Fogging or spraying for mosquitoes or biting flies around the yard and garden with an insecticide can be very harmful to pollinators. Even if flowering plants are avoided and applications are made after sunset, insecticides applied as a fog or mist can drift onto flowering plants within 100 meters or more depending on the wind speed and direction. The insecticide drift could contaminate pollen and nectar collected by bees for several days or weeks after it is applied, and the residue on leaves can be toxic to caterpillars for weeks or months. Caterpillars of some species of butterflies are extremely sensitive to insecticide residue on leaves.

A recent research study that analyzed pollen collected by honey bees in rural Indiana found fairly high levels of two pyrethroid insecticides often used for mosquito or nuisance insect control. It is not known how the insecticides were applied or for what purpose, but it does point out the need for more research on the impact of mosquito and nuisance insect sprays on pollinators, including bee larvae fed contaminated pollen, and caterpillars feeding on sprayed leaves.

Mosquito control or abatement programs in the United States’ north central region play an important role in monitoring for equine encephalitis, west Nile virus and other mosquito-borne diseases, and in suppressing mosquito populations by minimizing breeding habitats. For example, the state of Illinois has 21 public mosquito abatement districts. Most programs make the reduction of breeding sites for mosquito larvae a top priority, followed by treatment of spring flood pools and other breeding sites with Bacillus sphaericus (B.s.), Bacillus thuringiensis var. israelensis (Bti) and Saccharopolyspora spinosa (Spinosad). These larvicides are very selective and pose little risk to people, pets, pollinators and other non-target organisms.

Most programs also monitor adult mosquito activity and apply a broad-spectrum pyrethroid insecticide with truck-mounted ultra-low volume sprayers. In some programs nearly every inhabited neighborhood in the district is treated. Many programs attempt to minimize the impact on pollinators by spraying in the late evening when mosquitoes are active and bees have stopped visiting flowers. They may also avoid spraying trees and shrubs that are flowering.

The number of sprays applied for adult mosquitoes each year varies considerably depending on the district, abundance of breeding sites and weather conditions. As an example, the 2018 annual report of one mosquito district in Illinois sprayed prallethrin or sumithrin for adult mosquitoes on 22 nights in 2018. The truck-mounted sprayers in this district treated 75,000 acres and covered 2,081 miles of road.

Research studies with caged honey bees or honey bee hives have demonstrated that the ultra-low volume (ULV) applications made in the evening after bees are done foraging cause very little bee mortality. However, applications made during the day or to highly attractive flowers can be harmful to honey bees and other pollinators. It is not well-understood how much mosquito sprays affect native bees, but research with pesticides in agricultural systems consistently indicate that bumble bees and other native bees tend to be more sensitive than honey bees. Mosquito sprays are known to be harmful to monarch butterfly caterpillars feeding on milkweed plants within a 100 meters of the spray path, and in the Florida Keys, they have caused declines in populations of at last three species of butterflies.

Biting fly control around livestock rearing facilities are critical to the health and comfort of farm animals and farmers. Insecticidal ear tags and coat treatments have little impact on pollinators. Because pollinators are not frequent visitors to livestock holding areas pyrethroid sprays applied for fly control are unlikely to have much impact unless the spray drifts onto flowering plants or milkweed. Applying insecticides for biting flies under calm conditions and not when the wind will cause significant drifting can help minimize the impact on pollinators.

Read the next article in this publication series: Considerations for disease management

Or return to the beginning of this publication: How to protect and increase pollinators in your landscape