Mermithids of terrestrial insects have a broader host range than those that parasitize aquatic insects. Because of the difficulty in manipulating mermithids in the laboratory, much of what we know about host specificity is derived from observations of naturally infected insects. Mermis nigrescens is a parasite of grasshoppers but has also been reported from Dermaptera, Coleoptera, and Lepidoptera and is known to parasitize 61 species of insects (Poinar, 1979). Agamermis decaudata is principally a parasite of grasshoppers of the Acrididae and Tettigoniidae families but has also been collected from at least one coleopteran; and F. leipsandra has been reported from seven species in five genera of coleopterans and lepidopterans (Poinar, 1979).

Host range is more restricted in aquatic mermithids, often to a single or a few species of closely related hosts. Specificity of blackfly mermithids is difficult to assess because of systematics problems within the parasite and host groups. Host lists give the impression that blackfly mermithids have a broad host range. However, it is likely that several species of parasites are involved (Molloy, 1981). On several occasions one species of blackfly has been reported infected in a stream while others remain uninfected or parasitized at substantially reduced levels (Colbo and Porter, 1980; Molloy and Jamnback, 1975). Blackfly mermithids have not been reported from other insects inhabiting the same stream. Willis (1971) reported an Amphimermis sp. in naiads of two species of damselflies while at least one species of damselfly and several species of dragonflies were not parasitized. Similarly, Poinar and Petersen (1978) reported Drilomermis leioderma in larvae of the beetle Cybister fimbriolatus while all other insects in the habitat were free of this parasite; a mosquito mermithid also present in the same pool was not found in other insect species.

The most extensive host specificity studies have been made with mosquito mermithids. With the exception of two species of Chaoboridae (closely related to mosquitoes) mermithids parasitic in mosquitoes have not been found parasitizing other organisms in nature (Galloway and Brust, 1979).

In the laboratory, R. culicivorax develops in most species of Culicidae. In tests against 87 species of mosquitoes in 13 genera, susceptibility was generally highest in Anopheles, but at least one anopheline species was highly refractory and another moderately so; both exhibited the ability to encapsulate and melanize the nematode. Similar patterns of individual species showing some form of refractiveness were seen for the Aedes, Culex, and Psorophora species. Anopheles sinensis was found to be highly susceptible to Romanomermisjingdeensis, a mosquito species that is completely refractory to R. culicivorax; further, R. jingdeensis failed to produce significant levels of parasitism in Cx. q inquefasciatus, a highly susceptible host of R. culicivorax (Xinshi et al., 1983). These data are sufficient to demonstrate the complexity of mermithid-host interactions and the need to know these interactions if effective use of a mermithid species is to be accomplished.

In tests against nontarget organisms, R. culicivorax was able to penetrate early instars of some Chaoboridae, Chironomidae, and Simuliidae but failed to complete development. No other organisms were found to be susceptible to attack even under very heavy challenges (Ignoffo et al., 1973). Considerable research has been done with R. culicivorax as a possible biological control agent for blackflies (Finney, 1975; Poinar et al., 1979a). Though early-instar simuliids could be infected under laboratory conditions, successful parasite development could not be demonstrated. Romanomermis culicivorax has little invasiveness in moving water, and thus, it becomes impractical to attempt the control of an insect that inhabits a running water environment with a parasite adapted to the nonmoving water habitat of mosquitoes (Finney and Mokry, 1980). Though R. culicivorax has a wide range of mosquito hosts, other species parasitic in mosquitoes vary widely in range of suitable hosts. One species, Strelkovimermis peterseni, was found to be generically specific for Anopheles mosquitoes and failed to develop in culicine hosts (Petersen and Chapman, 1970). Further, mermithids that complete their parasitic development only in adult hosts are usually specific for a single host species (P. culicus for Aedes sollicitans, Culicimermis culcivora for Aedes communes, Empidomermis cozii for Anophelesfunestus, and Empidomermis riouxi for Aedes detritus). However, at least one species, Culicimermis schakovii, readily develops in several Aedes species.