Varroa mites multiply in bee larvae. After they come out of the cell when the bee is fully formed, they sit on the adult bees and suck hemolymph.
It was observed many years ago that during the brood period of the bees, 2/3 of the mites was found in the capped bee brood cells while 1/3 was on the bees.
If the mites had been sitting longer time on the bees than they did, before they returned into a brood cell, a greater proportion than 1/3 had been found on the bees. If they had been sitting less time there would have been a smaller proportion found on adult bees. The shorter the time the varroa mites are sitting on adult bees, the faster they return into a new brood cell to reproduce. This would increase the speed of varroa reproduction in the bee colony.
It is thus from the beekeeper’s and the bee colony’s point of view desirable that the mites are sitting as long as possible on the bees, resulting in a slower development of the varroa population. So, if the proportion of mites had been ½ on the bees and ½ in capped brood, this would have been better than that found for a number of years ago when varroa mites had arrived.
In early December 2015, two professional beekeepers from the Spanish mainland came to the small island of La Palma, one of the Canary Islands, and lectured on the varroa problem (http://archiv.resistantbees.com/phoretische-varroen). One of them was Manuel Izquierdo Garcia, a biologist at the University of Seville. (Thanks Rüdiger Dietrich who drew my attention to this.)
30 years ago when varroa mites came to Spain, the proportion of mites on the bees was 1/3 and 2/3 in capped brood. During the past 30 years, the mites’ behavior have changed. You could say that during the 30 years of conventional treatment of bees to kill mites, the mites have responded by spending less time on the bees to accelerate their reproduction rate. They have also changed the place on the bees they usually sit, from the abdomen to the middle part of the bee.
The result of this change has resulted in 15% are found on the bees (previously 33%) and 85% in the capped brood (previously 66%).
Mites are sitting shorter time on the adult bees. Thus you find at a given time 15% of the mites on the bees today and 85 % in the brood. This have increased the reproduction rate of the varroa population.
This change has consequences for beekeeping. It explains why we in Europe have had to increase treatment to kill mites. There are examples of recommendations in several countries where the fight starts in spring and continues throughout the season. And anyway, or should one say, maybe sometimes also because of this, the bees have difficulties to survive.
Some types of treatment will also be less effective due to this change. Treating with powdered sugar, only kills the mites sitting on the bees. One must fight very often if powdered sugar should have any effect of relevance.
If there are still small areas of capped brood when one uses oxalic acid against the mites, the oxalic woun’t have the effect one wants. This becomes more relevant when climate change means warmer winters, as it will be more common with brood in winter times, the time when oxalic usually are used. It becomes even more important keeping bees that really have brood-free periods during winter, also for treatment free beekeepers.
Treatment is a dead end
It is becoming increasingly clear that it is a dead end using all kinds of chemicals against varroa mites. And it is with the increased reproduction rate of the mites more difficult to select resistant bees and get areas with treatment free bees – which is the solution.
Focus on varroa resistance
All this show how important it is to focus on producing as varroa resistant bees as possible and develop management methods without chemicals. It is important that all beekeepers understand the problem and are involved at least somewhat.
Every beekeeper can at least try to identify which of his or her bee colonies are the least good in resisting varroa mites and replace the queen(s) in those. The simplest way is to just remove the queen in such a colony and let the bees rear a new of their own. It is not the best method, but a start. Then you can make more steps in improving your bees, depending on interest and opportunities.