A Sustainable Approach to Reducing Pesticide and Antibiotic Use in Honey Bee Colonies
The incorporation of the trait, Suppression of Mite Reproduction (SMR), into our line of honey bees bred for hygienic behavior significantly reduced infestation of Varroa destructor mites within two commercial beekeeping operations relative to the pure hygienic line and a control line. We demonstrated that the mechanism of SMR is through adult bee removal of pupae on which mites are reproductively successful, more so than through physiological suppression of mite reproduction by bee pupae. We are developing a web-based course for beekeepers that emphasizes treating colonies for diseases and mites only as a last resort.
Our goal is to breed honey bees, Apis mellifera, resistant to diseases and parasitic mites to reduce the amount of antibiotic and pesticide use in bee colonies, and to ensure that our breeding methods and stock are accessible to beekeepers everywhere.
Our first objective is to test hybrid crosses between a line of bees resistant to disease (bees bred for hygienic behavior) and a line resistant to the parasitic mite Varroa destructor (bees bred for "Suppression of Mite Reproduction" or SMR). Our previous research indicated that bees bred for hygienic behavior are only partially resistant to these mites. Thus, our aim is to increase the level of mite resistance while retaining the disease resistance and high honey production of the hygienic line. The crosses will be tested both in apiaries at the University of Minnesota and in apiaries owned by Mr. Darrel Rufer, a commercial beekeeper based in Minnesota.
Our second objective is to determine how bees selected for SMR are able to suppress the reproduction of the mite. We will test the hypothesis that there are behavioral incompatibilities between the mite and the bee larva during the first 12-24 hours after the bee larva is capped with wax which preclude the mite from initiating oogenesis (egg development).
Our third objective is to develop an interactive web-based course on sustainable methods of controlling diseases and mite pests of honey bees. The main emphasis will be on promoting the use of resistant bee stocks as the foundation for integrated pest management strategies. This will be the only such course available on-line to beekeepers, and is a crucial link between our research and its successful implementation.
Objective 1: The HYG/SMR breeder colonies we chose for 2004 all came from the colonies that were transported to ND in 2003, and we chose them from among the colonies that produced the most honey and had the lowest mite levels. Although we established colonies in MN and ND, we could only analyze results from the North Dakota apiaries because of an unfortunate mix-up in breeder queens for the MN apiaries which caused us to have to exclude the entire data set. However, the data from 110 colonies in ND demonstrated again that the HYG/SMR colonies had significantly lower mite loads on adults than both the HYG and Control colonies (ANOVA: P = 0.042), were significantly more hygienic than both the hygienic colonies and the controls (P 0.05), and had equivalent colony strengths and brood viability (P > 0.05). These results indicate that we have made progress on increasing mite resistance while maintaining hygienic behavior and good brood viability. We will be submitting these findings for publication a peer-reviewed journal soon.
Objective 2: Originally, we hypothesized that the mechanism for Suppression of Mite Reproduction was due to behavioral incompatibilities between the mite and the bee larva during the first 12-24 hours after the bee larva is capped with wax which preclude the mite from initiating oogenesis (egg development). In late summer of 2002, we began testing a new hypothesis about the mechanism of Suppression of Mite Reproduction. While working with the SMR line, we discovered that it is also highly hygienic. This finding is curious and unexpected because Dr. Harbo, who bred the SMR line, did not select for hygienic behavior. He bred for colonies in which the mites were not reproducing on worker pupae. Apparently, the presence of the hygienic trait is a by-product of his selection. From our previous research we know that hygienic bees tend to remove infested pupae after the mite has initiated reproduction (when there are offspring in the cell). Noticing that the SMR line was hygienic, we hypothesized that they might be removing infested pupae with reproductive mites, leaving only the pupae with non-reproductive mites. When capped cells containing pre-emergence worker pupae were opened to inspect for mite infestation and successful reproduction, it would appear that the mites did not reproduce, when actually any that did reproduce were previously removed by the bees.
Our findings from 2004 showed that the SMR colonies uncapped and removed significantly more mite-infested pupae than the HYG colonies. Both lines of bees removed significantly more infested pupae than control pupae (Nested ANOVA: P = 0.0438). Within the SMR colonies, the mites on pupae that were not removed by the bees had significantly lower reproductive success (fewer viable female offspring) compared to mites remaining in the HYG colonies. There was no significant difference in reproductive success between remaining SMR and HYG mites within each line (P > 0.05 each).
When the bees were not allowed to remove mite-infested pupae, the reproductive success of mites collected from SMR colonies (SMR mites) was significantly lower than mites collected from HYG colonies (HYG mites). (Nested ANOVA P = 0.007). In addition, the reproductive success of HYG mites was significantly reduced on SMR pupae relative to on HYG pupae (P=0.0163).
These findings show that the main mechanism for Suppression of Mite Reproduction (SMR) involves the active detection and removal by adult bees of pupae on which Varroa destructor produces viable female offspring, and the non-removal of pupae on which the mite does not produce viable female offspring. In this sense, adult SMR bees are applying direct selection pressure on the mites by actively suppressing their reproductive success. Over time, the mites that remain in SMR colonies have reduced reproductive potential.
Another minor component of the SMR trait involves some unknown physiological or behavioral factor associated with SMR worker brood that reduces the reproductive success of mites. This component was originally thought to be the most important factor in explaining the resistance of SMR bees.
In comparison with our colonies bred for hygienic behavior, colonies bred for SMR removed significantly more infested pupae, and were significantly more selective in removing pupae with reproductive mites. We speculate that this difference is because we selected colonies for hygienic behavior based on an indirect assay (removal of freeze-killed brood; Spivak and Downey, 1998). The SMR line was selected based on the presence of non-reproductive mites in worker brood (Harbo and Hoopingarner, 1997), a direct and specific assay for the bees’ ability to disrupt the reproduction of mites with high reproductive success.
Objective 3: We have completed the front end of the web-based course, which includes the home page, video welcome. We have also completed sections on Healthy Bees, the Strategy, Resources, some Frequently Asked Questions (FAQs), and one disease/pest module on chalkbrood disease. The course can be viewed at:
Before proceeding with the rest of the disease and pest modules, we wanted to obtain some user feedback, so we sent the web site and survey form to extension personnel and beekeepers.
Impacts and Contributions/Outcomes
With the introduction of the destructive parasitic mite, Varroa destructor, into the U.S., beekeepers have had to resort to using pesticides within their beehives to prevent devastating and expensive colony losses. Colony losses have more than doubled in many cases, even with the use of the pesticides, due to reinfestation and increase in the overall stress imposed on the colonies by the mites. Operating costs have increased dramatically because the pesticides are expensive and more labor is required to apply them. The most sustainable solution is to reduce the dependency on pesticides by breeding bees that can defend themselves against the mites and diseases. These bees would be healthier and more able to withstand the stress of continued mite infestation and disease exposure.
Our research demonstrates that colonies bred for hygienic behavior have good resistance to chalkbrood and American foulbrood diseases, and partial resistance to Varroa mites, particularly under low mite infestation pressure. Combining the hygienic trait with the SMR trait increased mite and disease resistance by increasing the bees' ability to remove infested brood. Our goal is to eliminate the use of pyrethroids and organophosphates entirely in beehives by using resistant stock, or minimally to reduce the number of treatments required. Most beekeepers treat their colonies twice annually with the synthetic pyrethroid, fluvalinate, or with the organophosphate, coumaphos. In the last several years, the mites have developed resistance to both fluvalinate and coumaphos. If beekeepers could half the number of treatments they apply, this small reduction in pesticide use would reduce the added operating cost in half. Our web-based course will help disseminate knowledge and know-how to beekeepers on how best to reduce chemical use in bee hives using best beekeeping practices and bee breeding.