Assessing exposure and impacts of combined pesticide residues in soil for ground-nesting bees and bumblebee (Bombus spp.) queens

Solitary ground-nesting bees in agricultural landscapes may be at risk of chronic contact exposure with pesticide residues in soil during nest construction and immature development. Similarly, bumblebee (Bombus spp.) queens in temperate climates spend most of their lifecycle hibernating underground and may also be exposed to soil contaminants. In this thesis, I explore the extent and possible sublethal impacts of such pesticide exposure for wild bees, including potential synergistic effects from exposure to pesticide mixtures. Another important question I aimed to answer is whether bees can detect and avoid pesticide-contaminated soils, or whether they might be attracted to such residues. I used the common eastern bumblebee (Bombus impatiens) and the hoary squash bee (Eucera pruinosa) as agriculturally relevant model species to explore these questions through a combination of laboratory and semi-field experiments. I first quantified pesticide residues in agricultural soils sampled at suitable likely hibernation sites for bumblebee queens and found that hibernating queens are very likely to be exposed to complex mixtures of insecticides, herbicides, and fungicides, especially in apple orchard soils. Using these exposure data, I then performed a multiple-choice preference experiment and found that B. impatiens queens do not avoid pesticide-contaminated soils when selecting underground hibernation sites. Instead, queens in this experiment consistently avoided pesticide-free soil, a behavior that increases their likelihood of exposure to pesticide residues during hibernation. My research also showed that exposure to residues of the fungicide boscalid in soil during hibernation doubled the likelihood of bumblebee queens surviving the colony initiation period (after successful hibernation) and laying eggs, and revealed complex interactions between pesticide exposure and queen body mass on aspects of colony founding. In squash bees, impaired behaviour and reproduction were observed following exposure to ‘low toxicity’ pesticides. Female squash bees exposed to the fungicide Quadris Top (azoxystrobin and difenoconazole) collected less pollen, while those exposed to the insecticide Sivanto Prime (flupyradifurone) produced larger offspring. Moreover, pesticide co-exposure synergistically induced hyperactivity in female squash bees and reduced the number of offspring that emerged per nest. Together, these effects may have important consequences on population dynamics of bumblebees and solitary ground-nesting bees.