What do people mean when they use the term resistant stock? The term resistant can address several behavioral or genetic traits that allow a colony to continue functioning and survive if not given relief to pests or diseases through various treatments. The most effective traits I have seen are VSH (varroa sensitive hygiene), and a resistance to viral transmissions from mites. VSH is a set of traits that allow bees to detect reproductive mites in capped brood, and uncap and remove infested pupae. This breaks the mites reproductive life cycle without chemical intervention. It is reported that the bees will leave non-reproductive mites alone, and only address the ones that are raising young. This discovery was formerly called SMR or (supressed mite reproduction) due to the suppression of mite levels by addressing reproductive mites. The term was changed to VSH after it was discovered this was a variant of the known hygiene response in bees. VSH bees are extremely hygienic, which also gives them strong resistance to chalkbrood and American foulbrood. I am a big fan of VSH traits and have implemented them in our stock for many years with favorable results. A resistance mechanism that is often overlooked is viral resistance. I have seen many colonies that get a fairly high mite load, but you never see withered wings, and other obvious signs of viral infestation. The bees make surplus honey, survive the winter, and continue on despite elevated mite levels. Viral resistance is clearly at play. If you couple VSH bees, with bees that have a strong resistance to mite vectored virus' you have a pretty solid bee. Another known resistance mechanism is allogrooming, or autogrooming. This trait triggers bees to physically remove phoretic mites that are attached to nest mates by biting them and damaging their legs and or outer carapace. This is a pretty cool trait. Within the past few years, I have implemented known grooming lines via Purdue mite biter virgins which I inseminated with VSH semen. I'm hoping for brood nests displaying both traits, which I suspect would be favorable. Not only would the mites be under attack when trying to reproduce in capped brood, but would also be under increased threat when hanging onto bees in their phoretic phase. Time will tell if this pays off, but I suspect it will.