A tiered investigation of pesticide hazards and risks for bumble bees via laboratory methods

Wild bee populations are under increasing threat due to a range of interactive anthropogenic stressors. Because of the importance of bees as pollinators, recent efforts have aimed to reverse the trend towards their decline by unravelling the complexity and interconnectedness of bee-stressor interactions.
Here I focus on the role of pesticides, which are often seen as pivotal in this paradigm. Specifically, in this thesis I aim to explore effects, exposure and risk of pesticide use across bumble bee castes and sexes. Building on standardised test methods and focussing on novel agrochemicals, the work presented in this thesis focusses on elucidating bee-pesticide interactions via laboratory designs of increasing complexity and realism.
In chapter 1 I review the current literature related to bee declines and pesticide hazard, exposure, and risk assessment.
In Chapter 2 I used a standardised methodology to quantify and compare the acute lethal hazards of three pesticides across bee sexes and castes. A key finding of this study is that bumble bee castes and sexes vary significantly in relation to their sensitivity to pesticide exposure. Males were the most sensitive group, while queens showed the highest resilience to pesticide exposure. Bodyweight was a meaningful, but not the only predictor of pesticide toxicity in bees, highlighting that future research should focus on characterising other determinants of pesticide sensitivity beyond the ones explored in my study.
In Chapter 3 I used microcolonies to characterise lethal and reproductive dietary risks of pesticides, nutritional stress and their interaction in bumble bee workers. With this study, which aims to give an improved definition of exposure realism, I found sharp lethal and reproductive impacts of sulfoxaflor in bumble bees. Altogether, these results confirm previous findings that sulfoxaflor can impact bumble bee reproductive success and show that non-mitigated label uses of sulfoxaflor may have major impacts on bumble bee survival.
In Chapter 4 I aimed to address a key unexplored exposure route in bumble bee research by undertaking the first steps towards the development of a test method quantifying lethal and sublethal hazards to bumble bees exposed to chemicals via soil. By exposing queens in their solitary phase, I aimed to explore pesticide risks during a particularly vulnerable stage of their lifecycle. I measured queen hibernation survival, body weight change and abdominal fat content and found that none of these responses were affected by a field realistic soil exposure to the novel insecticide cyantraniliprole. This study may be an initial step towards developing a standardised method to test the effects of the soil-mediated pesticide exposure route in bumble bee queens.
In Chapter 5 I aimed to explore the highest level of ecological complexity in a laboratory design by exposing queens during their solitary phase and assessing their long-term survival and reproductive success. In this chapter I found evidence of long-term lethal impacts of cyantraniliprole on bumble bee queens, which highlight the crucial importance of testing whether field studies would confirm these results in real-use scenarios.
The results of this thesis are summarised in a concluding chapter, along with
recommendations for future research directions on how to advance risk assessment for pollinators, including bumble bees.
This thesis aims to explore the potential of laboratory designs to answer fundamental questions in today’s hazard and risk assessment by improving our mechanistic understanding of pesticide-bee interactions beyond the boundaries of regulatory data requirements. My findings underscore that bee responses to pesticide impacts vary across sexes and castes of bumble bees with impacts ranging from lethal to sublethal effects. This thesis highlights that a careful characterisation of exposure and its long-term impacts in bumble bee toxicity studies is crucial to understanding pesticide risks in a field-realistic context. Finally, characterising risks of pesticide use for bumble bee populations cannot overlook assessing pesticide impacts in vulnerable life stages of the bumble bee colony cycle. Altogether these results may have important research and policy implications and are likely to represent progress towards improving bumble bee health.