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Why Pesticides Pose Different Kinds of Risk to Non-Honey Bees

Hunt bumble bee - Bombus huntii

For years, risk assessments for pesticides have examined how bees might be exposed to them when used, but up to now these assessments have used honey bees (Apis mellifera) as the stand-in for all bees. But the bee realm is far more diverse than the Apis genus of honey bees, and entomologists and other researchers are now trying to improve risk assessment protocols to account for the differences between honey bees and their bumble bee, solitary bee, and stingless bee cousins. A new special collection in Environmental Entomology examines existing research on pesticide exposure routes for non-honey bees and how risk assessment protocols might be improved to account for the differences between honey bees and non-honey bees. (Photo credit: Joseph Berger, Bugwood.org)

As concern for pollinators—and bees, in particular—has grown in the public consciousness over the past decade, so too has interest within the scientific community grown to advance our understanding of various threats that bees face. Importantly, not only is this understanding growing more robust, but it is also becoming more nuanced.

Case in point: For years, risk assessments for pesticides have examined how bees might be exposed to them when used, but up to now these assessments have used honey bees (Apis mellifera) as the stand-in for all bees. But the bee realm is far more diverse than the Apis genus of honey bees, and entomologists and other researchers are now trying to improve risk assessment protocols to account for the differences between honey bees and their bumble bee, solitary bee, and stingless bee cousins.

In bumble bees, for instance, “the most important thing that honey bee risk assessments miss is the potential exposure of bumble bee queens to pesticides,” says Angela Gradish, Ph.D., a research associate at the University of Guelph School of Environmental Sciences. While bumble bee queens spend a great amount of time foraging outside their colony, honey bee queens live their entire lives inside their hives, being fed by workers. Thus, direct exposure of queens to pesticides is not considered in risk assessments. “This is significant because the loss of a bumble bee queen means the potential loss of an entire bumble bee colony,” Gradish says.

Gradish is the lead author on a new report published last week in Environmental Entomology that reviews our existing research on bumble bee pesticide exposure routes and compares them to that of honey bees. The paper is part of a special collection on pesticide exposure in non-honey bees in Environmental Entomology. The collection, edited by Theresa Pitts-Singer, Ph.D., of the U.S. Department of Agriculture’s Agricultural Research Service, is the result of a January 2017 workshop on the subject hosted by the U.S. Environmental Protection Agency.

The report on bumble bees explains their colony structure, developmental cycle, and foraging behavior, and illustrates their potential pesticide exposure routes for sprayed pesticides and seed treatments. In addition to the exposure to foraging queens, another notable difference in exposure risk between honey bees and bumble bees is via soil. Unlike honey bees, some bumble bees make their nests in soil and some bumble bee queens hibernate in soil, where they could come into contact with pesticide residues. This is another factor not accounted for in risk assessments based on honey bees.

bumblee bee pesticide exposure routes

A new special collection in Environmental Entomology examines existing research on pesticide exposure routes for non-honey bees and how risk assessment protocols might be improved to account for the differences between honey bees and their bumble bee, solitary bee, and stingless bee cousins. One report in the collection focuses on bumble bees (Bombus spp.) and features these conceptual models of pesticide exposure to bumble bees from foliar-applied systemic or nonsystemic pesticides (A) and systemic pesticide seed treatments (B). Black boxes indicate stressors and residue sources, solid gray boxes denote exposure matrices, and double-lined gray boxes represent receptors. (Image originally published in Gradish et al 2018, Environmental Entomology )

While recent research has examined the toxicity of certain pesticides to bumble bees and other non-Apis bees, future research will need to better quantify pesticide exposure levels out in the wild, Gradish says. “Risk is a function of both toxicity and exposure. So, without an understanding of exposure, we can’t estimate the risk of pesticides to bumble bees or determine if honey bee exposure estimates are similar to or protective of bumble bees,” she says.

Honey bees are comparatively less diverse than bumble bees, which comprise about 250 species. Much about them is still unknown. “For example, we generally don’t have much data on how much nectar or pollen individual bumble bee larvae or adults consume,” says Gradish. “But we need those data to estimate how much pesticide bumble bees may be consuming. So, before we can adapt the risk assessment process, we need to study and quantify some basic biological traits of bumble bees.”

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