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expert reaction to study looking at the use of the pesticide thiamethoxam and its effect on bumblebee populations

Publishing in Nature Ecology & Evolution, researchers looked at the neonicotinoid insecticide thiamethoxam and its impact on bumblebee colonies. They reported that exposure to thiamethoxam resulted in a reduction in the proportion of queen bees laying eggs and from this modelled possible effects on population dynamics.


Dr Peter Campbell, Senior Environmental Risk Assessor at Syngenta (which manufactures and sells products containing the neonic thiamethoxam), said:

“This is a very interesting paper which investigates under laboratory conditions, the effect of thiamethoxam in combination with a trypanosome parasite and variations in hibernation duration, on nest initiation and egg laying in the Bumble bee Bombus terrestris. The results of which are then incorporated into a model to estimate potential population level impacts.

“Although the laboratory experiment in this study reports a 26% reduction in the proportion of queens exposed to thiamethoxam that laid eggs, it must be noted that this study was carried out under artificial laboratory conditions, where the forced exposure period was 14 days. The authors themselves point out that to extrapolate this result to field populations “the experimental procedure…would ideally be carried out on populations in the field”.

“There are some important methodological points that also need to be considered before accepting the results at face value. For example the lack of proper replication of the egg laying experiment does not inform us of the natural variability of the egg laying response and secondly the hibernation periods used (6 & 12 weeks) are substantially lower than the normal hibernation periods (6-9 months) of bumble bees in the wild. So how representative the results of this laboratory experiment are to exposure and risk to freely foraging bumble bee queens under field conditions of use needs to be considered. Indeed the authors acknowledge this weakness and state that “the extent of pesticide exposure faced by bumblebee queens in the field, and how this impacts colony initiation under natural forging conditions required further investigation”.

“With regards to the modelling carried out in this study, the model used was not a mechanistic model but more a probabilistic approach that would not take into account environmental and biological factors as well as ecological feedback mechanisms that may regulate bumble bee populations in the field.  The authors themselves also point out that there was a lack of data for many of the parameters used within the model.  Therefore the reported population extinction predictions of this model need to treated with caution.  Indeed these reported increased population extinctions seem to conflict with conclusions and results from previously published papers on the status of natural bumble bee populations.  For example, Kleijn et al 2015 1, reported  Bombus terrestris as the most prevalent pollinator in Europe (see supplementary data) and Woodcock et al 2016 2 reported no effects on Bombus terrestris/lucorum populations in their modelling investigations based on UK wild pollinator abundance data for species foraging on oilseed rape.

“There were however some interesting positive findings from this study that are worth highlighting. For example, there was no reported effect of thiamethoxam on ability of queens to produce adult offspring.  Colony initiation was apparently brought forward for exposed queens, suggesting some sort of colony compensation response can be activated by Bumble bees. Another interesting result from this study was that there was no interaction between thiamethoxam with C. bombi infection, indicating that infected bees were not more susceptible to thiamethoxam, which confirms absence of interactions previously reported in other insecticides (pyrethroids).

“In conclusion, there are some interesting data and insights presented in this paper but great care needs to be taken in extrapolating the results and predictions from this combined laboratory experiment / probabilistic modelling approach to the performance of real bumble bee queens and populations in the field, particularly when there is little evidence to show that Bombus terrestris populations are actually on the decline. ”


1 Kleijn et al 2015 (Suppl Data): Delivery of crop pollination services is an insufficient argument for wild pollinator conservation10.1038. Nature Communications DOI: 10.1038/ncomms8414:

2 Woodcock et al 2016: Impacts of neonicotinoid use on long-term population changes in wild bees in England. Nature Communications. DOI: 10.1038/ncomms12459OI:


* ‘Pesticide reduces bumblebee colony initiation and increases probability of population extinction’ by Baron et al. was published in Nature Ecology & Evolution on Monday 14th August.

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