Varroa destructor, a small parasitic mite, is a real plague on honey bees which can decimate entire colonies. Identified as one of the causes of the decrease in the number of bees in the world, it is the subject of specific control measures included in the "pollinator plan 2020". By studying the defense behavior of bees against this parasite, researchers from INRAE, CNRS, the University of Rennes 1 and the University of Otago (New Zealand), discovered that the parasitized alveoli emit a specific cocktail of molecules which triggers, in certain bees, a hygienic behavior which consists in piercing and cleaning the contaminated alveoli, thus making it possible to preserve the colony. Their results, published on January 25 in Nature Chemical Biology, open up new perspectives in the fight against the Varroa destructor parasite by developing tests to select resistant colonies for beekeepers.

The Varroa destructor mite is a honey bee parasite native to Asia that has spread almost throughout the world. The honeybees Apis mellifera are particularly sensitive to it: the parasite can cause the loss of colonies in just a few months, and causes significant economic losses for beekeepers. While drug treatments have existed since the 1980s, they have several drawbacks, including the appearance of parasite resistance, adverse effects on bees and the persistence of certain compounds in honey or royal jelly. It has already been observed that some bee colonies are able to survive the mite in the absence of treatment, thus showing resistance to Varroa destructor. To reproduce, this parasite lodges in the alveoli of the hive where the larvae and pupae of bees grow. By analyzing their behavior, the scientists noticed that some bees are able to detect the parasite in the alveoli where it is present. These so-called hygienic bees then open the contaminated alveoli to clean them, sacrificing the developing nymph but making it possible to control the spread of the parasite and to preserve the colony. But how do these bees do to detect the parasite hidden in the alveoli?

To answer this, the researchers analyzed and compared alveoli parasitized and non-parasitized by Varroa varroa. They were able to identify six specific molecules in parasitized alveoli that had never been described in bees. They then synthesized1 these molecules and carried out various behavioral tests to find out whether this cocktail of molecules actually triggered the hygienic behavior of bees.

They first compared the behavior of bees when faced with parasitized alveoli and alveoli where the cocktail of molecules had been injected. In both cases, there was a positive reaction from the bees which cleaned the alveoli. Subsequently, they studied the behavior of different colonies which had different levels of hygiene towards Varroa. In colonies with a high level of hygiene, bees locate and clean the cells containing diseased or dead larvae, which is done less consistently in colonies with average hygiene. The results show that the most hygienic colonies have the strongest reaction to the alveoli containing the cocktail of molecules. In colonies, bees have different roles and functions, and only a part of them have this hygienic behavior of cleaning the cells. In a final test, the scientists compared the behavior of hygienic bees and unhygienic bees in the face of the cocktail of molecules. Result: if all bees are able to detect molecules at the level of their antennae, only hygienic bees will be able to integrate the information centrally in the brain and thus adopt a behavior of cleaning the contaminated alveoli.

The discovery of this cocktail of molecules specific to alveoli contaminated by Varroa destructor opens up new perspectives for beekeepers in the fight against this parasite. This would allow them to be able to identify and select the colonies that will be more resistant to the parasite by studying their reaction to the cocktail of molecules. INRAE ​​and the University of Otago have filed a patent on these molecules and their application. Research is currently underway to develop reliable tests that can be used by beekeepers to select colonies resistant to Varroa destructor.