Date of this Version
Dahlgren, L. (2014). Mechanisms of differential toxicity between honey bee (Apis mellifera L.) castes with an emphasis on coumaphos. PhD diss., University of Nebraska, Lincoln.
Acaricides are used to treat honey bee (Apis mellifera L.) colonies to control the parasitic Varroa mite (Varroa destructor Anderson & Trueman), a worldwide threat to honey bee health. As such, these compounds act as drugs to mitigate bee losses but may also stress the bees. This dissertation quantifies differences between queen and worker tolerance of five acaricides and clarifies the honey bee mechanism of tolerance for coumaphos. Selected acaricides were topically applied to adult queen and worker bees to generate dose-response curves and LD50s. Twenty-four hours after treatment, queens were 3-times more tolerant of tau-fluvalinate and 6-times more tolerant of thymol than workers when adjusted for body weight differences between workers (108 mg) and queens (180 mg). Queens survived the highest administered doses of fenpyroximate (1620 μg/g) and coumaphos (2700 μg/g) indicating that queens are at least 12-fold more tolerant of coumaphos and at least 40-fold more tolerant of fenpyroximate than workers. However, queens treated with as little as 54 μg/g of fenpyroximate exhibited reduced survival over 6 weeks following treatment. Amitraz was the only acaricide tested for which queens were not more tolerant than workers. Differences between queen and worker tolerance of coumaphos was explored further by topical bioassays with the metabolite coumaphos oxon. Coumaphos oxon was only half as toxic as the parent compound to Apis mellifera. This insensitivity to both coumaphos and coumaphos oxon is unique as both castes were highly susceptible to chlorpyrifos, with chlorpyrifos oxon being twice as toxic as the parent compound. Using Ellman’s assay, honey bee and house fly acetylcholinesterase were all shown to be similarly inhibited by both chlorpyrifos oxon and coumaphos oxon. Target site insensitivity is therefore not the mechanism of honey bee tolerance. Finally, bee metabolism was compared using GC/MS and LC-MS/MS at 24 hr time intervals for 5 days. Most notably, coumaphos oxon was not found. Queen and worker acetone rinse and worker internal recovered coumaphos concentrations decreased over time. Therefore, coumaphos is likely being metabolized to a less toxic coumaphos metabolite. Caste differences are likely due to metabolism, but further studies are needed to determine specific mechanisms.
Advisors: Marion Ellis & Blair Siegfried