Abstract
Beekeepers report significant honey bee deaths during and after almond bloom. These losses pose a major problem for the California almond industry because of its dependence on honey bees as pollinators. This research aimed to determine if combinations of pesticides applied during almond bloom during daylight hours were a possible explanation for these losses. In the present study, we aimed to mimic the spray application route of exposure to pesticides by using a Potter Spray Tower to treat adult honey bees with commonly encountered pesticides and pesticide combinations at multiples of the maximum recommended field application rates. Tested insecticides included Altacor® and Intrepid® and tested fungicides included Tilt®, Pristine®, Luna Sensation®, and Vangard®. Synergistic toxicity was observed when the fungicide Tilt® (a.i. propiconazole) was applied with the insecticide Altacor® (chlorantraniliprole), though neither caused significant mortality when applied individually. The study also looked at the effect of adding a spray adjuvant, Dyne-Amic®, to pesticide mixtures. Dyne-Amic® was toxic to honey bees at concentrations above the maximum recommended field application rate and toxicity was increased when combined with the fungicide Pristine® (pyraclostrobin and boscalid). Addition of Dyne-Amic® also increased toxicity of the Tilt® and Altacor® combination. These results suggest that application of Altacor® and Tilt® in combination with an adjuvant at the recommended field application rates could cause mortality in adult honey bees. These findings highlight a potential explanation for honey bee losses around almond bloom, emphasize that the safety of spray adjuvants to bees should not be assumed, and provide support for recommendations to protect bees from pesticides through application at night when bees are not foraging.
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