Studies on the bionomics of mermithid nematode parasites of blackflies in Newfoundland

Neomesomermis flumenalis is a univoltine nematode infecting the simuliids Prosimulium fuscum/mixtum during the winter months and the first (early spring) generations of the summer simuliids, primarily Simulium venustum, in May and early June. Infection of simuliid larvae occurred when water temperatures were 4–12 °C. Data extrapolated from late-instar larvae and pupae indicated that less than 0.03% and 0.06% of the P. fuscum/mixtum and S. venustum adults, respectively, were infected with this parasite, which is primarily a parasite of larval simuliids. Adult simuliids were not considered to be the source of the next generation of mermithids. The apparent host specificity of N. flumenalis is attributed to the close synchronization of the parasite life cycle with the life cycle of those simuliid species which overwinter in the larval stage and develop at temperatures less than 12 °C; temperatures above this level tend to be lethal to the preparasitic mermithids and simuliid species developing in the summer are thus not infected by this species. Male mermithids were more numerous than females in the Prosimulium populations but females outnumbered male mermithids in the S. venustum populations. When only a single mermithid parasitized a simuliid larva, the nematode was always a female; when more than one nematode parasitized a single larva, the nematodes were usually males.