Little and often makes much: identifying the time-reinforced toxicity of pesticides and their impacts on bees
Holder, Philippa Jane
Thesis or dissertation
University of Exeter
Reason for embargo
Currently unpublished data included
Bees provide important pollination services for crops and wild flowers, estimated to be valued at £120 billion to the global economy. However, declining bee populations have put these services in jeopardy. Pesticides are widely blamed, at least in part, for declines in both wild and managed bee species. Bees are exposed to dietary residues of pesticides when foraging on the nectar and pollen of treated bee-attractive crops. However, these residues are generally found at such low levels that it would not be feasible for a bee to ingest an acute lethal dose. Pesticides which exhibit time-reinforced toxicity could cause mortality to bees over an extended exposure period, though, as the damage they cause can increase exponentially over time. Currently, there is no test for time-reinforced toxicity included in bee risk assessments of pesticides. The overall aims of this thesis were to identify pesticides that exhibit time-reinforced toxicity and determine their effects on a range of demographically important sublethal endpoints in bees. Using a bioassay based on Haber’s Law, I identified fipronil as a pesticide exhibiting time-reinforced toxicity (TRT) in both the honey bee (Apis mellifera) and bumble bee (Bombus terrestris), from four widely-used candidate pesticides. Fipronil at field-relevant levels was found to significantly reduce the longevity and feeding of individual worker bumble bees and those in microcolonies. This nutrient limitation was postulated to be the cause of reduced fecundity of bumble bee microcolonies exposed to dietary fipronil at concentrations of 1 part per billion and less. The toxic effect of fipronil was dramatically increased when microcolonies were placed outside to forage for food, an effect documented by several other studies, and potentially due to an increase in metabolic rate from the need to fly. However, these effects were not observed in queenright Bombus terrestris colonies in the field. This disparity in effects may have been due to problems with exposure to fipronil rather than any possible resilience of colonies. The thesis findings highlight the need for time-reinforced toxicity testing in bees to be integrated into current risk assessment protocols for pesticides. My work in this thesis has provided validation for the use of the TRT bioassay in future risk assessments of pesticides. Current-use pesticides that exhibit TRT, in this case fipronil, pose a serious threat to both wild and managed bees, impacting on demographically important endpoints including feeding and reproduction. Further research, continuing on from the work in this thesis, is needed to ascertain the impacts of TRT pesticides at both colony and population levels. Determining the mechanisms of TRT pesticides will also be key to protecting bees from the danger they pose.
PhD in Biological Sciences