How can I minimize the impact of pesticides on native bees?

Answer: Insecticides not only kill pollinators, but sub-lethal doses can affect their foraging and nesting behaviors, often preventing plant pollination and bee reproduction (Thompson, 2003; Decourtye et. al., 2004; and Desneaux et. al., 2007). Herbicides can kill plants that pollinators depend on when crops are not in bloom, thus reducing the amount of forage available (Kremen et. al., 2002 and Tscharntke et. al., 2005).

In general, while pesticide labels may list hazards to honey bees, potential dangers to native bees are often not listed. For example, many native bees are much smaller in size than honey bees and are affected by lower doses. Also, honey bee colonies may be covered or moved from a field, whereas wild native bees will continue to forage and nest in spray areas.

If pesticides cannot be avoided, they should be applied directly on target plants to prevent drift. Broad-spectrum chemicals should be avoided if at all possible. In addition to active ingredients, consider the formulation of pesticides; generally dusts and fine powders that may become trapped in the pollen, collecting hairs of bees and consequently fed to developing larvae are more dangerous than liquid formulations (Vaughan et. al., 2007).

Crops should not be sprayed while in bloom. Nighttime spraying, when bees are not foraging, is another way to reduce bee mortality. Periods of low temperatures may also be good for spraying because many bees are less active. However, the residual toxicity of many pesticides tends to last longer in cool temperatures. For example, dewy nights may cause an insecticide to remain wet on the foliage and be more toxic to bees the following morning, so exercise caution (Vaughan et. al., 2007 and Johansen and Mayer, 1990).

Spray drift presents another threat to foraging native bees. Drift can occur either as spray droplets or vapors, as happens when a volatile liquid changes to a gas. Factors affecting drift include temperature (including temperature inversions), wind, application method, equipment settings and spray formulation (Ozkan, 2000).

Spray application methods and equipment settings also strongly influence the potential for drift. Since small droplets are most likely to drift long distances, aerial applications and mist blowers should be avoided. Standard boom sprayers should be operated at the lowest effective pressure and with the nozzles set as low as possible. For example, drop nozzles can be used to deliver insecticide within the crop canopy where it is less likely to be carried by wind currents. Regardless of the chemical or type of application equipment used, sprayers should be properly calibrated to ensure that excess amounts of pesticide are not applied (Ozkan, 2000).

Alternatives to conventional insecticides include the use of selective products that target a narrow range of insects, such as Bacillus thuringiensis (Bt) for moth caterpillars, although even these products can be detrimental when they drift. Other alternatives for some pests include bug vacuums, pheromones for mating disruption and kaolin clay barriers for fruit crops. Several ATTRA publications are available to assist farmers with implementing non-chemical pest control alternatives. See the ATTRA Biorationals: Ecological Pest Management Database for information on specific product and pests. Finally, remember that many of the habitat features that support pollinators will also host beneficial insects that help control pests naturally, reducing the need for pesticides.

Ready to learn more? The ATTRA publication Alternative Pollinators: Native Bees is an excellent resource to get you started. It provides information and resources on how to plan for, protect, and create habitat for native bees in agricultural settings.

References:

Decourtye, A., J. Devillers, E. Genecque, K. LeMenach, H. Budzinski, S. Cluzeau, and M. H. Pham-Delegue. 2004. Comparative sublethal toxicity of nine pesticides on olfactory performances of the honeybee Apis mellifera. Pesticide Biochemistry andPhysiology 78:83-92.

Desneaux, N., A. Decourtye, and J. Delpuech. 2007. The sublethal effects of pesticides on beneficial arthropods. Annual Review of Entomology 52:81-106.

Johansen, E. W., and D. F. Mayer. 1990. Pollinator Protection: A Bee and Pesticide Handbook. Cheshire,
CT: Wicwas Press.

Kremen, C., N. M. Williams, and R. W. Thorp. 2002. Crop pollination from native bees at risk from
agricultural intensification. Proceedings of the National Academy of Sciences 99:16812-16816.
Ozkan, H. E. 2000. Reducing Spray Drift. Ohio State University Extension Bulletin. 816-00. Columbus, OH.

Thompson, H. M. 2003. Behavioural effects of pesticides use in bees – their potential for use in risk assessment. Ecotoxicology 12:317-330.

Tscharntke, T., A.M. Klein, A. Kruess, I. Steffan-Dewenter, and C. Thies. 2005. Landscape perspectives on agricultural intensification and biodiversity – ecosystem service management. Ecology Letters 8:857-874.

Vaughan, M., M. Shepherd, C. Kremen, and S. Hoffman Black. 2007. Farming for Bees: Guidelines for Providing Native Bee Habitat on Farms. 44 pp. Portland: The Xerces Society for Invertebrate Conservation.