\We should have done this decades ago. Yet here we are and in need of a mandate that protects insects in our agriculture.
Alternate strategies need to be honed and well applied.
all good finally when work like this comes out.
Digging below the surface: Hidden risks for ground-nesting bees
Science
14 Nov 2024
https://www.science.org/doi/10.1126/science.adt8998
Metrics
Modern intensive agriculture faces a critical paradox: The very pesticides designed to protect our crops endanger essential pollinators that sustain their productivity (1). As human reliance on pollinatordependent crops grows (1), it becomes more urgent than ever to reconcile the need for crop protection with pollinator preservation. The stakes are high, as pollinators, such as bees, are vital to food security and biodiversity (3).
Pesticide environmental risk assessments (ERAs) aim to approve only those agrochemicals that pose low environmental risks. Yet as mounting scientific evidence highlights adverse effects of authorized products on bees, it becomes clear that current ERAs fail to adequately protect insect pollinators (1, 4, 5). Part of the issue is a reliance on the western honey bee (Apis mellifera), by pesticide regulatory authorities worldwide, as the model species to assess risks to all bees (6). This specific focus is problematic owing to substantial differences in life history traits, exposure routes, and vulnerabilities among bee species (7). For example, whereas most of the world’s 20,000+ bee species are solitary, honey bees live in large colonies that benefit from social detoxification strategies, which buffer pesticide impacts (8).
Failures to detect and document pesticide impacts on wild bees arise from multiple other deficiencies, which include incomplete consideration of potential long-term and sublethal effects, overlooked exposure routes specific to wild bees, and failures to account for possible coexposure to multiple pesticides (1, 4). While completing my PhD at the University of Guelph, I addressed each of these pitfalls to help develop ERAs more suitable for a wider range of bee taxa.
Current ERAs overlook soil pesticide residues as a threat to pollinators because honey bees rarely interact with soil. In contrast, >80% of bee species nest or overwinter underground, which exposes them to these residues (9). My thesis used two agriculturally relevant model species: the common eastern bumble bee, Bombus impatiens, and the hoary squash bee, Xenoglossa pruinosa (see the figure), to study wild ground-dwelling bees.
Bumble bees (Bombus spp.) are social insects. In temperate climates, queens overwinter underground for 6 to 9 months before establishing new colonies in the spring (10). Yet it was unknown how exposure to pesticide-contaminated soils might affect bumble bee queens during this critical phase. In contrast, the hoary squash bee is a solitary species that specializes on Cucurbita plants (pumpkins and squash). Females excavate individual underground nests within crop fields, which makes them highly vulnerable to soil pesticide residues (10).
To ensure field realism, I conducted a literature review to understand the extent of pesticide exposure for bees (12). Then, I generated the first field exposure estimates for overwintering bumble bee queens to pesticide residues in agricultural soils (10). I identified high risks of exposure to multiple pesticide residues for bumble bee queens that overwinter in agricultural soils in eastern Canada, especially in apple orchards. Orchard soils at suitable overwintering sites contained mixtures of up to 29 pesticides, and 95% of samples contained at least one insecticide, herbicide, and fungicide.
Pesticide exposure is dependent not only on the extent of agrochemical contamination in the landscape but also on how bees choose to use available resources. A key question is whether bumble bee queens can detect and avoid pesticide-contaminated soils, or whether they might be attracted to them. To address this question, I conducted a large-scale multiple-choice experiment where newly emerged bumble bee queens were offered numerous boxes of soil treated with different pesticides, alongside untreated soil, within large mesh-covered enclosures (13). I then analyzed the distribution of queen overwintering burrows (hibernacula) among these soil treatments.
I found that bumble bee queens did not avoid residues of commonly encountered pesticides when selecting overwintering sites; rather, they were seemingly attracted to pesticide-contaminated soils. This apparent preference increases their likelihood of exposure to and potential risk from pesticide residues while they overwinter underground. The mechanisms that drive these preferences are currently unclear, but further research on avoidance behavior or sensory perception of pesticides by bees will help elucidate them.
From exposure to impact
Ground-dwelling bees, such as bumble bee queens (A) and the hoary squash bee (B), are commonly exposed to pesticide residues in soil. This exposure can lead to lethal and sublethal impacts on behavior and brood production
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My research also investigated two newer classes of insecticides marketed as safer alternatives to bee-toxic neonicotinoids—diamides (chlorantraniliprole, cyantraniliprole) and butenolides (flupyradifurone)—in combination with commonly used broadspectrum fungicides. In the absence of established protocols, I developed a methodology to evaluate how pesticide residues in soil affect overwintering bumble bee queens. In a fully crossed experiment, I exposed bumble bee queens to soil treated with field-realistic concentrations of two diamide insecticides and two fungicides (boscalid and difenoconazole), alone or combined, over a 30-week overwintering period and examined the effect on queen overwintering survival and subsequent colony initiation success (14).
My research revealed size-dependent effects for bumble bee queens exposed to cyantraniliprole-treated soil. Heavier queens exhibited increased mortality and delayed brood emergence and produced smaller workers. These larger queens likely face greater vulnerability to cyantraniliprole because of their greater muscle mass (cyantraniliprole disrupts insect muscle function), greater surface area in contact with soil, and lower metabolic detoxification capacity. My findings suggest that cyantraniliprole impairs heavier queens’ ability to feed their brood and highlight potential cascading impacts on bumble bee populations, as larger queen size is key to successful overwintering and colony establishment in the spring.
In another study, I examined the effects of chronic exposure to two pesticides commonly used on squash crops—the soil-applied insecticide Sivanto Prime (flupyradifurone) and the fungicide Quadris Top (azoxystrobin and difenoconazole)—both alone and combined, on ground-nesting squash bees (15). In this fully crossed semi-field study, female squash bees were exposed to treated squash crops within large outdoor enclosures to replicate real-world conditions. Exposure to the fungicide reduced pollen collection by female squash bees, whereas coexposure to both pesticides synergistically induced hyperactivity and reduced the number of offspring that emerged from each nest. These results highlight potentially serious consequences for squash bees. For example, reduced offspring production per nest due to pesticide coexposure could contribute to population declines.
My research reveals a clear potential for ground-dwelling bees to be affected by field-realistic (co)exposure to soil pesticide residues. These findings urgently call for a reassessment of global pesticide regulations to create a safer, more sustainable agricultural future that protects both our crops and the pollinators they rely on.
Acknowledgments
The author thanks her PhD supervisor, N. Raine, for his mentorship and support; her advisory committee members for their guidance; and her colleagues and collaborators for their valuable contributions. This work was supported by scholarships from the Arrell Food Institute at the University of Guelph, the Fonds de recherche du Québec–Nature et technologies (FRQNT), and the Ontario Agricultural College of the University of Guelph.
