Comparative Susceptibility of Larval Mosquitoes Exposed Separately by Instar or in Mixed Populations to the Nematode Romanomermis culicivorax.

The mermithid nematode Romanomeris culicivorax has been released many times to control natural populations of larval mosquitoes. Generally, infection levels resulting from such releases were highest in secondand third-instar and lowest in first-instar hosts when all instars were present in the habitat (1,3). This contrasts sharply with well.established results from laboratory studies which showed that firstand second-instar hosts were highly susceptible to infection, third-instar hosts about half as susceptible, and fourth-instar hosts only slightly susceptible (2). The apparently reduced incidence of parasitism of first-instar hosts in field trials was attributed, in part, to continuous hatching of new mosquito larvae after the numbers of infective parasites declined (2). Additionally, it was reported in the 1977 Annual Report of the Vector Control Research Centre, Pondicherry (Indian Council of Medical Research 1978), that early instar larvae in mixed populations were less susceptible than older larvae to infection by Roman. omermis iyengari. Therefore, laboratory