Abstract: Nematodes of the Mermithidae family are obligate endo-entomoparasitic of arthropods as insects. These nematodes are appealing for biocontrol due to their tendency to infect a large percentage of host populations. Mermithids have a simple life cycle consisting of 4 molts. Specifically, these nematodes hatch as infective juveniles (the only parasitic stage), who swims vigorously and hunt for an appropriate host and spend few days inside their host. The juveniles complete their development into postparasites while inside the host, and emerge from the host to dig into the floors of these mosquito pools changing to a free-living post-parasitic stage where they undergo the final two molts. The adult stages who is also a curious one, as the chance of finding a mate should be difficult in aquatic environment, mate, and lay eggs. For many years, mermithids have received attention for their usefulness as biological alternatives for the chemical insecticides, due to their potential killing efficacy, ease of rearing in mass quantities and their wide but specific host range. More importantly, these organisms are very safe to the environment and to non-target organisms, and no known resistance has developed against these parasites. Romanomermis and Strelkovimermis are mermithid nematodes that parasitize mosquito larvae. The behavioral and biological relationships of these mermithids with Culex pipiens were investigated. Particularly, differences in host penetration, emergence patterns, search behaviors for the host/mates, and population regulation are described. Results indicated that both nematodes, show different entrance and emergence sites. searching behavior has been shown to be directed, as they can detect the presence of their hosts and change their spatial distribution to attack them. Also, as parasite load increased, host heart rate and nematode survival decreased significantly, and both show discriminating against previously infected hosts. Species differences could be useful in developing production and release strategies.
