Among all of the things in this world that threaten the survival of honey bee colonies, none is a bigger threat than the parasite known as varroa destructor. Although these mites are small, they can be seen quite easily with the naked eye. They are red to brown in color, about 1/16th inch long and oval shaped. Varroa mites attach themselves to the bodies of adult bees, essentially biting them and living off of their hemolymph, or blood. Varroa mites frequently populate themselves to such high levels within a bee colony that the resulting severe weakening of the bees causes colony collapse from viruses.
When varroa mites first found their way to North America in the 1980's, they quickly wreaked havoc on apis mellifera, in domesticated colonies as well as in feral ones. The feral bee population, being made up mostly of swarms from domesticated colonies, was greatly reduced and commercial beekeepers struggled to deal with the problem as they watched their bees die and their livelihoods fade away. It was during this time that beekeepers began to rely heavily upon chemicals in order to control varroa mite populations and keep their colonies alive. Fluvalinate (Apistan) was one of the first such chemicals used, followed by coumaphos (a toxic nerve agent used in serin nerve gas), contained in products such as Check Mite+. Different and sometimes more toxic chemicals have been required as time has passed due to the fact that some mites have built resistance to certain chemicals, rendering some products ineffective.
Unlike the pests we talked about previously, varroa mites are not usually kept in check by strong colonies. In fact, strong colonies often allow varroa to thrive because strong colonies usually have lots of brood being raised and it is in the capped brood cells that the varroa mites breed and multiply. Before we discuss varroa mites any further, please take about 11 minutes to watch this very informative video.
So...now that you are thoroughly disgusted with these nasty little creatures, let's discuss what we've learned about them by having watched the video. First, we've learned that mites are born and bred inside of a capped brood cell which is at the same time, of course, occupied by a developing bee. This means that in order for the mites to survive and thrive by successfully birthing and breeding, there must be ample space inside of the cell for them to accomplish this. Knowing this, it stands to reason that adult varroa mites would find drone cells, which are considerably larger than worker cells, much more attractive for their purposes. Indeed, this is exactly what scientists have found. Because drone cells are quite large (about 6.6mm), they not only give the mites lots of room to work but also remain capped for the longest amount of time, which gives the mites 2-3 extra days to mature before the bee emerges. Scientists have verified that adult female varroa mites use the pheromone produced by drone larvae to identify and occupy these cells just before they are capped by the adult workers. Because of this, varroa mites can nearly always be found occupying drone cells before they are found occupying worker cells.
Another thing we learned from the video is that, on average, the reproductive cycle of a varroa mite produces only 1 new mite. This is a relatively poor survival rate for new mites and of these surviving mites, some are males (which will not mate again) and some are virgin females (which will never mate). The fact that there are low survival/breeding success rates among new varroa mites is promising because it suggests that it may be possible to reduce those rates even further; possibly to near zero.
Nature's Delicate Balance
Let's take a few minutes to reflect on what we know about parasites in general. There are thousands of known parasites in the world today feeding on everything from plants, to fish, mammals of all types; even humans. The thing about parasites is that since they are totally dependent on their host for survival, killing the host is certainly undesirable. Parasites and their hosts often survive together indefinitely; that is the balance of nature. We must then ask ourselves why the parasitic mite varroa destructor, which can only survive in a honeybee colony and nowhere else, would overpopulate a hive to the point of killing all of the bees and consequently, itself? A parasite which kills its host and causes its own demise certainly seems to go against what we know about this natural balancing act between parasite and host. The most likely answer to this question is the same as the answer to so many similar questions about why nature isn't working. It's that man has intervened in nature itself, upsetting the balance on which survival relies.
The first way that man's intervention in nature has brought on the varroa mite disaster is obvious. It is almost certainly due to world trade that varroa are even here in the first place. However, this does not explain why the honeybees in North America have had such a difficult time coexisting with them. Eastern European honeybees have survived mite infestations for centuries, as have African honeybees. Some of this is undoubtedly due to a different hygienic behavior, brought on by evolution and exposure to them. But as all honeybees exhibit significant hygienic behavior, is there possibly some other reason for our bees' failure to cope? Many believe there is. Continue to Cell Size and Regression to learn more.