Exploitation of cold temperature as defence against parasitoids in bumblebees

PARASITES can modify host behaviour for their own benefit after infection^1-4. Whether these changes also minimize loss of host fitness is less well known^5-7. Although parasitoids are abundant⁸, few cases are known of behavioural changes in host-parasitoid systems⁹. Bumblebees (Bombus spp., Apidae, Hymenoptera) are primitively eusocial insects with an annual life cycle. Reproduction occurs in late summer when males and young queens are released^10,11. Parasitoid conopid flies (Conopidae, Diptera) are abundant in summer, and up to 70% of field-caught bumblebee workers may be parasitized¹². Development of the parasitoid usually takes ^10-12 days and ends with the host's death when the larva pupates. Here we report a novel strategy used against these parasitoids, based on the exploitation of temperature effects on parasite development. We demonstrate that parasitized workers of the bumblebee Bombus terrestris L. stay in the field overnight rather than returning to their nest. This behaviour retards the development of the parasite and reduces the chances of successful development. In choice experiments, parasitized foragers actively seek out cold temperatures. This behaviour therefore enhances colony success by prolonging the life of parasitized workers while at the same time reducing parasitoid fitness.