Studies on the outcomes of pesticide exposure on bumble bees have traditionally evaluated the pesticides individually. Poorly known is the effect of simultaneous exposure to multiple pesticides, which has high likelihood in agricultural landscapes. Four experiments were conducted to determine if simultaneous exposure to sub-lethal doses of the insecticide Assail® (30% acetamiprid) and the fungicide Orbit® or Tilt® (41.8% propiconazole) would cause detrimental effects on survival and colony productivity of the common eastern bumble bee, Bombus impatiens (Cresson) (Hymenoptera: Apidae). Survival of workers exposed to propiconazole was marginally reduced (Experiment 1), whereas gyne production increased in colonies exposed to acetamiprid (Experiment 2). There was a trend for fewer larvae and workers in colonies exposed to acetamiprid and a mixture of acetamiprid and propiconazole (Experiment 3). Higher wax moth infestations and densities were observed in colonies exposed to the mixture of acetamiprid and propiconazole (Experiment 4). In conclusion, exposure to the two pesticides resulted in variable outcomes in B. impatiens. Synergy between acetamiprid and propiconazole was explained as an indirect effect of enhanced wax worm infestation. We found an increased level of wax worm infestation in colonies exposed to a mixture of Assail and Tilt compared to colonies not exposed to Assail or Tilt (control) or exposed only to Assail. While this study does not provide evidence that B. impatiens are at high risk when simultaneously exposed to field rates of these two pesticides, we suggest that wild blueberry growers practice integrated pest management as a means of reducing any amount of risk to these important bumble bees.
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