This graph shows the last two years of varroa mite tests for one of my hives. It was doing pretty great until the end of September, when the percentage of mites-to-bees exploded—going from under 1% to over 8% in less than one month. This demonstrates a couple things. First, you’ve got to be testing your hives for mites throughout the year. As long as your bees can fly, you need to monitor their mite loads. Too many people do a couple OAV treatments, or apply Apiguard, MAQS, etc. after pulling honey supers, and then assume they’re all set for the winter. But this is clearly not the case. If I’d have ignored this colony after the sub-1% test on September 7th, it would almost certainly die this winter. And in the spring I’d be saying, “It wasn’t mites that killed them, because the mite load was very low going into winter.” But it was the mites that killed them, despite my genuine belief to the contrary.
The second thing that jumps out about this graph is what appears to be a mite bomb. “Mite bomb” is a colloquial term referring to a honey bee colony that has been overwhelmed by varroa mites. The colony is collapsing from disease and becomes vulnerable to robbing. A stronger colony comes in and robs the dying colony’s food stores, taking that honey back to their hive. Except they don’t just take honey—during the robbing varroa mites are busy crawling onto the robber bees, hitching rides to the invaders’ hive.
On the upside, fall wildflowers (including these white asters) are still blooming, providing both nectar and pollen
So to determine whether this was a mite bomb or just an explosion of this hive’s internal mite population, I get to try some math. On September 7th, I had an ectoparasitic (phoretic) mite load of 0.94%. Assuming a worst-case scenario with the maximum amount of brood in the hive, the colony would have a true mite load of about 1.70% (this ballpark number includes mites under capped cells). Using the r-value published by Randy Oliver of .021 (varroa birth rate minus death rate), and assuming the hive’s population dropped from 40,000 to 25,000 between September 7th and October 5th (which is a stretch based on my observations), the expected mite load at the end of the period is 4.87%. That’s about half of what actually happened, which leads me to believe this spike in mites was caused by a mite bomb. Of course, I won’t rule out that my numbers are off—but to make my amateur math work in favor of a purely internal mite explosion, the population on October 5th would have to be just 15,000 bees, and that was not the case with this particular hive. Another piece of anecdotal evidence is that this was a huge deviation from all of my other hives—none of the others, even the similarly strong ones, had such a sharp increase in mite loads over such a short period of time.
Some people claim that mite bombs aren’t a real thing. They “prove” this by reminding us that bees die from parasitic mite syndrome (PMS) during times of the year when it’s too cold to fly, so they can’t possibly be robbing other colonies. But I would point out that the colony in the graph above is still very strong right now, and I have no doubt they would make it deep into winter without any intervention on my part. So there could be months between a mite bomb and a colony’s ultimate demise. Unless I’m missing something, various arguments about cold climates disproving the existence of mite bombs don’t hold up.
More October flowers, including cosmos sensation and nearly-spent goldenrod
To be fair, the mite bomb in this example could very well be my fault. It may have been a swarm I lost in the last 2-3 years. By now, those swarms, after making a home in the forests surrounding my bee yard, are probably succumbing to mites. Those dying feral colonies are easy pickin’ for my strong colonies, and as a result the mites get to move into my apiary and live on by terrorizing another hive.
So what do I do with this hive? I treated it with oxalic acid vaporization last weekend, and I’ll be sugar rolling it tomorrow to get a fresh gauge on the mite load. I’m sure the mites will still be too high, and I’ll OAV it again. This’ll probably continue for the next few weeks until the mites are under control. Then I just cross my fingers that the winter crop of bees didn’t suffer too much damage to successfully survive until next spring.