No. 42 Spring, 1997 Page 2

Resistance to Varroa: Research Update

When we started to look for resistance back in 1992 it was decided that we needed to find colonies that had survived at least for some time without chemical treatments. The obvious first choice would be wild (feral) colonies. The difficulty with these colonies is that it hard to get at the queen. While the drones would supply some of the genes, you can not be certain that they come from the colony. Thus, we chose to use beekeeper colonies that had not been treated, either by neglect or by choice. The first characteristic that we screened these colonies was the percentage of damaged mites (grooming behavior) found on sticky boards. We were able to find several colonies with a relatively high index. It became clear after the first subsequent winter that this character was not sufficient to keep the colonies alive by itself. The research literature indicated that hygienic behavior (cleaning out brood cells that had mites) was also used by bees to rid the colony of mites. So we decided to add this character to the selection process. At this juncture we again decided that we needed more genetic traits to add to the arsenal.

Since mites reproduce during the sealed (pupal) stage of growth, research had been directed at finding strains of bees that had a shorter post-capping stage. One of the leading researchers on this type of selection was Dr. John Harbo of the USDA Baton Rouge Laboratory. Thus, I decided to merge our selections with those of his. This way we might have three mechanisms of resistance – grooming and hygienic behavior along with short post-capping period. John is a world leader in artificial insemination, especially using a single drone for the source of sperm. In 1995 he made many crosses combining these different stocks. The queens were all single drone inseminated, to reduce the variability. They were introduced into nucleus colonies that were all started from bees taken from the same (50 lb.) package of bees. The bees put into each nuc were weighed to know how many bees were added, and sampled for the number of mites. Thus, we knew how many bees and mites we started with in each nucleus.

After about 40 days to allow the bees from each queen to populate the nuc we tested them for hygienic and grooming behaviors. Then after 70 days (1995) or 90 days (1996) we closed up the nucs at night and the following morning weighed the bees, took a sample of bees and brood. Then we could determine the final number of bees, total mites on adults, and by sampling the brood the number of mites in the brood. By calculating these totals we could determine how much the mite population grew during this period. Each cycle of reproduction (about 3 weeks) of a female mite could at least double the population. We should then expect (from what the research literature told us) 2.5 to 3 times as many mites at the end. We did have two or three of the stocks that actually had the same number or fewer mites! That was exciting. Then upon analysis we had a new surprise. The trait that was the most important was one that we had not developed. The character was one of non reproduction of the mites after the females entered the brood cell. Research is a little mixed as to what might be going on with these mites. Is it something that they pick up while feeding on adult bees, or is it something that they don’t pick up? The result is fine– its just that at the moment we don’t quite know why the mites don’t reproduce.

What happened to all the traits that we put into these stocks? Well, here we were a little disappointed. In 1995 only short post-capping period showed any measurable effect. In 1996 hygienic behavior also had a significant addition to the resistance. We are now thinking that it is possible that these types of characters may have to be at high levels in order to be effective. That is, that many, many bees would have to carry these traits in order to be very effective. And it may be that they are only ever going to be slightly additive to the overall varroa resistance.

Where are we now in the selection process? We have had two positive years of results, and we measured the heritability of the non-reproductive trait as 0.4 so it should be able to be increased in intensity through selection. John Harbo is now making additional crosses with the aim of increasing the frequency of these genes, and establishing the stock for release to beekeepers. Once these are reasonably fixed the stock will be released to queen breeders who will produce queens for your colonies. This release will now probably take place in 1998. Maybe none too soon. The tales of varroa resistance to Apistan^Æ is increasing. The varroa resistant honey bee stock may not be the whole answer but it surely may allow us to cut down on the number and amount of chemicals that we are currently using to control this pest.

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^{Document author(s): Roger Hoopingarner
E-Mail: hoopinga@msue.msu.edu
URL: http://www.ent.msu.edu/bplus/page2.html}