Factors for reinvasion

Robbing

Several years ago, it was common that there were always noticeable, even many bees flying around the ventilation and the door to the extraction room. It has become less and less bees every year. This year there have been virtually none. However wasps have been in abundance. Even though I often walked through the door in July and extracted some of the honey then. And it was no honey flow then.
Last year’s breeding queen no 1 gave many new queens working the colonies this year. That has apparently had a big effect this year. Last year, I used a lot of thymol as wingless bees appeared in many colonies. This year I barely needed to use any thymol at all. I find it hard to draw any other conclusion than that my bees have become more resistant to varroa.
However, there are a few colonies in some apiaries which are close to other Beekeepers bees where the mite level suddenly increased quite some in late July/August onwards for a few weeks. It seems to indicate so-called silent robbery from my colonies from hives that are affected by larger amounts of mites so that they do not maintain a good entrance defense.
Then when I was told about the problems some beekeepers had in Finland with imported Italian bees when it comes to how quick they were to rob and as often as these colonies must be treated against Varroa, I made a connection here.
Bees that are not inclined to rob have less problems with reinvasion of mites. Bees that have less problems with reinvasion can be called more resistant to Varroa.

Drifting

Another factor that is important for reinvasion is the tendency to return to wrong hive, to drift. I have sold a few queens to countries further south in Europe. I got some feedback recently from a beekeeper in Germany who this year introduced two Elgon queens with differing color, one light and one dark. (One of the features I do not attach great importance to is the color and therefore it can vary a lot.)
Her carniolan bees drift a lot, she has noted. Now she split a hive in two and put the two parts very close together with the entrances adjacent. She noticed that Elgon colony with striped bees contains only striped bees. The hive with without beige striped bees, had only dark bees, no bees with beige stripes. None of these colonies’ bees thus drifted to the neighbor.
It seems that part of the Elgon bees’ better resistance can be explained that they are less inclined to rob and that they drift less.