Distribution and specificity of Mermithidae (Nematoda) infecting Simuliidae (Diptera) in Newfoundland

A number of simuliid populations infected with mermithids were studied to elucidate the nematode's life history patterns and the effect of this on the infected host's distribution. Some of the mermithids were very host specific, while others infected all simuliid species exposed to some degree. An extensive study showed that mermithids infecting Prosimulium mixtum failed to infect Stegopterna mutata which may have been due to the preparasites inability to recognize St. mutata as a potential host, or due to host behaviour, rather than microdistribution or physiology.In mermithid species where the majority complete their parasitic phase and emerge from the larval simuliid, the heaviest infections occur towards the downstream end of the host distribution. On the other hand those mermithid species which emerge only from adult simuliids have a high infection rate in the larval simuliids at the upstream end of host distribution. Infection occurs in the early simuliid instars, primarily first and second, with perhaps some in the older larvae. The rate of infection and the number of larvae per host changed between the time of simuliid hatch and the final host instar at most sites. However, the infection pattern in late instar larvae in any one stream appeared to be relatively stable from year to year.In the present study all mermithid infections of adult hosts were confined to the female fly irrespective of the mermithids life history pattern.