By Charlotte Stoddart
21 October 2012
Bees, the most important pollinators of crops, are in trouble. All over the world, their populations are decreasing and scientists and farmers want to know why. In some cases, such as the widely reported colony collapses in North America in 2006, it is probably down to disease. But a blooming crop of research suggests that pesticides are also to blame1–3.
Earlier this year, two studies published in Science showed that colonies are severely affected when bees are exposed to neonicotinoid pesticides of the kind commonly sprayed on crops. In one study1, exposure led to a significant loss of queens in colonies of bumblebees (Bombus terrestris). In the other2, on honeybees (Apis mellifera), the insecticide interfered with the foragers’ ability to navigate back to the hive.
Now, in a study published in Nature3, researchers at Royal Holloway, University of London, in Egham, UK, show that low-level exposure to a combination of two pesticides is more harmful to bumblebee colonies than either pesticide on its own. The results suggest that current methods for regulating pesticides are inadequate because they consider only lethal doses of single pesticides. As ecologist Nigel Raine explains in the video, low doses of pesticides have subtle effects on individual bees and can seriously harm colonies. He hopes that his work will feed into consultations on pesticide regulations that are happening now in Europe.
ABSTRACT: Reported widespread declines of wild and managed insect pollinators have serious consequences for global ecosystem services and agricultural production1, 2, 3. Bees contribute approximately 80% of insect pollination, so it is important to understand and mitigate the causes of current declines in bee populations 4, 5, 6. Recent studies have implicated the role of pesticides in these declines, as exposure to these chemicals has been associated with changes in bee behaviour7, 8, 9, 10, 11 and reductions in colony queen production12. However, the key link between changes in individual behaviour and the consequent impact at the colony level has not been shown. Social bee colonies depend on the collective performance of many individual workers. Thus, although field-level pesticide concentrations can have subtle or sublethal effects at the individual level8, it is not known whether bee societies can buffer such effects or whether it results in a severe cumulative effect at the colony level. Furthermore, widespread agricultural intensification means that bees are exposed to numerous pesticides when foraging13, 14, 15, yet the possible combinatorial effects of pesticide exposure have rarely been investigated16, 17. Here we show that chronic exposure of bumblebees to two pesticides (neonicotinoid and pyrethroid) at concentrations that could approximate field-level exposure impairs natural foraging behaviour and increases worker mortality leading to significant reductions in brood development and colony success. We found that worker foraging performance, particularly pollen collecting efficiency, was significantly reduced with observed knock-on effects for forager recruitment, worker losses and overall worker productivity. Moreover, we provide evidence that combinatorial exposure to pesticides increases the propensity of colonies to fail.