Individual worker behaviour and colony success are both affected when bees are exposed to a combination of pesticides, according to research conducted by Dr Richard Gill and Dr Nigel Raine at Royal Holloway, University of London.

This research, published in Nature, investigated social bumblebee colonies which rely on the collective performance of numerous individual worker bees. It showed that chronic exposure to two commonly-used pesticides (a neonicotinoid and a pyrethroid), at concentrations approximating field-level exposure, impaired natural foraging behaviour and increased worker mortality. This led to significant reductions in colony success, and increased rates of colony failure.

Bees are typically exposed to numerous pesticides when collecting pollen and nectar from crops, and this is the first study to examine the potential effects of exposure to a combination of pesticides under realistic field conditions.

While recent studies have investigated the role of pesticides on either individual bee behaviours, or the impact on whole colonies, how changes to individual behaviour could have knock-on effects for the colony has not previously been shown.

Dr Raine said: "There is an urgent need to understand the reasons behind current bee declines as they are essential pollinators of many agricultural crops and wild flowers. We rely on these insects to produce most of the food we eat and maintain the landscapes we enjoy".

"Previous studies investigating the possible role of pesticides in current bee declines have focused on honeybees, but it is also crucial that we understand how pesticides affect other bee species," added Dr Gill.

This study mimicked realistic scenarios in which 40 early stage bumblebee colonies received four-week exposure to two pesticides that are frequently encountered when foraging on flowering crops: the neonicotinoid 'Imidacloprid' and the pyrethroid 'lambda-cyhalothrin'.

Imidacloprid was provided in a sucrose solution at levels that could be found in nectar and lambda-cyhalothrin was administered following label guidance for field spray applications.

Bees were able to forage in the field providing a realistic behavioural setting, and the foraging behaviour of individual workers was recorded using radio frequency identification (RFID) tagging technology.

The researchers found that bees exposed to imidacloprid were less able to forage effectively, particularly when it came to collecting pollen. This meant treated colonies had less food available to them, so could not raise as many new workers.

On average, the percentage of workers leaving the colony and then getting lost was 55% higher in those receiving imidacloprid than those that were not exposed to pesticides. The results of this study also indicate that combinatorial exposure to pesticides increases the tendency of bee colonies to fail.

Dr Gill commented that: "The novelty of this study is that we show how the sublethal effects of pesticide exposure affects individual bee behaviour with serious knock-on consequences for the performance of the colony as a whole".

Dr Raine added: "Policymakers need to consider the evidence and work together with regulatory bodies to minimize the risk to all bees caused by pesticides, not just honeybees. Currently pesticide usage is approved based on tests looking at single pesticides. However, our evidence shows that the risk of exposure to multiple pesticides needs to be considered, as this can seriously affect colony success".

The Insect Pollinators Initiative is joint-funded by the Biotechnology and Biological Sciences Research Council, Defra, the Natural Environment Research Council (NERC), the Scottish Government and the Wellcome Trust. It is managed under the auspices of the Living with Environmental Change (LWEC) partnership.