Pesticide Risk to Bees Increases with Temperature Change: Study

Some pesticides, particularly neonicotinoids, are known to have an impact on bees and other essential insects and are suspected to be contributing to population decreases. Yet, bees' reported responses to this threat appear to vary around the globe, implying that other interacting processes are at work. 

Imperial College London researchers have now demonstrated that environmental temperature can influence the extent to which pesticides modify a suite of bumblebee behaviours critical to their survival and ability to pollinate crops. The findings were published today in the journal Global Change Biology.

The researchers observed six bumblebee behaviours at three temperatures (21, 27, and 30°C) when exposed to two pesticides (the neonicotinoid imidacloprid and the sulfoximine sulfoxaflor). At lower temperatures, imidacloprid had a stronger effect on four of the behaviours: responsiveness, likelihood of movement, walking rate, and food consumption rate. This shows that cold snaps may exacerbate pesticide toxicity on nest-related behaviours. Nevertheless, at the highest temperature, imidacloprid had the greatest effect on a key behaviour - the distance the bees could fly.

This association demonstrated a significant drop-off, with flying distance measuring the same between 21 and 27°C before dropping drastically after reaching 30°C. "The drop-off in flight performance at the highest temperature signals a 'tipping point' in the bees' ability to handle the combined temperature and pesticide exposure," stated lead researcher Dr Richard Gill from Imperial's Department of Biological Sciences (Silwood Park).

This apparent cliff-edge impact occurs over only three degrees, which alters our perception of pesticide risk dynamics given that such temperature variations occur often over the course of a day. "Furthermore, the frequency with which bees will be exposed to insecticides and high temperatures as a result of climate change is expected to grow." Our research can assist to determine the best pesticide doses and application periods for different climatic locations around the world, thereby protecting pollinators like bees.

Flight distance is critical for pollination because it supports foraging potential and contributes to food security via crop pollination. Although the tropics are often hotter, insect pollinator populations in more temperate latitudes, including the UK, may be more sensitive to pesticide effects due to broader temperature ranges. Bees pollinate a wide range of vital cereal crops, as well as legumes and fruit trees. Demand for their pollination services will rise as our food supply diversifies, but so will the pressures bees experience as a result of climate change and greater insecticide usage.

According to the researchers, this work measuring the links between temperature and pesticide damage should aid in modelling pesticide risks throughout different locations of the world as climate changes. "Our findings suggest that environmental context is critical when assessing pesticide toxicity, particularly when projecting bee reactions under future climate change," said first author Daniel Kenna of Imperial's Department of Biological Sciences (Silwood Park).

"Our results are essential for establishing a toxicity forecast framework, allowing us to predict how bee populations will respond to climate change while living in dense agricultural environments," said co-author Dr Peter Graystock of Imperial's Department of Biological Sciences (Silwood Park). The researchers plan to conduct more extensive investigations throughout the temperature gradient to see how toxicity effects scale with temperature and where tipping points may exist across a variety of species.