First reported in Europe in the 1970’s, the parasitic mite Varroa destructor has since created a lot of damage and is still a big threat for beekeeping. In 2007, the bee research group of Wageningen University started a selection program on 2 groups of colonies kept isolated in Tiengemeten island and Amsterdam Water dunes. The colonies were kept without Varroa treatment and selected on survival, growth and reproduction criteria. Upon stopping treatments against varroa in the 2 groups, the naturally selected colonies rapidly acquired resistance to Varroa mites and are now able to survive without any treatment against the mite.
Therefore it is now known that a resistance building process occurred but whether grooming (adult bees cleaning themselves and each other), inhibition of mite reproduction (through chemicals produced by pupae) or sensitive hygiene (adult bees detecting and removing parasitized pupae) was responsible for it remained an unanswered question.
Varroa sensitive hygiene
In order to estimate the importance of Varroa sensitive hygiene (VSH) on this resistance, the bee research group of Wageningen University set up an experiment that took place during spring and summer 2015. The main hypothesis was that VSH is used as a control mechanism and is at least partly responsible for resistance. To estimate VSH rates, artificial introduction of mites in the brood cells was performed. Potential bias coming from the brood were avoided using neutral brood coming from sister Buckfast colonies, and mites of same origin were used for all introductions (collected in a group of colonies dedicated to mite rearing).
Mites were collected using the powder sugar method, and introduced in freshly capped cells (within 6 hours post capping) by opening the cells with a razor blade (see figures).After introduction of the mites, brood frames were returned to their original colonies to let the bees seal the cells properly. After one day, the accepted cells (manipulated cells recapped by the bees) were marked and the frames were moved to colonies from the tested groups (Water dunes, Tiengemeten and Control). The frames spent the next 7 days in the group colonies and were collected again on the tenth or eleventh day post capping, before emergence of adults (usually on day 12 post capping). The number of removed pupae was assessed and when the pupa was still present, the cell was opened to check if the mite was still in, and whether it had reproduced or not.
The results concerning the Waterdunes group were in line with our hypothesis: 40% of the infested pupae were removed from the brood frame. The results concerning the other selection were quite surprising: higher rates of VSH were found in the control group (24% of the infested pupae removed), that is sensitive to varroa (because it was not selected, and twice per year the a mites were treated), than in the Tiengemeten group (15%), that has been selected and is tolerant or resistant.
However, this varroa resistance in the Tiengemeten group might be explained by another resistance mechanism: inhibition of mite reproduction. In previous experiments, a significant number of non-reproducing mites were found in the colonies of Tiengemeten, as well as a lower number of female offspring Although more trials need to be done, this mechanism could explain the tolerance of colonies from Tiengemeten selection.
It is very interesting to see that two groups of honey bees exposed to high parasite pressure and selected on the same criteria developed two different strategies to control mite population growth in such a short time. These results are very encouraging and stand for a natural selection approach as a relevant strategy on the struggle against Varroa mite.