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Title: Influence of varroa mite (Varroa destructor) infestation levels and management practices on insecticide sensitivity in the honey bee (Apis mellifera)

item Rinkevich, Frank
item Danka, Robert
item HEALY, KRISTEN - Louisiana State University Agcenter

Submitted to: Insects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2017
Publication Date: 1/11/2017
Citation: Rinkevich Jr, F.D., Danka, R.G., Healy, K.B. 2017. Influence of varroa mite (Varroa destructor) infestation levels and management practices on insecticide sensitivity in the honey bee (Apis mellifera). Insects. 8(1),9.

Interpretive Summary: Varroa mites affect many aspects of honey bee health. High Varroa infestation levels result in reduced body weight, which is a critical factor for determining insecticide sensitivity. Reducing Varroa mite levels often involves using chemical treatments which may synergize insecticide sensitivity. Therefore, we evaluated if varroa mite infestation levels and management practices can affect insecticide sensitivity. Colonies were established in a control group, another group treated with amitraz, and another group used IPM methods such as screened bottom boards, powdered sugar treatments, and drone brood mite trapping. Our results show that varroa mites infestation levels measured at the colony level did not affect insecticide sensitivity, while measuring varroa mite infestation levels on bees used in bioassays showed a significant impact on insecticide sensitivity. Measurements of colony health showed that honey bees treated with amitraz were much healthier than control or IPM treated colonies. There were seasonal differences in insecticide sensitivity with fall bees being less sensitive than spring bees. These results show that varroa mite infestation significantly affect honey bee colonies. Potential concerns of insecticide synergism with amitraz should be minimal at the large benefit of reduced varroa mite infestation levels.

Technical Abstract: Varroa mites may cause devastating colony losses throughout the year and especially over winter. In addition to killing honey bees by feeding directly on bodily fluids, these parasites transmit many viral diseases, increase the susceptibility of the honey bee to pathogens, as well as disrupt biochemical and developmental processes. A variety of chemical, mechanical, and cultural practices may be implemented to mitigate mite infestation. While miticide applications are typically the most consistent and efficacious Varroa mite management method, increased sensitivity of honey bees to insecticides via miticide synergism and the eventual evolution of miticide resistance in Varroa mites are reasonable concerns. We used discriminating potency bioassays to test if mite infestation levels affected sensitivity to three commonly used insecticides. With no other factors considered, higher Varroa mite infestation levels significantly increased sensitivity to naled and imidacloprid, but not to phenothrin. Varroa mite infestation levels measured from newly emerged adults was significantly higher than from sampling from samples of bees on unknown age within the colony. To test the effects of mite infestation levels and mite management practices on insecticide sensitivity, colonies of Italian honey bee were treated with amitraz (Apivar®) according to the labeled instructions, IPM techniques such as screened bottom boards, drone brood trapping, and powdered sugar grooming stimulation, as well as an untreated control group. Sensitivity to phenothrin, amitraz, and clothianidin was assessed on a monthly basis from May through October. Measurements of colony health such as adult bee population, brood quantity, brood quality, queen presence, mite infestation levels, pollen collection rates and honey bee weight were also recorded. Surprisingly, mite infestation levels did not significantly affect the sensitivity to the pesticides we tested. Phenothrin sensitivity was significantly increased by day of the year, pollen collection rate, but significantly decreased by bee weight and queen retention. There was a significant negative relationship between amitraz sensitivity and bee weight. Clothianidin sensitivity was significantly affected by treatment type (amitraz, IPM>Control), and declined with day of the year, brood quality, pollen collection rate, and bee weight. Varroa mite infestation significantly decreased brood quality and honey bee weight. Varroa mite infestation levels were significantly determined by treatment (amitraz < IPM, Control) and day of the year. These results show that insecticide sensitivity is dynamic throughout the year with fall bees being less sensitive than spring bees likely due to physiological differences between those cohorts. The observation that larger bees and higher pollen collection rates reduce insecticide sensitivity underscores the importance of nutrition on colony health. In-hive amitraz treatment according to the labeled use pattern did not synergize sensitivity to the pesticides tested and should alleviate concern over potential synergistic effects of amitraz observed in laboratory studies. Since IPM practices were largely ineffective at reducing Varroa mite infestation levels, reliance on chemical methods of Varroa mite management is likely to be prominent in the near future. However, these products must be used judiciously so the long term effectiveness of these compounds could be maximized. These data demonstrate the complex and dynamics variables that contribute to honey bee colony health. It underscores the importance of controlling for as many of these variables as possible in order to accurately determine the effects of each of these factors as they act alone or in concert with others.