In vitro effects of fungicides on the susceptibility of Honeybee (Apis mellifera) larvae to European foulbrood.

Pesticide exposure has been implicated in the immunosuppression of honeybees (Apis mellifera) and suspected to increase susceptibility to European foulbrood (EFB). EFB, caused by the bacterium Melissococcus plutonius, produces increased mortality in honeybee larvae in colonies under environmental and nutritional stress, particularly in association with commercial blueberry pollination. The effects of exposure to formulated fungicide products commonly used in blueberry production on susceptibility of honeybee larvae to EFB during blueberry pollination is currently unknown. Using an in vitro larval infection model of EFB, we tested the effects of chronic larval exposure to field-relevant concentrations of the formulated blueberry fungicides Captan® and Kenja® on larval mortality from M. plutonius infection. Surprisingly, we found that chronic exposure to Captan® or Kenja® during development significantly (PM. plutonius. However, larvae chronically exposed to a combination of Captan® and Kenja® did not experience a significant difference in survival relative to infected controls. These in vitro results suggest that chronic exposure of honeybee colonies to formulated fungicide products during blueberry pollination does not predispose these colonies to EFB. Additional colony-level studies are necessary to verify the field-relevance of these in vitro results.