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Watershed Friendly Mosquito Control

Routine spraying of individual lawns with non-selective insecticides to reduce mosquito populations can easily enter waterways, harming non-target organisms.

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Updated:

August 17, 2023

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Culex pipiens - the most common mosquito in Pennsylvania. Stechmücke (Culex pipiens) by vobebis is marked with CC0 1.0.

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Aquatic macroinvertebrates are important to stream health and aquatic food webs. These macroinvertebrates, such as stoneflies, caddisflies, mayflies, and more, are at the base of food web relationships that allow cycling of materials and nutrients within well-functioning aquatic systems. They eat organic matter such as leaves, plant material, woody debris, algae, and bacteria, and are then eaten by other organisms or emerge from the water and transport those nutrients back to terrestrial ecosystems. Without benthic macroinvertebrates, there is a disruption in food webs for fish, birds, amphibians, reptiles, and other wildlife in and adjacent to waterways.

Many land uses, such as urban development, construction, agriculture, and residential land management practices can disturb the ecology of waterways, by introducing pollutants or sediments that impair the ability of aquatic insects to live or reproduce. One such activity is the broad-scale application of insecticides, such as pyrethroids, that are considered reasonably safe for mammals, but are very toxic to many aquatic organisms.

This harm to non-target organisms is particularly problematic when the pesticide application is non-selective and is done in a manner that is not effective at controlling the target organism. This is the case with many residential mosquito control services that generally spray backyards with pyrethroid insecticides, which are synthetic versions of an insecticide produced by a type of chrysanthemum flower.

Why is general, non-targeted spraying a concern? These treatments target only adult mosquitoes. But adults are the most difficult life stage of mosquitoes to manage because they are mobile, widely distributed, and spend much of their time resting in protected areas. As a result, adult mosquito treatments must use large amounts of insecticide over large areas to be effective. These insecticides are not specific to mosquitoes, so the treatment is likely to kill any other insect in the treated area. In many cases, the products used to manage mosquitoes remain active for one to two months, so the hazards to non-target insects remain long after the treatment is applied. In addition, continuous exposure to the same insecticides over many generations can result in the development of insecticide resistance, making future control more difficult. Insecticide applications to eliminate adult mosquitoes fit into mosquito IPM, but only when justified by heavy biting pressure and/or disease transmission risk, based on systematic mosquito and disease surveillance.

Despite careful application, sometimes pesticides move from the target and contaminate adjacent areas, including creeks, streams, and other water bodies. Unfortunately, pyrethroids break down more slowly in water, especially where there is less sunlight, and persist for much longer in aquatic sediments. This increases harmful impacts on non-target organisms when residue enters waterways.

The most effective and watershed-friendly mosquito control uses integrated pest management (IPM) that specifically targets the mosquito life cycle and logical points where mosquitoes breed or rest. IPM is a decision-making process that weighs the risks and benefits of many control methods against the risks created by the pest. This can reduce mosquito populations and the likelihood of mosquito-borne illness while minimizing hazards to pollinators, aquatic macroinvertebrates, and other non-target organisms.

Recommended strategies for managing mosquitoes include removing any areas of standing water or access to standing water. This should be done at least once a week to disrupt the reproductive cycle by preventing mosquitoes from laying eggs. Make sure to eliminate, empty, clean, tightly cover, screen with mesh smaller than an adult mosquito, or discard any outdoor objects that may hold water. This can include old tires, buckets or containers, rain barrels or cisterns, planter bases, toys, kiddy pools, lids, birdbaths, trash or recycling containers, open plumbing vents, or even clogged gutters where water can pool. Septic tank owners should inspect for any cracks, gaps, or openings and repair them to prevent mosquito entry.

To eliminate larvae in standing water or small ponds that cannot be drained, covered, or removed, adding products derived from naturally occurring bacteria called Bacillus thuringiensis israeliensis (Bti) or spinosad, in granules or dunks can be effective. For isolated ponds or water gardens, small insect-eating fish may be stocked, with care to avoid introducing non-native and potentially destructive species and following regulations from the PA Fish and Boat Commission.

In addition to removing mosquito breeding sites, removing resting sites for adult mosquitoes may decrease pressure from some mosquito species. Adult mosquitoes rest in areas protected from sunlight, extreme temperatures, wind, and drying out. These may include woodpiles, hollow trees, dense vegetation, undersides of decks and porches, and crawlspaces and basements. However, many species have long flight ranges and can find resting sites elsewhere.

Community-wide, mass deployment of mosquito traps may provide relief from some species of mosquitoes with little to no non-target effects, though many mosquito species will not be captured in high enough numbers to significantly affect biting pressure. The traps consist of a container of water that attracts egg-laying mosquitoes and are most effective against species that favor small breeding containers and have short flight ranges, such as the Asian tiger mosquito. These traps typically work best if at least 80% of homes in a residential neighborhood use at least two traps in their yards.

For adult mosquitoes, wearing protective clothing, such as long pants and sleeves, and/or using insect repellents on skin or clothing can prevent bites. Additional interventions include limiting outdoor activities at peak mosquito biting times, screening areas, and using electric fans or repellent emitters where people gather after dusk. Focus any insecticide treatments only on areas where adult mosquitoes rest or seek shelter during the day, like piles of wood or debris. Any areas that are immediately adjacent to waterways or that include plants that are frequented by beneficial insects and pollinators should be avoided.

Written in consultation with Jamie Kopco, Pesticide Education Specialist at Penn State University, and Amy Korman, Extension Educator, Green Industry, Penn State Extension