Here are links to the USDA Bee Lab findings on Varroa-Sensitive Hygiene in honey bees
Varroa Sensitive Hygiene (VSH) Publications
VSH is an important mechanism of resistance to varroa mites. VSH is very similar or the same as hygienic behavior that honey bees use to combat American foulbrood, chalkbrood, and the eggs and larvae of wax moths and small hive beetles.
Another characteristic of VSH bees is a reduced fertility of mites, when compared to non-VSH bees. In a colony, mite fertility is reduced several weeks after introduction of VSH queens into non-selected colonies. This led to the original name of the trait, Suppressed Mite Reproduction (SMR). This name describes the trait (or traits) selected in the experimental population of bees. The name of the trait was later replaced by Varroa Sensitive Hygiene (VSH). This is due to the finding that the primary mechanism of the trait is the removal of infested pupae from capped brood cells.
Varroa Sensitive Hygiene, (VSH) identified and researched at the USDA Bee Lab in Baton Rouge, is neither a line nor a breed of bee. Rather, it is identified as a “trait.” Backcrossing suggests that the trait involves a small group of genes, perhaps only two.
While the VSH trait was first identified in honeybees in the U.S., current research supports the theory that the VSH alleles (alternate forms of genes) may occur in bee populations worldwide. Moreover, the VSH alleles can be bred into any honey bee population to produce VSH-based resistance
In 1992, as varroa mites were becoming widespread in the United States, Dr. John Harbo at the USDA’s Honey Bee Laboratory in Baton Rouge was assigned the task of breeding bees for resistance to varroa. In 1995 he and Dr. Roger Hoopingarner at Michigan State University discovered the VSH trait. During the next ten years Dr. Harbo led USDA research to understand how this trait works and to evaluate its effectiveness in field colonies. The VSH trait was released to the beekeeping industry in 2001.
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Important Points in Understanding VSH
1. Varroa Sensitive Hygiene (VSH) is not the same as freeze-kill or pin-kill hygiene and cannot be measured with those methods.
2. Bees with the VSH trait express mite resistance by disrupting mite reproduction in worker brood that is aged 4-6 days post-capping. Mites on adult bees and mites in younger and older stages of worker brood are not affected.
3. Very useful in measuring VSH is the fact that VSH bees do not disturb a varroa-infested cell if the mite is not producing progeny. In a typical varroa-susceptible colony non-reproducing mites commonly occupy about 5-15 percent of the varroa infested cells. When one finds a higher proportion of infested cells with non-reproducing mites, that colony has some VSH alleles. If the only varroa infested cells in a sample contain non-reproducing mites, the bees are 100% VSH.
4. The VSH trait is expressed only by adult worker bees that are at least one week old. Therefore, when a VSH queen is introduced to a varroa-susceptible colony of bees, the mite population will continue to grow for the next 35 days.
5. It takes about 20 days for all of the mites to pass through this three-day vulnerable period in their reproduction. Therefore, it will take at least 55 days following the introduction of a VSH queen for her progeny to fully control a mite population.
6. VSH does not result in poor brood pattern unless one introduces a VSH queen to a colony with a high rate of varroa infestation. In that case, one may notice 3 weeks of spotty brood that begins about 7 weeks after a VSH queen is introduced. After the 10thweek, VSH bees will have reduced the mite population and brood removal will move into maintenance mode with minimal removal. If a VSH queen produces a poor brood pattern in her first month, the issue is not caused by VSH.
7. The VSH trait does not seem to control mites in drone brood, but we do not recommend drone brood removal. It is not necessary with VSH.
8. VSH is not a pedigreed stock. The VSH trait represents only a tiny fraction of a bee’s genome, so bees with VSH can be as diverse as any existing bee population and the trait can be added to any stock.
Below is a clip of VSH bees removing mites from an infested cell