Since the early 2000’s, there have been alarming reports on significant annual losses, due to multiple stressors, in European honey bee populations. These include the extensive use of pesticides on agricultural land, the loss of natural food resources and aggressions by viruses and parasites. Amongst these, the varroa mite, which is native to Asia, has parasitized European honey bees for about four decades. Selection towards varroa-resistant honeybees is a strategy towards healthy colonies that could mimic the observation of colonies surviving infestation in natural conditions. "One of the ways to infer colony resistance is to look for reproduction failure of the varroa mite ," explains Sonia Eynard, a postdoctoral researcher in the GenPhySE unit (INRAE, ENVT, INP-ENSAT).

First of all, we must understand…

In order to build an effective targeted selection program, the resistance mechanism must be well understood and accurately measured. Several mechanisms contribute to reproduction failure: poor fertility of the parasite, inhibition of the varroa breeding cycle by the larvae, and/or adult bee behaviour consisting in specifically cleaning the varroa infested brood cells. With the Plant Health and Environment team of INRAE Avignon, Sonia Eynard evaluated the repeatability and variability of a measure of reproduction failure called MNR (Mite Non Reproduction), estimated by dissecting brood cells, and compared it to other estimates of resistance, such as the targeted cleaning behaviour VSH (Varroa Sensitive Hygiene). However, they were unable to establish a correlation between MNR and VSH. This evaluation of varroa resistance measurement methods highlights the complexity of this trait, with all that this implies in terms of selection difficulties.

... for a successful selection

Although resistance to varroa infestation is an interesting selection target, issues concerning the variability and repeatability of MNR measurements, especially when performed under field conditions, suggest this trait to be used with caution.

Further research on varroa resistance is ongoing, particularly within the BeeStrong project, whose aim is to identify genetic markers towards resistance, in order to provide beekeepers with an efficient and pragmatic selection tool.