Conditional mutualism between Allodynerus delphinalis (Hymenoptera: Vespidae) and Ensliniella parasitica (Astigmata: Winterschmidtiidae) may determine maximum parasitic mite infestation.

Mutualism is a prominent interaction within ecosystems, yet most may actually be conditional. The symbiotic mite, Ensliniella parasitica Vitzthum, ingests the hemolymph of juvenile potter wasps, Allodynerus delphinalis (Giraud), but also protects them from a natural enemy, Melittobia acasta, and is transported to new nests in host pockets specialized for this purpose (i.e., acarinaria). Thus, two different antagonisms from the mite may arise: commensalistic cheating is expected without the natural enemy of the host, and parasitism is predicted with excessive numbers of the mite. However, facultative parasitism mediated by mutualism has rarely been studied in any organism. We found no significant differences in juvenile mortality, nesting rate, or fecundity between mite-free and naturally mite-laden juveniles. However, when overloaded with mites ( approximately 1.5-2.5 times more mites than the maximum number per wasp larva in the field), the developmental period of the male wasp was significantly delayed, and juvenile wasp mortality increased to 30%. These results show that mutualism mediated by parasitism may revert to parasitism, suggesting that either or both organisms in a mutualism mediated by parasitism need population control of the parasite to avoid the risk of parasitism.