Possible regulation of the reproduction of the honey bee mite Varroa jacobsoni (Mesostigmata: Acari) by a host's hormone: Juvenile hormone III

With its first haemolymph meal from honey bee larvae the parasitic mite Varroa jacobsoni obtains different amounts of juvenile hormone III depending on the species and sex of the larva. Drone larvae of both Apis mellifera and A. cerana contain more than 5 ng/ml juvenile hormone in the haemolymph during the first 60 h after brood cell capping. Worker brood of A. mellifera contains 3–7 ng/ml whereas A. cerana does not reach 1 ng/ml during the first day after sealing. Juvenile hormone concentrations lower than 4 ng/ml seem to be one factor to prevent reproduction of the mites. Varroa, having parasitized on winter bees (low juvenile hormone titre) show 8% reproduction after beeing transfered to suitable brood cells. Mites from old summer bees (high juvenile hormone titre) show 76% reproduction. Varroa from winter bees transfered to larvae which were treated with juvenile hormone show 18%, and mites parasitizing winter bees sprayed with juvenile hormone showed 39% reproduction. Juvenile hormone effects could be demonstrated down to the cellular level in oögenesis. Our results suggest a model for a 2-fold influence of host juvenile hormone on Varroa: The first influence on the adult bee which might be necessary for the cell-entering behaviour and preparation of the ovary. The second one on the bee larva obviously triggers yolk incorporation into the eggs of the mite and induces oviposition.