And if the frequent swarming is an essential part of their defense against varroa?
It is not. This can be shown by very low varroa counts at any time of year. I pulled drone larvae from a colony a year or two ago because someone wanted to know how high my counts were. I found a varroa in the cell with the 127th drone uncapped. My bees show VSH and allogrooming traits.
This list is far from complete. A recent dutch study showed at least 2 more mechanisms for resistance.
Varroa Selective Hygiene - disrupts the reproductive cycle of the varroa mite
a. Detect infested larvae
b. Uncap infested larvae
c. Remove infested larvae
d. selection involves testing for hygienic behavior and removal of infested larvae
Allogrooming - bees grooming each other to remove mites
a. Varroa mauling - chewing and biting the mites which kills them
b. Selection involves monitoring for chewed mites on the bottom board
Breaks in brood rearing - during brood breaks, varroa cannot reproduce.
a. Heavy pollen collection - bees that collect pollen heavily are more sensitive to lack of pollen and shut down brood rearing earlier.
b. Sensitive to nectar dearth - bees that react to nectar shortage by breaking the brood cycle.
c. Swarming is one type brood break but is not as effective as other strategies.
d. Selection involves monitoring for bees that shut down brood rearing when pollen is unavailable.
Reduced days to worker maturity - fewer days gives mites less time to reproduce
a. some worker bees mature in 19 days vs standard 21
b. using small cell foundation and timing brood emergence
c. Selection involves identifying the small percentage of colonies that mature workers in fewer days.
Mite entombment - bee larvae shed cocoons in such a way that mites are trapped.
a. some colonies show more entombed mites.
b. selection involves counting entombed mites after a brood cycle and breeding from colonies with higher counts.
Superinfection exclusion - A variant of DWV that is less virulent protects infected bees from other strains.
a. Identified in a British apiary after years of breeding from survivor colonies.
b. Selection is by breeding from survivors that live with large mite populations.