Introduction

There are approximately 175 species of mosquitoes in the continental United States. For management in and around schools, mosquito species can be divided into two types; nuisance species and vector species. Vector species can spread disease and thus require more vigilance than nuisance species. About 40 species of mosquitoes found in the US are medically important because they may transmit various forms of encephalitis, yellow fever, malaria, dengue and other conditions. Of these, the encephalitis group is currently the most significant for the continental US, including West Nile virus, Eastern equine encephalitis, Western equine encephalitis, St. Louis encephalitis and California encephalitis.

Table 1 Mosquito species most likely to be found in schools including diseases vectored, larval habitat, adult activity periods and geographic distribution.

Management strategies for mosquitoes vary depending upon which species are present and whether mosquito-vectored diseases pose a serious public health threat. Some states have relatively few mosquito species, e.g., West Virginia with 29, while others are particularly blessed, e.g., Texas with 84. Climatic differences between regions as well as unusual weather patterns impact mosquito status. For example, in northern areas where mosquito season begins in June and is over by October, management of mosquitoes at schools is more easily accomplished. Tropical and subtropical areas, and schools with year-round calendars will have a longer, more challenging season.

Effective management requires understanding the life cycle, behavior and dispersal ranges of mosquito species. Some of the most common nuisance and vector species breed only in artificial containers and fly no more than a half mile from the breeding site. Other species breed only in salt marshes and are capable of flying five to ten miles or more. Identification of species can save a great deal of trouble and reduce under or over-reaction to potential problems.

Mosquitoes pass through four distinct life stages: egg, larva, pupa and adult. Adult females bite animals including humans to obtain blood. Blood provides protein for forming eggs. A female lives two weeks on average, during which she may lay eggs in standing water up to ten times, 50-500 eggs at a time. Suitable water for egg laying can include swamps, storm retention basins, culverts, ponds, lakes, natural tree holes, hollow stumps or artificial containers such as pots, cans, tires, plastic covers, or plugged rain gutters. In general, anything that can hold water for four to seven days or more can potentially become a breeding site.

Eggs are deposited either individually or in groups called rafts on the surface of water or on soil where flooding will produce puddles or pools. Most eggs hatch within 48 hours. Larvae are called wrigglers reflecting their movement in the water. Wrigglers feed on organic debris and microorganisms, and breathe at the surface of the water through tubes. After molting several times, a pupa is formed. Pupae are C-shaped and sometimes called tumblers because they will somersault below the surface of the water when disturbed. Adults emerge from the pupal stage. As long as watery habitat is available, the population gradually increases. The entire life cycle varies from four to 30 days, depending on the species.

The recommendations below for mosquitoes in schools generally apply to the most common domestic mosquitoes that share the following characteristics:

a. widespread geographically; b. breed in and around buildings in artificial containers; c. always associated with humans; d. typically small (<1/2 mile) flight range; e. capable of transmitting disease.

These species are relatively easily managed by school personnel by eliminating larval habitat around buildings. However, your location may require special attention to other species. For example, malaria is reintroduced to the US regularly but currently only poses a very limited health threat. The malaria mosquito, Aedes quadrimaculata, occurs in only a few places and does not typically breed in and around buildings. Thus, it is not included for discussion here.

Some mosquitoes found around marsh habitats are capable of flying many miles, in which case, control may need to be area-wide. If a school is near such sources and intervention is necessary, efforts will have to be coordinated with county or state mosquito abatement authorities. Refer to your state's department of health for updates on medically important mosquito species in your area.

Monitoring and inspection for mosquitoes

The most effective monitoring method for mosquitoes is to look for larvae prior to the emergence of adults. Larvae and pupae of common domestic species are found primarily in standing water in artificial containers located around the school building itself. These sources can be inspected using a dipper to capture larvae and pupae if present. A sketch or plot plan of the school grounds is helpful in recording locations where surveillance may be needed.

If adult mosquitoes are present, they will find you! If mosquito-borne diseases are a concern in your area, capture several intact adults and preserve them in a vial of alcohol for identification by mosquito specialists. State or county public health agencies, or pest control companies in mosquito-prone areas may have specialists on staff.

Cultural and physical options for mosquito management

In general, identification and elimination of mosquito breeding sites is more effective and less hazardous than attempting to eliminate adults. Elimination of such pools on a weekly basis brings the population down before they become adutls. Adults are difficult to control by any means, chemical or not. Least-hazardous adult control methods include predators, traps, bug-zappers etc. but they do not effectively reduce mosquito populations. In all but the most extreme cases of mosquito infestations, widespread spraying of pesticides for adult mosquitoes around schools poses an unacceptable risk of exposure to non-target organisms including humans.

Keep in mind that during warm weather, mosquitoes can breed in any puddle of water that lasts more than four to seven days, depending on the temperature.

Source elimination strategies.

• Identify anything outside that can hold water such as plastic, cans, containers and pots. Dispose of, turn over, or drill holes in such containers.
• Turn over wheelbarrows and other water-holding tools when not in use. Do not allow water to become stagnant in birdbaths, ornamental pools or other outside areas.
• Regularly inspect and clean out gutters and drainpipes.
• Cover dumpsters, trash and recycling receptacles to prevent water accumulation.
• Be aware of nearby piles of used tires, which have become important mosquito breeding sites. Remove or have holes drilled in them to drain water.

Habitat manipulation strategies for mosquitoes.

• Eliminate adult resting sites.
• Cut back or remove dense brush and other vegetation from around buildings.
• Keep grassy areas mowed.
• Promote natural breezes to discourage mosquito occurrence.
• Alter the landscaping to eliminate standing water.

Strategies to avoid adult mosquitoes and bites.

• Make sure window and door screens are in good repair. Advise students to take the following precautions:
• Reduce outdoor exposure, especially at dawn, dusk and in the early evening during peak periods of mosquito activity in your location. (Exception: day-biting species.)
• Avoid areas where mosquitoes tend to concentrate— tall grass, margins of wooded areas, or heavily wooded areas in dense vegetation.
• Avoid wearing dark colors. Some mosquitoes and other biting flies are attracted to dark greens, browns and black. They are less attracted to light-colored clothing, especially whites and yellows.

Table 2 Commonly used products for physical, cultural or mechanical management of mosquitoes and uses.

Biological Control

Biological organisms used to control mosquitoes include predators of larvae and adult mosquitoes, or formulations of naturally occurring mosquito parasites or diseases. The latter are registered by the EPA as pesticides and are covered in the next section.

Many naturally-occurring fish are predators of mosquito larvae. The killifish species Gambusia holbrooki and G. affinis (Cyprinodontidae) are native to the southern and eastern US and have been used quite successfully for larval control in many situations. However, when translocated to new environments, these fish may compete unfavorably with local fish and other aquatic species. Thus, Gambusia spp. should be used selectively in self-contained water bodies that are not fed or drained by natural waterways. These include ornamental ponds, abandoned pools, mine pits, livestock waterers, fountains or large birdbaths. Releasing Gambusia into waterways is illegal in some states. Efficacy and recommended stocking rates for Gambusia affinis are reviewed at www.rci.rutgers.edu/~insects/gamb2.htm

While predators of adult mosquitoes such as bats and purple martins can be encouraged, they are opportunistic feeders and so will feed on many insects and may not have a noticeable impact.

Pesticide options for mosquitoes

Many states have laws governing the use of both chemical and biological pesticides in and around schools or other specific environments. This is particularly true in the case of mosquito control which may involve applications of pesticides to natural bodies of water and thus pose environmental hazards, and be regulated or managed under state and local mosquito control jurisdictions. It is important to be informed about these factors prior to using pesticide options.

If students are going to be in areas of high mosquito activity, advise their parents of this fact so that precautions can be taken. Insect repellents are considered to be pesticides by the EPA and as such, are not appropriate for application by staff to students. Precautions should be taken to avoid toxic repellents such as DEET. Alternative repellents are available.

Larvicides, pesticides used to kill immature mosquitoes, are typically more effective and target-specific than pesticides that target the adults. Habitat modification is more permanent and preferred where possible. Larvicides include bacteria specific to mosquito and fly larvae, insect growth regulators (IGRs), and chitin synthesis inhibitors (Table 8.38). Conventional larvicides include several non-petroleum oils and monomolecular films.

The timing of larvicide applications depends on the product. Bacterial toxins must be consumed by the larvae and are usually applied well before the fourth molt. IGRs must be applied later in the life cycle to upset the molting process. Chitin synthesis inhibitors are effective throughout the entire larval life cycle. Monomolecular films prevent the insect from remaining at the surface of the water by reducing surface tension, causing the larvae and pupae to die. Non-petroleum oils kill larvae and pupae by suffocation. Conventional insecticides kill larvae at all stages and can be applied whenever larvae are present.

Pesticides targeting mosquito adults and applied from the ground or air are generally the least efficient approach and considered a last resort when all other methods have failed. They are often applied as ultra-low-volume sprays in which small amounts of insecticide are dispersed either by truck-mounted equipment or from fixed-wing or rotary aircraft. Pesticide droplets must contact the mosquito to be effective.

Table 3

Commonly used larvicidal products for mosquitoes.

Biological formulations.

CAUTION-label formulations that are more toxic and/or have greater exposure potential.

Organophosphate products applied to water for larval control are not recommended. (e.g. temephos, Abate®) due to both human exposure hazards and strong potential for widespread non-target impacts. Similarly, aerosol spraying, thermal fogging and/or UltraLowVolume (ULV) fogging for adult mosquitoes with organophosphate, carbamate or pyrethroid products is strongly discouraged, especially on school grounds. Such tactics should only be initiated as a last resort by state mosquito abatement personnel as part of a strategic disease vector management program. Should your school grounds be subject to such spraying, it will be important to close down all ventilation intakes, be sure students are not present and advise parents of the date and time of such applications.

Emerging issues, new strategies and priorities for mosquitoes

The need for effective mosquito management tools will follow the pace set by the introduction of new mosquito species and new disease emergence in the US. As such, mosquito management is a moving target. Effective mosquito management requires increased knowledge, understanding of mosquito biology, communication with the public and coordination between managers at the school, community, county and state levels.

Priorities for mosquitoes.

• Education
• Importance of outdoor clutter control and proper waste handling to avoid standing water and mosquito breeding.
• Importance of avoiding compaction and promoting infiltration in landscapes including turf to avoid standing water and mosquito breeding.
• Efficacy of prevention plus larviciding as an alternative to fogging.

Additional resources for mosquito management

American Mosquito Control Association www.mosquito.org/mosquito-information/control.aspx

Arizona Cooperative Extension. 2004. Scorpions and Mosquitos. Pest Press. cals.arizona.edu/urbanipm/pest_press/2004/may.pdf (PDF)

Florida Coordinating Council on Mosquito Control. 1998. Mosquito Control Benefits and Risks: Integrated Mosquito Management. In Florida Mosquito Control White Paper. mosquito.ifas.ufl.edu/Integrated_Mosquito_Management.htm

Long, K. 2006. IPM for Pennsylvania Schools: A How-to Manual. PA IPM Program.

National Park Service. Mosquitoes. In Integrated Pest Management Manual. www.nature.nps.gov/biology/ipm/manual/mosquito.cfm

O‘Neill, J. 1997. Chapter 18. Mosquitoes. Pp. 837-880. In Handbook of Pest Control. A. Mallis, ed.

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