We examined manipulation of mosquito behavior by the parasitic mermithid nematode, Strelkovimermis spiculatus. This nematode species typically infects early instar host larvae and emerges after parasitic development to kill last-instar larvae. Parasitized adults, however, have occasionally been reported from field collections. We obtained low rates (1.7 to 11.5%) of parasitized adults in laboratory exposures only when Culex pipiens pipiens fourth-instar larvae nearing pupation were exposed to infective nematodes. This did not allow an adequate interval for parasitic development in immature host stages. Parasitized adult females in a multiple-choice assay were three times more likely to seek water than a blood source (63.1 vs. 20.5%), whereas uninfected females were twice as likely to seek blood than water (64%3.9 vs. 32.6%). This altered host behavior benefits the parasite by providing the only mechanism for dispersal and colonization of new host habitats while concurrently avoiding risks from the defensive behaviors associated with blood-feeding. Behavioral alternation in Cx. p. pipiens larval hosts was also 35 examined using larvae infected as second instars to allow for a normal duration of parasitic 36 development. As larvae neared pupation and parasite emergence, parasitized larvae became more spatially aggregated than unparasitized larvae. This altered host behavior benefits the parasite by providing a corresponding increase in post-parasite aggregation, which facilitates formation of large mating clusters and concomitantly reproductive success. Parasites derive fitness gains by overriding host autonomy, whereas hosts have zero fitness once parasitism is established, suggesting a coevolutionary response is inoperative and that the behavioral modifications may be adaptive.

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