| Summary: | Neonicotinoid and fungicide exposure has been linked to immunosuppression and increased susceptibility to disease in honeybees (<i>Apis mellifera</i>). European foulbrood, caused by the bacterium <i>Melissococcus plutonius</i>, is a disease of honeybee larvae which causes economic hardship for commercial beekeepers, in particular those whose colonies pollinate blueberries. We report for the first time in Canada, an atypical variant of <i>M. plutonius</i> isolated from a blueberry-pollinating colony. With this isolate, we used an <i>in vitro</i> larval infection system to study the effects of pesticide exposure on the development of European foulbrood disease. Pesticide doses tested were excessive (thiamethoxam and pyrimethanil) or maximal field-relevant (propiconazole and boscalid). We found that chronic exposure to the combination of thiamethoxam and propiconazole significantly decreased the survival of larvae infected with <i>M. plutonius</i>, while larvae chronically exposed to thiamethoxam and/or boscalid or pyrimethanil did not experience significant increases in mortality from <i>M. plutonius</i> infection <i>in vitro</i>. Based on these results, individual, calculated field-realistic residues of thiamethoxam and/or boscalid or pyrimethanil are unlikely to increase mortality from European foulbrood disease in honeybee worker brood, while the effects of field-relevant exposure to thiamethoxam and propiconazole on larval mortality from European foulbrood warrant further study.
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