A Sustainable Approach to Reducing Pesticide and Antibiotic Use in Honey Bee Colonies
We demonstrated that honey bee resistance to Varroa destructor mites depends on the interplay between hygienic behavior (adult bees that physically remove mite-infested pupae) and physiological suppression of mite reproduction by bee pupae (when the mites feed on pupae). The incorporation of the trait Suppression of Mite Reproduction (SMR) into our line of bees bred for hygienic behavior significantly reduced mite infestation within two commercial beekeeping operations relative to the pure hygienic line and a control (unselected) line. After one year, the HYG/SMR colonies did not require any pesticide or antibiotic treatments to control the mites or other diseases.
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 that 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 online to beekeepers, and is a crucial link between our research and its successful implementation.
Objective 1: Over the last two years, we have been incorporating the SMR trait into the hygienic line by establishing colonies that contain “hybrid” worker bees, because many colonies in the SMR line have low brood viability. A relatively high percentage of larvae and pupae never complete development, which could mean that the SMR trait is deleterious to both bee brood and mites, or that the line was overly inbred to “fix” the trait. However, crosses between SMR and other lines have better brood viability.
We began in 2001 by making reciprocal crosses between the hygienic and SMR lines (hygienic queens were inseminated with semen from SMR males and vice versa). From the reciprocal crosses, we bred new daughter queens from the colonies with the best brood viability, and produced subsequent crosses with varying degrees of the SMR trait (by using varying numbers of drones from both lines to inseminate queens from each line).
This summer (2003), we tested one of the crosses in apiaries in Minnesota and North Dakota, in collaboration with two commercial beekeepers, Mr. Darrel Rufer and Mr. Bill Klett, who are part of a group of MN and ND beekeepers (the “East TX Cartel”) that transport their colonies to eastern TX each winter, and have been using our hygienic line for over six years. The majority of the drones in their colonies carry the hygienic alleles, as they are progeny of the hygienic queens we have been donating to them. The queens produced from the experimental cross mated naturally with hygienic drones in their apiaries to produce worker progeny that were approximately 67% hygienic and 33% SMR (each worker bee had this 67:33 genotypic mixture, hereafter called Hyg/SMR). Our results indicate that the Hyg/SMR colonies had significantly fewer mites than control colonies (MN: P = 0.0118; ND: P = 0.0291), while the hygienic colonies had intermediate mite levels (see Appendix). Importantly, the mite levels in half of the Hyg/SMR colonies did not increase over the summer, in contrast to the levels in the hygienic and control colonies, which rose in every colony. The brood viability in the Hyg/SMR colonies was good, and was statistically equivalent among the three groups. The Hyg/SMR colonies also removed as much freeze-killed brood within 24 hours as the hygienic line. These results indicate that we have made progress on increasing mite resistance while maintaining hygienic behavior and good brood viability.
In 2004, we will continue our efforts to increase mite resistance in the hygienic line by cooperating with the same beekeepers who will breed daughter queens from among the Hyg/SMR colonies in their commercial apiaries that have demonstrated the greatest mite resistance, most rapid hygienic behavior, highest honey production, and best brood viability.
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). We proposed to observe and record the movements of the mite on larvae through transparent, glass cells during the first 12-24 hours after the mite enters the cell, to determine if the movements of mites when they do not reproduce differ from those when the mite reproduces successfully. We recorded the movements of five successfully reproducing mites, and five unsuccessfully reproducing mites, and found no differences between them. Thus, we switched emphasis of this objective, as follows:
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 preliminary data suggests that SMR colonies removed an average of 92% of infested pupae, hygienic colonies removed 89%, and control colonies removed 50% (which was significantly less than the SMR and HYG colonies). The mites remaining on the pupae not removed by the bees did reproduce, but this depended on whether the mites were from SMR or non-SMR colonies. Mites from SMR colonies had less reproductive success (fewer offspring) on pupae from their own colonies than on pupae from non-SMR colonies (12.5% vs. 48%; X2 = 3.54; P = 0.06). Mites from non-SMR colonies were equally reproductive on SMR or non-SMR pupae (71.4% vs. 72.3%; P > 0.05).
These results indicate there are two mechanisms at play: removal of infested pupae and an “anti-reproduction factor” in SMR pupae. In 2004, we will continue to unravel this problem to understand how the SMR trait manifests itself in bee colonies.
Objective 3. Due to budget cuts and extensive restructuring of the Extension Service at the University of Minnesota, the people that were assisting us in developing interactive web-based course on sustainable methods of controlling diseases and mite pests of honey bees were laid off. However, we have gathered a new group of experts, and will have a pilot module of this course ready for demonstration within three months.
Impacts and Contributions/Outcomes
The fact that we have identified a number of “breeder” colonies in the apiaries of the commercial beekeepers has far-reaching implications. A measure of success of our research program is whether the beekeepers adopt the new technology, in this case, increasing disease and mite resistance through bee breeding. Until now, we have been providing breeder (instrumentally inseminated) queen to the beekeepers. However, a more sustainable solution is for the beekeepers to select from among their best colonies instead of relying on University breeder queens. By breeding from their colonies that express both hygienic behavior and SMR, and requeening their colonies with these daughters, the daughters will produce drones that also carry both the hygienic and SMR traits for mating in subsequent generations. Thus they do not need to instrumentally inseminate queens to propagate the hygienic and SMR traits.
Our cooperating beekeepers sell their resistant queen bees to beekeepers in the North Central region and throughout the United States either directly, or through B&B Honey Farm, a regional beekeeping supplier based in Houston, MN, so the stock is being distributed widely and successfully.