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Reduced growth rate and fewer queen bees.

Professor Dave Goulson of Sterling University led the British study says neonicotinoid insecticides have been implicated in the decline of bee populations as they occur at trace levels in the nectar and pollen of crop plants. In the study they exposed colonies of the bumble bee Bombus terrestris in the lab to field-realistic levels of the neonicotinoid imidacloprid, and then allowed them to develop naturally under field conditions. Treated colonies had a significantly reduced growth rate and suffered an 85% reduction in production of new queens compared to control colonies. Given the scale of use of neonicotinoids, they suggest that they may be having a considerable negative impact on wild bumble bee populations across the developed world.

Disorientated bees can’t find their way home

In the French study they found nonlethal exposure of honey bees to thiamethoxam causes high mortality due to homing failure at levels that could put a colony at risk of collapse. Simulated exposure events on free-ranging foragers suggest that homing is impaired by thiamethoxam intoxication. They state that these experiments offer new insights into the consequences of common neonicotinoid pesticides used worldwide.

Doubts over insecticide activity

But there are other factors influencing the demise of bees; the decline in wild flowers in the arable landscape and the fact that bees are increasingly succumbing to mites, viruses, fungi and other pathogens.

Within the scientific community there are doubts about the validity of these two studies and David Fischer, an ecotoxicologist at Bayer CropScience, told the New York Times that  the new experiments had design flaws and conflicting results. In the French study, he said, the honeybees got far too much neonicotinoid. “I think they selected an improper dose level,” Dr. Fischer said.

Dr. Goulson’s study on bumblebees might warrant a “closer look,” Dr. Fischer said, but he argued that the weight of evidence still points to mites and viruses as the most likely candidates for bee declines.

Neonicotinoid insecticides

Acetamiprid – marketed in UK mainly for horticultural crops

Clothianidin – marketed in UK as a seed dressing

Imidacloprid – marketed in UK as a seed dressing

Thiacloprid - marketed in UK mainly for horticultural crops but also oilseed rape as Biscaya

Thiamethoxam – marketed in UK mainly for horticultural crops

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