I have been asked to explain why I believe large colonies make winter mite management easier. It’s important to note that I said easier and not easy. You still have to pay attention and you still have to do something.
It is interesting to me that people who prefer triple-deep hives frequently report successful mite management, and people who have not tried them claim that triples are mite factories. In my own case, I had been successfully overwintering 80 to 100 percent of my hives for many years until last year, when I switched from ApiLife Var to HopGuard. I “misinterpreted” the instructions, treated all my hives in the same insufficient way, and ended up losing most colonies to mites. I just happened to have a few triples in that group, and oddly enough, it was the triples that survived despite huge mite loads.
Okay, my story is merely anecdotal, but it piqued my curiosity. I began asking others about their experience with triples and heard many similar stories. I also did a lot of reading about the population dynamics of both mites and bees. So I will explain my theory but, alas, it is only a theory.
First, I agree that triples are mite factories but only because more bees produce more mites. For most of the year, mites/bee is no different in small colonies than in big ones. There is a maximum mite/bee ratio that can be reached before the colony just collapses—big or small.
The timing of mite treatments
Mite treatments have to be performed at the right time regardless of hive size. That time is when brood production is lowest, usually late summer or early fall depending on where you live. A small number of brood cells provide few places for the mites to breed, so most of them are riding around on the backs of bees. These adult mites are exposed to the mite treatment and are killed by it.
Some colonies have virtually no brood for a short period in late fall, so if you can time your treatments right, you can get about a 95% kill rate. Some mites will survive, of course, and others will ride into the hive on visiting bees, but for the moment at least you have very few mites.
Hygienic behavior plays a part
All bees have some hygienic behavior, and bees can and do rid themselves of some adult mites and even pull infected brood out of cells and dispose of them. If you have two colonies—one small and one large—and both have nearly zero brood for that brief period—the large colony will have many fewer mites/bee than the small one. This gives the large hive a distinct advantage. Although both hives will experience an increase in mites subsequent to the treatment, the large colony has more bees to deal with each individual mite as they go into winter.
Although a large colony has a larger brood nest than a small colony, large colonies tend to have a smaller proportion of brood to adult bees. This is probably due to the fact that the colony is so large—enough bees to keep warm, enough bees to defend the hive, enough bees for hive duties, and certainly enough mouths to feed—that there is no compelling reason to make it larger or even to keep it as large. If the number of brood cells per adult bee stays low for an extended period, the winter cluster is in a better position to keep mite numbers down using only standard amounts of hygienic behavior. No matter what the task, large colonies nearly always out-perform small ones.
Can mites drown in a gene pool?
Related to this is simple genetics. A large population has a bigger pool of genetic traits, so in a large colony there is a greater probability that there are some bees that can successfully deal with mites. Remember that although the bees in a colony have the same mother, they have a variety of fathers, so there can be quite a bit of genetic variation. More bees mean more genetic variation and a higher probability that some bees will have good hygienic behavior.
During the winter neither small nor large hives have drone brood, which means mite built-up over winter is slower than in spring and summer. The reduced rate of mite build up allows the larger colony to maintain its advantage over the smaller one until colony expansion and drone production begin in the spring.
The ratio is key
Once spring expansion begins, however, the large colony will produce mites like crazy—lots of brood and especially lots of drones. If left alone at this time, mites will begin to overwhelm the colony. But by this time, the large colony can be split, re-queened, treated, or whatever the beekeeper prefers. The point is, the colony made it through the winter because of a high bee to mite ratio at the critical time.
In this scenario, it is always the ratios at specific times of year that are important, never the specific numbers. The ratio of mites to bees and the ratio of bees to brood during the fall and winter are what tip the balance in favor of the large colony surviving until spring.