Semiochemicals influencing the host-finding behaviour of Varroa destructor.

Studies of Varroa destructor orientation to honey bees were undertaken to isolate discrete chemical compounds that elicit host-finding activity. Petri dish bioassays were used to study cues that evoked invasion behaviour into simulated brood cells and a Y-tube olfactometer was used to evaluate varroa orientation to olfactory volatiles. In Petri dish bioassays, mites were highly attracted to live L5 worker larvae and to live and freshly freeze-killed nurse bees. Olfactometer bioassays indicated olfactory orientation to the same type of hosts, however mites were not attracted to the odour produced by live pollen foragers. The odour of forager hexane extracts also interfered with the ability of mites to localize and infest a restrained nurse bee host. Varroa mites oriented to the odour produced by newly emerged bees (<16 h old) when choosing against a clean airstream, however in choices between the odours of newly emerged workers and nurses, mites readily oriented to nurses when newly emerged workers were <3 h old. The odour produced by newly emerged workers 18-20 h of age was equally as attractive to mites as that of nurse bees, suggesting a changing profile of volatiles is produced as newly emerged workers age. Through fractionation and isolation of active components of nurse bee-derived solvent washes, two honey bee Nasonov pheromone components, geraniol and nerolic acid, were shown to confuse mite orientation. We suggest that V. destructor may detect relative concentrations of these compounds in order to discriminate between adult bee hosts, and preferentially parasitize nurse bees over older workers in honey bee colonies. The volatile profile of newly emerged worker bees also may serve as an initial stimulus for mites to disperse before being guided by allomonal cues produced by older workers to locate nurses. Fatty acid esters, previously identified as putative kairomones for varroa, proved to be inactive in both types of bioassays.