. Weed killers may be used in some cases to eliminate or prevent emergent plant growth. Refer to Purdue Extension Publication WS-21 “Aquatic Pest Management” http://www.agcom.purdue.edu/AgCom/Pubs/WS/WS-21.html
for further information on the use of aquatic herbicides.
4. Stock small lakes and ponds with top-feeding minnows if allowable.
5. Improve wetlands and marshes to encourage development of mosquito predators (e.g., frogs, predatory insects, predatory fish).
CHEMICAL MANAGEMENT TECHNIQUES
The use of chemicals is, at best, a temporary expedient that should be limited to only those situations for which no other alternatives exist. In general, chemical control can be divided into two major operations. The first, larviciding, is the most efficient and effective, and should be the backbone of any good chemical program. The second, adulticiding, is less efficient and as such should be used strictly for supplemental or emergency purposes. The detection of active transmission of mosquito-borne disease is an example of such an emergency. The Indiana State Department of Health routinely monitors levels of arborvirus transmission throughout the state and may be contacted for information on the status of disease transmission.
A number of insecticides have been registered for use in mosquito control. The relative value of chemical control varies with the mosquito species and the location conditions where control is to be applied. Because each situation differs, care must be taken to select the proper insecticide for a particular situation. Some of these factors include:
• Effectiveness against target species (resistance problems);
• Relative toxicity to humans and domestic animals (impact on non-target organisms);
• Contamination of food, garden, or fruit;
• Availability in quantities needed;
• Need for residual action in some situations;
• Chemical stability;
• Ease of preparation;
• Corrosiveness; and
• Offensive odor, staining, etc.
Resistance can be a problem in mosquito control, especially when using organo-phosphate and pyrethroid compounds. However, before assuming that resistance is the cause of poor control, it must be established that poor control is not caused by other factors such as improper identification of mosquitoes, spray techniques, lack of knowledge about insect habits, or faulty source reduction procedures. Any decrease in susceptibility should be substantiated in carefully controlled tests before seeking another toxicant or considering a change of procedure.
PESTICIDE SAFETY MEASURES
The key to the safety of humans and other nontarget organisms is knowledge of the hazards involved in handling and applying pesticides.
All pesticides must be handled in such a way that any possibility of harm to nontarget organisms (including humans), either through contamination of food and water or by contact, is kept to a minimum. Before using any pesticide it is essential to first READ THE LABEL. In preparing and applying the pesticide, FOLLOW ALL DIRECTIONS CAREFULLY.
• Wear protective clothing to avoid prolonged or dangerous exposure to pesticides.
• Take care to avoid contamination of foods or drinking water of human and animals.
• Keep application equipment clean and in good condition.
• Store pesticides only in their original containers with the proper label and out of reach of children and animals.
• Dispose of empty containers properly, and know the emergency measures for treating accidental poisoning and cleaning up of spills or other pesticide contamination.
Many chemical insecticides registered for use in mosquito control are toxic to birds, fish, and other wildlife, so appropriate precautions must be taken. In addition, most of these insecticides are toxic to bees exposed to direct treatment or to residues on crops. In making applications, care should be exercised to avoid getting any of these insecticides on food or feed crop areas. Instructions on the label will give precautions or restrictions while using insecticides for mosquito control.
Mosquito breeding sites that are undesirable or impossible to alter or eliminate may be treated with an appropriate larvicide. Table 1 lists the insecticides recommended for use as mosquito larvicides in Indiana. The application of larvicides should only be made at sites where mosquito larvae of the proper target species are present. In addition, the degree of control obtained with larvicide applications often depends upon the amount of pollution and the type and amount of vegetative cover present. Some of the insecticides listed in Table 1 thus have a range of application rates.
Where cover is heavy, granular formulations frequently provide better control than emulsions or oil sprays. Repeated treatments with some of these insecticides may be needed, especially after heavy rainfall. Generally, three or four treatments each season will be needed. For proper mixing instructions, application rates, and precautions, all label directions should be read and followed carefully. Application rates may vary depending on the extent of vegetative cover and/or degree of pollution of the water to be treated.
Granular larvicides can be applied from the air if the plane does not have to fly over populated areas. Granules can also be applied by crank-operated spreaders similar to those used for spreading seeds and fertilizers. Knapsack or other hand sprayers that can be carried by field workers may be used for liquid formulations. Power sprayers may be satisfactory if advantage is taken of the wind so that the larvicide drifts into larval-infested water areas. Larvicide treatment of fish-bearing waters should be avoided. Briquet, granular, and pellet formulations are often preferred for use in catch basins and in containers not easily disposed of.
Various application methods are available for adult mosquito control. Thermal fogs can provide a rapid, temporary control of adult mosquitoes with little residual effect. Thermal fog generators break up the insecticide by means of hot gases or superheated steam to produce a fog or smoke. This method is effective only where there is little or no wind in the evening or night. Malathion (Fyfanon) is recommended for use with thermal fogging equipment.
A more effective method of ground application is the use of ULV (ultra low volume) cold aerosol equipment. These machines produce a very tiny droplet of high concentrate insecticide, which results in a greater area coverage with less dosage. This type of application is designed to kill active adult mosquitoes and provides little or no residual control. Like the thermal fog generator, the cold aerosol machine should be used during the time the adult mosquitoes are most active. This means from twilight until about midnight, when atmospheric conditions are usually best (lack of wind). ULV application is generally the preferred space treatment for adult mosquito control.
The cold aerosol method has certain advantages over thermal fog generators. Less insecticide is applied, resulting in fewer pollution problems. Smaller holding tanks and consequently smaller vehicles are needed because smaller quantities of insecticide are used. There is less of a traffic hazard compared with thermal fog applications, which reduce visibility. ULV ground applications, however, are somewhat less effective than thermal fogs in heavy vegetation, because the larger ULV droplets tend to be filtered out more rapidly. Insecticides recommended for use with ULV ground equipment include: permethrin (Biomist), chlorpyrifos (Dursban, Mosquitomist), sumethrin (Anvil), malathion (Fyfanon), synergized pyrethrins, and resmethrin (Scourge). (Note that resmethrin is a restricted use insectcide.)
Application by fixed-wing aircraft or helicopter for adult mosquito control is also a common practice. It is useful under emergency conditions or if areas to be controlled are too large or are inaccessible for economical treatment with ground powered equipment. Best results are obtained in areas without dense tree cover so that spray particles can penetrate the low shrub zone where the greatest mosquito activity occurs. To obtain uniform coverage of an area, careful observance of preplanned flight patterns, altitudes, and air speeds is essential.
Applications should not be made over a food or feed crop area or populated areas unless the insecticide is labelled for that use. Label directions regarding application over fish-bearing waters should be followed. The same insecticides for use with ground ULV equipment can be considered for aerial treatment. For proper mixing instructions, application rates, and precautions for any insecticides used, all label directions should be read and followed carefully.
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