13 January 2024

It’s quite frequent to hear reports from beekeepers who credibly claim to have done a proper summer treatment but find extremely high numbers of varroa mites after a winter treatment. Or even damaged colonies from the mite. That’s usually due to varroa reinfestation. This happens by drifting of workers from highly infested colonies or by robbing. In the latter case, workers from strong colonies enter weaker colonies to rob their stores. In both cases, varroa mites from one colony get into another one. Re-infestation it’s called because the mites come back (which is said by the prefix re-) after a treatment.

High colony density is a risk factor for reinfestation. And by the way, this is also the reason why I sometimes spend quite a bit of time searching for a good apiary site in clinical studies. It’s another reason to coordinate treatments in regions with many beekeepers – besides avoiding resistance against varroa treatments.

The effect of varroa reinfestation

How many mites can enter honey bee colonies by such reinfestation events wasn’t clear for quite a while. My colleague Eva Frey worked on this during her doctoral thesis. She worked on a quite unusual place: A military training area. This area isn’t accessible to the public usually, so it was isolated from other beekeepers. It’s also a quite vast area, so ideal to study varroa reinfestation in isolated conditions.

She placed four heavily infested “mite donors” in this area. Then they put 10 “receiver” colonies in 1-1.5km distance. The receivers were continuously treated against varroa to monitor the incoming varroa mites. From August to October, she assessed the colony strength of the receivers every three weeks and recorded the invading varroa mites every 7-12 days. All this under difficult circumstances: She had to call when she wanted to enter the area and ask for permission. She wasn’t allowed to move outside some designated roads to access the colonies. Obviously for her own safety – it was a military training area after all. An active one.

During these two months, 85 to 444 mites made it from the donors to the receivers. This may not seem a lot. But consider that these mites enter brood cells and still reproduce in late summer and autumn. In addition, remember that the infestation doubles every month as long as there is brood. So, this number would have increased if these receiver colonies hadn’t been treated continuously. The other interesting finding was to see that 1-1.5km of distance didn’t protect the receiver colonies. So, your neighbours, even if they’re not immediately next door, can give… some naughty “present”.

Too many honey bee colonies?

I already mentioned that colony density in an area can favour varroa reinfestation. Eva also studied this in more detail. This time, she placed honey bee colonies in two areas, 14 at each site. In one area, there was a high colony density, in the other this density was lower. However, these were more realistic management conditions than found in the military training area, for sure. At each apiary, she treated half of the colonies continuously, like in the study before. The other half was treated before the experiment and then left untreated until its end.

From end of July to November – this time without the risk of being shot – she monitored the colonies. Again, she monitored the colonies regularly during this period. In contrast to the distance in the previous study, the number of neighbouring colonies had a significant effect on the invasion rates of mites. At the low density apiary, 72 to 248 mites entered the colonies over the 3.5 months of the study. This was significantly lower than the 226 to 1,171 mites at the high density apiary… Again, think of the doubling each month as long as there’s still brood. We can get in trouble at these places, even if you treat correctly.

By the way, this kind of experiment, with a simple design but highly informative, is what we call “elegant” in science.

Drifters or robbers?

Let me finish with another study which looked at the main cause for this phenomenon. As I said, both drifting and robbing can cause reinfestation. In this study, the researchers placed six colonies with very low infestation around a cluster of highly infested colonies. They then observed the movement of the bees between the colonies and their mite levels. The highly infested colonies collapsed during the study and they could see that this was the moment when the mites began to arrive in the low infested colonies. Therefore, robbing seems to be the more important cause for reinfestation than drifting.

So, why is this important for practical beekeeping? Well, because varroa reinfestation can hit even those who treat correctly, as I already mentioned at the beginning. It’s important to understand that good practices, in this case treating against varroa, affect also others. Try to coordinate the treatments with your neighbours. And if the “don’t believe” that varroa is a thing (still happens…), try to find another apiary. Varroa reinfestation is also one of the reasons why I’m not that convinced about resistance breeding. But that’s for next time.

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