The honey bee, Apis mellifera, provides about $15 billion USD in crop value each year in the U.S. alone in the form of pollination services. Since 2006, commercial beekeepers have reported an average annual overwintering loss of about 28.6% of all managed colonies. There are many factors that are thought to contribute to colony loss including bee-specific pests (e.g. the Varroa destructor mite), bee-specific pathogens (e.g. Nosema fungus), modern beekeeping practices, diminished genetic variability, poor queens, climate change, and exposure to agricultural pesticides. While not the single cause of colony loss, the neonicotinoid insecticides elicit sublethal effects to honey bees that could increase their sensitivities to other stressors that affect colony health. Previous studies found that honey bees have differential sensitivities to the neonicotinoid insecticide imidacloprid, which suggest a mechanism of tolerance to the insecticide in certain colonies. In this study, I examined the imidacloprid sensitivity of honey bees collected from different colonies. After determining a range of LC50 values in the tested colonies, I examined the metabolic detoxification activities of honey bees collected from two colonies that represented the highest and lowest LC50 values, between which there was a 36-fold difference in their LC50 values. I discovered that of the three main families of metabolic detoxification enzymes, general esterases, cytochrome P450 monooxygenases, and glutathione S-transferases (GSTs), a reduction of GST activity with diethyl maleate (DEM) significantly increased imidacloprid-mediated mortality to the honey bees. A comparative analysis of GST kinetic activity from imidacloprid-susceptible and -insensitive honey bees revealed a lower bimolecular inhibition rate constant (ki) for imidacloprid-insensitive individuals (5.07 ± 0.098 nmol/min/mg protein) compared to the imidacloprid-sensitive honey bees (17.23 ± 1.235 nmol/min/mg protein). The IC50 of DEM estimated for bees from each colony showed that the imidacloprid-susceptible honey bees possess a higher IC50 (10 μM) than that of the tolerant honey bees (3 μM). These data suggest that the GSTs in the imidacloprid-tolerant honey bees might be a more efficient detoxification mechanism for the conjugation and elimination of imidacloprid, or imidacloprid metabolites, compared to that of imidacloprid-susceptible honey bees. Therefore, I hypothesize that the differences in metabolic detoxification enzyme activities of honey bees collected from different colonies can result in the differential toxicities of honey bees exposed to neonicotinoid insecticides, such as imidacloprid. However, a thorough examination of imidacloprid detoxification in honey bees is warranted to confirm this hypothesis. / Master of Science in Life Sciences / Honey bees are the most important crop pollinator known to humans. The domestication and use of these insects constitutes a multi-billion dollar industry. Their pollination services alone are a necessary part of modern day agriculture. One of the concerns raised today with regard to honey bee health is their exposure to insecticides used widely in modern agriculture to manage crop pests and protect our food supply from devastating crop loss. One insecticide family that has gained much attention lately are the neonicotinoids. These insecticides are reported to elicit sublethal effects to honey bees that can affect colony health. Some of the more widely used neonicotinoids include, but are not limited to, imidacloprid, thiacloprid, and acetamiprid. The goal of this study was to examine the acute toxicity of imidacloprid to honey bees collected from different colonies and to compare the metabolic detoxification enzyme activities of the honey bees to understand the mechanism(s) of imidacloprid sensitivity in the honey bees. Here, I report a 36-fold difference in the acute toxicity of imidacloprid to the honey bees collected from different colonies. A comparison of glutathione <i>S</i>-transferases activities in imidacloprid-susceptible and -tolerant honey bees suggest that these metabolic detoxification enzymes may assist in the conjugation of imidacloprid, or associated metabolites, and thus facilitate the removal of the insecticide from the honey bees.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/73220 |
| Date | 14 October 2016 |
| Creators | Langberg, Kurt |
| Contributors | Entomology, Anderson, Troy D., Bloomquist, Jeffrey R., Brewster, Carlyle C., Weaver, Michael John, Rueppell, Olav |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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