A reader asked how the VSH bees detect mites within the cell. So far, I cannot find a detailed explanation. Many scientific papers discuss various aspects of mite removal and efficiency, but the ones I read didn’t answer this specific question.
Varroa sensitive hygiene was originally discovered by the USDA Honey Bee Breeding, Genetics and Physiology Laboratory in Baton Rouge, LA. After discovery of the trait in nature, they bred lines of bees with amplified hygienic behavior. According to the USDA, VSH bees only remove pupae infected with mites that have begun to reproduce; they do not remove pupae with mites that are not reproducing or that are sterile.
The USDA online literature says the bees “sniff out” reproductive mites but it doesn’t say more than that. According to Glenn Apiaries, a company that sells VSH breeder queens, VSH bees also show activity against tracheal mites, American foulbrood, chalkbrood, wax moths, and small hive beetles.
Based on that information it sounds as if the VSH bees are recognizing all sorts of wildlife that doesn’t belong in the hive. Odor is the most likely mechanism, but so far I haven’t found a specific citation.