The efficacy of melanotic encapsulation as a defense against parasite-induced mortality in snowpool mosquitoes (Diptera: Culicidae) infected by mermithid nematodes

Abstract

The effects of host reactions to parasitism were studied in two species of mosquitoes (Diptera: Culicidae) infected with mermithid nematodes (Romanomermis sp.). No immune reactions were observed in Aedes impiger and virtually all (98.5%) infected larvae died before pupation. Mermithid infections of Aedes nevadensis resulted in high host mortalities (99.3% failed to pupate) when none of the parasites were encapsulated. Melanotic encapsulations of the nematodes were found in 67% of infected Aedes nevadensis, and larvae of this species that encapsulated all of their parasites had the same rate of survival as uninfected conspecifics. Survivorship of Aedes nevadensis larvae that encapsulated some but not all of their Romanomermis parasites was intermediate. Encapsulation also suppressed parasite-induced reductions in host size, and larvae that encapsulated all parasites were significantly larger than other Aedes nevadensis. This is the first quantitative demonstration of the efficacy of mosquito immune responses as defenses against mermithid parasites.