Varroa, king of cuticular mimicry

The adaptive strategies of imitation make mimicry a model of a complex coevolutionary mechanism involving a first species acting as a model (e.g. Apis cerana) and a second, imitating species (e.g. Varroa destructor), very often parasitic. Varroa destructor, a parasitic mite of bees, has the ability to mimic the chemical composition of its host’s cuticle; moreover, it is also capable of modifying this composition according to the species it parasitizes.

Researchers from the Institute for Insect Biology (CNRS/University François Rabelais of Tours) and the “Abeilles et environnement” laboratory of INRA, in collaboration with American and Chinese colleagues, have demonstrated that Varroa destructor, a parasitic mite of bees capable of mimicking the chemical composition of its host’s cuticle, is also able to modify this composition according to the species it parasitizes. This remarkable adaptive capacity could explain how this parasite of the Asian honey bee was able to colonize the European honey bee during the twentieth century, thereby contributing to the species’ decline.

The mite Varroa destructor is an ectoparasite of the domestic honey bee, meaning that it lives on its surface. It causes numerous health problems within colonies. It enters brood cells containing bee larvae and feeds on their hemolymph. It also parasitizes pupae and adult bees, contributing to the decline currently observed in this species and causing significant economic losses in beekeeping. The original host of this mite is Apis cerana, the Asian honey bee; however, it has become a serious threat to the European honey bee (Apis mellifera), which it began to parasitize in the 1940s–1950s and which is less resistant to its attacks. Asian honey bees indeed display behaviors (grooming of adults and inspection of larvae by workers) that allow them to detect and eliminate the parasite. These behaviors are less common in honey bees (Apis mellifera, which convert nectar into honey), and without chemical treatment their colonies die within two to three years.

The cuticle, that is, the external envelope of the bee, is composed of a mixture of about fifty lipid compounds—hydrocarbons—that serve, among other functions, in chemical communication. Bees are able to recognize the composition of a cuticle and thus identify the species or age of an individual. This also allows them to detect the presence of parasites, whose cuticle differs. Previous studies have nevertheless shown that the mite Varroa destructor can mimic the cuticular hydrocarbons of its host and thereby evade the hygienic behavior of bees.

In these new studies, researchers from the Institute for Insect Biology (CNRS, University François Rabelais of Tours) and the “Abeilles et environnement” laboratory of INRA, in collaboration with American and Chinese colleagues, investigated the ability of mites, depending on their origin, to mimic the cuticle composition of a new host of a different species. To this end, mites living in colonies of Asian honey bees were transferred onto larvae of European honey bees, and vice versa.

Varroa destructor and chemical mimicry

Their results, published in the journal Biology Letters, show that the parasites are able to imitate both hosts, even when they are artificially transferred. Indeed, the proportions of cuticular hydrocarbons in the mites change after transfer in order to mimic the cuticle of their new host. Chemical mimicry is therefore maintained, and this remarkable adaptive capacity could explain how this parasite of the Asian honey bee was able to colonize the European honey bee.

Analysis of the cuticles also revealed that mites originating from colonies of Asian honey bees are better mimics than those originating from European honey bees. The long coevolution between Varroa destructor and Apis cerana appears to have enabled the mites to become more effective in their chemical mimicry, while Asian honey bees developed behaviors better adapted to combating the parasite. Conversely, the relatively recent host shift of the mite to Apis mellifera explains why the European honey bee has difficulty detecting the parasite. This host–parasite system thus provides a clear illustration of the “arms race” engaged in by two organisms over the course of their shared evolutionary history.