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expert reaction to study on exposure to pesticides and the effect on learning in bees

One of the concerns of the use of pesticides has been its impact on insect pollinators in ecosystems. Publishing in the Proceedings of the Royal Society B journal a group of researchers have reported that the pesticides that they tested affected the learning of honeybee and bumblebee differently and that this should be accounted for when creating accurate pollination models.


Dr Christian Maus, Global Pollinator Safety Manager, Bayer CropScience, said:

“The exposure regime applied in this study is highly artificial and entails an exposure of bees to the test substance that would not occur under realistic field conditions.

“The tested concentration of clothianidin is relatively high; concentrations as the tested one may occur in nectar or pollen of seed-treated but they are clearly higher than the concentrations that are typically observed, especially when taking into considerations more extensive data sets; therefore, a chronic, exclusive exposure to this concentration is exaggerated and does not reflect realistic exposure conditions.

“The test methods applied are not validated, in so far it is not clear whether the results would be reproducible.

“It would not be surprising if the behavioral endpoints of different species were differently influenced by stressors; however, this would not imply that observed differences are of biological significance at the individual or colony level, nor would it allow conclusions to be drawn as to extrapolation between different species as such.

“However, if the honey bee would in fact react more sensitively to pesticide stressors in this test compared to bumblebees, this would confirm the suitability of the honey bee as a good surrogate species

“There have already been various tests that examined effects of neonicotinoids on the memory, learning, and orientation of bees. In no case have effects on the individual been observed to corresponded with measurable effects at colony level.

“The PER Test and other test designs related to effects to memory and learning in bees have been shown to be rather unspecific, effects can apparently be caused by a great variety of stressors.”


Dr Chris Hartfield, bee health specialist and chief horticulture adviser at the NFU, said:

“Much of the evidence around the harmful effects of neonicotinoids relies on studies where bees have been dosed artificially with the insecticide – this latest study is another case in point. Ten or more bees were kept in containers not much bigger than a regular baked bean tin for around two weeks, and allowed to feed freely on a sugar solution laced with a neonicotinoid. This is clearly a very unnatural situation for bees. Even under these unnatural conditions, which no doubt stress the bees, it was found that while exposure to the neonicotinoid negatively affected learning in honeybees, there were no adverse effects on bumblebees.

“The big unanswered question remains whether the harmful impacts observed in studies based on artificially dosing bees occur in real-life field situations and cause the population declines we are concerned about.

“Farmers understand the importance of pollinators, and they are doing more than any other group to help provide for bees – in 2015 there was 3,526 hectares of land which farmers voluntarily dedicated to pollen and nectar mixes. A further 19,000 hectares, under agri-environment schemes, have been put aside for the same thing. These large areas provide the food and habitat on farmland that bee populations need to thrive.”


Prof. Nick Birch, senior entomologist and IPPM expert, James Hutton Institute, said: 

“The new paper is interesting and adds to existing knowledge about how different bee species respond to sub-lethal pesticide exposure in combination with an additional stressor, the bee parasite Nosema. These findings are not particularly novel in themselves but add evidence to existing lab and field data on multiple stressor impacts on pollinators. Because the authors only tested limited populations of both honeybee and bumblebees, derived from commercial sources, I view the results as preliminary and needing confirmation under more realistic field conditions.

“To confirm these interesting lab bioassay findings, the authors would need to test several regionally diverse populations of each bee species; it is already known that honeybee populations vary in sensitivity to pesticides, so known intra-specific variation needs to be taken into account when comparing between bee species, especially when based on limited sample sizes.

“Overall, the paper represents an interesting first step under artificial (lab) conditions and using a simplified bioassay to simulate more complex bee behaviour patterns. We would need further field based studies using many more populations of each bee species before making robust conclusions. The future implications of these and other recent results point towards the need for testing pesticides against more non-target pollinator species, rather than relying on a few lab-reared surrogate species.”


Dr Christopher Connolly, Reader at Centre for Environmental Change and Human Resilience, University of Dundee, said:

“This paper is interesting as it suggests that insects may differ in their sensitivity to neonicotinoids. In which case, it is important to identify which insects may be at risk and adapt the use of neonicotinoids accordingly. In stark contrast to other studies, where imidacloprid and thiamethoxam were found to be toxic to bumblebee colonies, this study raises the possibility that clothianidin may not exert the same sub-lethal effects on bumblebee learning and memory and so might not be toxic to bumblebee colonies. This needs to be confirmed experimentally.

“If such conclusions are correct, then this is extremely important for the environmental safety of the neonicotinoids. More widely, it highlights an important scientific fact that has been forgotten – we cannot reliably generalise evidence gained on individual pesticides or species and apply this to all others, regardless of how similar they (chemicals or insects) appear to be. These basic principles underlie pharmacology, a highly relevant discipline that is rarely consulted on such issues. ”


‘Chronic neonicotinoid pesticide exposure and parasite stress differentially affects learning in honeybees and bumblebees’ by Piiroinen et al published in Proceedings of the Royal Society B on Wednesday 6th April. 


Declared interests

Dr Christian Maus is Global Pollinator Safety Manager for Bayer CropScience, the company that manufactures clothianidin.

Dr Chris Hartfield: None received

Prof. Nick Birch: I don’t have any DoIs in connection with this paper. I work on biopesticides and IPM and do look at non-target impacts of pesticides and GM crops on pests and beneficial insects (pollinators, predators, parasitoids). My work is funded by RESAS (Scottish Government) and the EU currently.”

Dr Christopher Connolly: I have no interests to declare beyond working on neonicotinoids.

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