Effects of clothianidin and flupyradifurone on bumblebee colony development at ambient temperature

Neonicotinoids has raised serious concerns regarding their adverse effects on non- target organisms. These concerns led to the ban of the three most common neonicotinoids from outdoor use in the EU in 2018. A replacement insecticide was made available for the European market in 2015; the butenolide flupyradifurone. Although neonicotinoids and flupyradifurone share the same mode of action, the latter is marketed as pollinator-friendly. Studies performed for risk assessment of chemicals are often under constant and favourable conditions, which is rarely the case when organisms are in their natural habitat. The present thesis aimed to explore the effects of clothianidin and flupyradifurone using measures of colony development of the buff-tailed bumblebee (Bombus terrestris audax) under fluctuating ambient temperature. Also, the effect size difference between clothianidin and the pollinator-friendly flupyradifurone was investigated. Bumblebees were exposed to low and high (4× low) sublethal concentrations of each insecticide separately through artificial nectar over 21 days followed by 21 days post-exposure, giving 42 days in total to develop. The experiment ran sequentially from May until August, giving a natural ambient temperature gradient with low temperatures in May and higher temperatures in August. The effect of temperature on toxicity depended on the insecticide, the exposure concentrations and the endpoints assessed. For example, colonies exposed to low concentrations of flupyradifurone had a lower number of bees compared to control at day 42 under warmer conditions (-37%). In comparison, colder conditions gave a higher number of bees than control (+55%). However, at high exposure concentrations of flupyradifurone, a reduced number of bees compared to control was seen under cold conditions (-47%) and an increase in bee number under warm conditions (+28%). Exposure to low concentrations of clothianidin led to an increase in weight and the number of bees compared to control. In contrast, high exposure concentrations of clothianidin turned out to be detrimental at all temperature regimes. These results show the complex interactions chemicals can have with the dynamic environment. Clothianidin generally had a larger effect size difference than flupyradifurone. This can be interpreted as clothianidin being more toxic than its replacement chemical. However, this measure of effect size difference hides the interaction with temperature, which turned out to be essential for the toxicity of flupyradifurone.