Location: Bee Research Laboratory
Project Number: 8042-21000-291-10-I
Project Type: Interagency Reimbursable Agreement
Start Date: Feb 1, 2020
End Date: Jan 30, 2022
Varroa mites rely on honey bees for food and their own reproduction. The rapid reproductive output of Varroa mites may be aided by host-derived proteins that the mite may sequester and directly provision their developing oocytes. While feeding, Varroa mites (plus saliva) can cause physical damage to honey bee pupae and adults and vector honey bee viruses. The resultant disease, varroosis, negatively impacts honey bees, leading to bee mortality and colony loss. Horizontal transmission of the disease among bees is more understood than transmission of mite vectors between colonies. Like many honey bee diseases, varroosis on a colony level may be transmitted horizontally. Researchers believe that high mite-infested colonies may represent a source of mites that infest neighboring colonies. These ‘Varroa bombs’ may be responsible for spreading mite populations among colonies and leading to their demise. This possible mechanism of mite transmission may involve perception and response to chemical cues. For example, when mite populations exceed a hypothesized threshold, but before colony collapse, chemical cues may alter behaviors of mites toward becoming phoretic, whereby they seek out foragers for a ride to a potential new residence. Chemical agents have been employed to control Varroa mites with mixed results; mites have gained resistance or tolerance to many varroacides, and beekeepers have specifically complained that current varroacides are less effective when applied late in the season prior to overwintering. Our unpublished data suggest that as fall approaches, mites have significantly increased longevity compared to summer mites, similar to the pattern observed for their host honey bees. Questions such as whether mites utilize their hosts’ biochemical machinery (e.g., host-derived antioxidant vitellogenin) to increase tolerance to chemical exposure, or whether they have their own physiological mechanisms, have yet to be addressed. Components of mite saliva are likely an important aspect of their ability to obtain food, and may be important for aiding transmission of honey bee pathogens. Surprisingly little is known about the identities of saliva constituents and their singular effects on honey bees, and whether/how saliva constituents change based on cues perceived by Varroa mites. Understanding both the chemical constituents of saliva and their effects on honey bees and the ability of mites to acquire food, would provide key targets for effecting control of these damaging mites. The goal of the research is an integrated effort against Varroa mites using the our expertise in honey bees, mite biology, and chemical ecology. Four distinct objectives are: 1) Refine understanding of Varroa mite ecology, including the roles of chemical cues that direct mite movements among colonies; 2) Identify the roles of seasonality and mite/host physiology underlying variable susceptibility of Varroa mite to toxicants; 3) Identify and characterize components of mite saliva, and 4) Provide data and analyses that identify the contribution of host-derived proteins to Varroa mite reproduction, and their movements within Varroa mites and/or their eggs.
For Objective 1, the focus will be on understanding whether and how colony mite loads, which may be correlated to levels of in-hive chemical cues, affect movements of mites among colonies. First, we will leverage the many colonies used as ‘mite farms’ on USDA-ARS grounds and monitor/measure their mite levels. Then, repeating the study from FY2019, we will sample in-coming and out-going honey bees and determine the number of mites they carry. Finally, we will artificially increase/decrease colony mite load and re-evaluate mite movements among colonies. A hypothesis for this work based on results from FY2019 states there will be a net emigration of mites from colonies with lower manipulated mite loads and net immigration of mites to colonies having manipulated higher mite loads. For Objective 2, we will use established toxicological methods to determine the Varroa mite LC50 for key varroacides, including amitraz, fluvalinate, and coumaphos at several points over the spring/summer and fall/winter seasons. At these same time points, we will determine the antioxidant capacity, activities of several antioxidant/detoxification enzymes, and the expression of the genes producing these enzymes for both the mites and their honey bee hosts. We will also quantify the levels of host-derived proteins (vitellogenin, heat shock proteins, etc.) in both hosts and mites, and use these data as explanatory variables in analyses of the mites’ response to chemical exposure. For Objective 3, protocols are in place for obtaining quantities of mite saliva, and for isolating and identifying its protein constituents, and microbes ‘carried’ by mite saliva. We also have equipment necessary to expose mites to chemical cues that mimic changes in mite populations (dependent on results from Objective 1). We will analyze samples of mite saliva collected when exposed to no chemical cues, and also when Varroa mites are exposed to chemical (volatile and or tactile) cues and mine the resulting proteomic data for those associated with specific chemical cues. For Objective 4, we will use in-house protocols and resources to track movements of host-derived resources (e.g., proteins, lipids) ingested by mites through their bodies and into developing eggs. Tissue preparations of mites treated with tissue-specific stains and/or with host specific antibodies will be collected along a time series (from ingestion to egg laying, an approximately 30-hour time span) and analyzed using microscopy, will help decipher the major locations (sinks) for specific tissues within the parasites. The lyrate is hypothesized to be a sink of many of the host-derived proteins sequestered by Varroa mites.