Mermithids generally have localized, discontiguous distribution. This is especially true for mermithids that develop in larval hosts. Romanomermis culicivorax was found in only seven pools out of several thousand examined in southwestern Louisiana. This is understandable because the hosts are killed prior to emergence from the habitat, thus greatly reducing the nematodes ability to disperse.

Generally, observations of mermithid parasitism are based on isolated collections and often give the impression of higher parasite activity than actually exists. In extended studies, levels of parasitism are usually found to vary greatly. In a 27-month survey of five habitats containing populations of R. culicivorax, mean parasitism of Anopheles crucians for individual habitats ranged from 8 to 42%, with parasitism of individual populations as high as 80%. Similar but lower levels of parasitism were observed for several other mosquito species (Petersen and Willis, 1971). Similarly, Phelps and De Foliart (1964) observed parasitism of blackflies, Simulium vittatum, in four Wisconsin streams over a 1-year period. Parasitism typically ranged from 10 to 90% and the authors suggested that localized parasite populations probably attained levels of parasitism sufficient to virtually eliminate some host populations. Similar population reductions of blackflies were reported by Welch and Rubtsov (1965).

Parasitism of adult aquatic insects usually occurs over a wider geographical area and often reaches very high levels but fluctuates greatly over time. Steiner (1924) observed 80% parasitism of adult Ae. vexans in British Columbia in 1920, but only 20% the following year. Trpis et al. (1968) reported 100% parasitism of the same host in the same locality in 1967. Similar observations were made on parasitism of Ae. sollicitans adults by P. culicis in southwestern Lousiana. During an entire year only collections made in January failed to produce infected hosts; mean monthly parasitism ranged from 0 to 48% with individual populations reaching 96%. During a period of high parasitism, a survey was made along 80 km of coastline and revealed parasitism ranging from 48 to 94% for seven populations (Petersen et al., 1967).

Similar isolated high levels of parasitism are frequently observed in terrestrial insects. Christie (1936) reported a heavy outbreak of grasshoppers in Wisconsin in 1923-1925 with a correspondingly heavy infestation of M. nigrescens. The author concluded that the parasites were an important factor in terminating the grasshopper problem. Similarly, Mongkolkiti and Hosford (1971) reported that a population of the grasshopper Hesperotettix viridis pratensis was totally destroyed prior to egg laying by M. nigrescens. Other species of grasshoppers in this population were only partially affected or not affected by the mermithid.

Though reports of natural insect control are impressive and suggest the potential of mermithids as biological control agents, these parasites generally do not influence significant control over host populations. In a review of blackfly-mermithid literature, Molloy (1981) reported that parasitism in most blackfly populations is moderate, ranging from 3 to 15%, and is perennial with only rare and highly localized epizootics. This pattern probably holds true for most mermithid-host associations.