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To understand the infection strategies of the mosquito parasitic mermithid nematodes Romanomermis iyengari (Welch, 1964) and Strelkovimermis spiculatus (Poinar and Camino, 1986), we have investigated the foraging behaviors of the pre-parasites of both species. Romanomermis iyengari with body length of 0.99 ± 0.071 mm (twice as S. spiculatus 0.55 ± 0.01 mm) swims faster (2.76 ± 0.13 cm/min) than S. spiculatus (1.61 ± 0.09 cm/min). The nematode pre-parasites of R. iyengari showed negative phototaxis in responding to natural light, which caused them to migrate to shade where mosquito larvae tend to aggregate. This behavior helped the nematode to gain significantly higher parasitism when shade was present (37.7 ± 2.74%) than no shade (11.3 ± 0.95%). In contrast, S. spiculatus pre-parasites had no response to natural light and therefore, shade contributed no parasitism gain to this species. The host searching pattern data indicated that the two species used different strategies for host searching. The pre-parasites of R. iyengari tended to do long range search and responded to host aggregation habitat but slowed down and do more localized search when host present. However, the species of S. spiculatus , tended to do localized movement when hosts were absent regardless shade. Once host larvae present in nearby location, the nematodes responded to the host cue and moved faster toward their hosts. The active foraging behavior of R. iyengari may suggest that the species is more suitable for mosquito control in clear and moving water such as streams. However, the passive stay and wait behavior of S. spiculatus might be more suitable against mosquitoes in small sized water bodies such as temporarily flooded or stagnant ponds.

Superparasitism is a common phenomenon in mosquito-parasitic mermithid nematodes. Multiple nematodes are needed in a single host to produce males. Host selection behavior and intraspecific competition among Romanomermis iyengari and Strelkovimermis spiculatus were investigated against their host, Culex pipiens pipiens in laboratory experiments. In a choice assay between previously infected and uninfected host larvae, infectious preparasites of both nematode species could distinguish not only between infected and uninfected hosts, but even between different parasite loads in showing a strong preference for uninfected hosts or hosts with a low parasite load. Host heart rate declined briefly immediately after parasitism. Superparasitism resulted in increased parasite mortality. Scramble competition within mosquito larvae for limited host nutrients, coupled with a skewed sex ratio favoring males, is assumed to lead to parasite population decline and subsequently toward host-parasite population equilibrium. The ability of mermithid preparasites to accurately assess parasite load likely plays an important role in host population dynamics and regulation.

Mating aggregations in the mosquito parasitic nematode, Strelkovimermis spiculatus, were investigated in the laboratory. Female postparasites, through their attraction of males and, remarkably, other females, drive the formation of mating clusters. Clusters may grow in size by merging with other individual or clusters. Female molting to the adult stage and reproductive success are enhanced in larger clusters. Male mating behavior is initiated when the female begins to molt to the adult stage by shedding dual juvenile cuticles posteriorly. Males coil their tail around the adult cuticle, migrating progressively along the female in intimate synchrony with the molting cuticle until the vulva is exposed and mating can occur. The first arriving male is assured of access to a virgin female, as his intermediate location between the vulva and subsequently arriving males blocks these competitors. Males deposit an adhesive gelatinous copulatory plug into and over the vulva before departing the female. Fecundity was greater in larger mating clusters, but this was a function of a greater rate of molting which is a prerequisite for mating. Males compete for virgin females by emerging and molting to the adult stage earlier than females. Mating aggregations have previously only been examined in snakes, but these studies have tended to be observational as snakes offer a challenging system for study. The relatively easy to culture and manipulate mermithid system may offer a model for experimental studies of male-male competition, protandry, copulatory plugs and female choice in mating clusters.

Pyriproxyfen, a juvenile hormone analogue, diflubenzuron, a chitin synthesis inhibitor, and azadirachtin, an ecdysone agonist, are three insect growth regulators (IGRs) considered as selective and effective insecticides for mosquitoes. Romanomermis iyengari (Welch) is a mosquito-parasitic mermithid that can provide biological control against many medically important mosquito species. The compatibility of these two control tactics was tested by evaluating the sublethal effects of exposure to IGR on nematode developmental stages (preparasitic, parasitic, and preparasitic + parasitic) using Culex pipiens larvae as the host. Sublethal concentrations of IGRs were 90 % emergence inhibition of host mosquito. Preparasitic exposure to pyriproxyfen, azadirachtin, and diflurbenzuron had no effect on infectivity, parasite load, sex ratio, or male size but reduced nematode female length and increased male sex ratio at one parasite/larva. When IGRs treatments were made against the parasitic and preparasitic + parasitic stages, pyriproxyfen and azadirachtin reduced R. iyengari infectivity, parasite load, and male nematode length, whereas pyriproxyfen exposure increased male sex ratio and reduced the female R. iyengari length. Thus, IGRs have significant negative impacts on different stages of mosquito mermithid that can destabilize the balance of host-parasite population interaction. Therefore, IGRs should be used with caution in mosquito habitats where these parasites have established.

The responses of eggs in diapause and the infective stage of the nematode, Strelkovimermis spiculatus ,t o larvae of its host, Culex pipiens pipiens, were investigated in the laboratory. The results indicated that the presence of the host induced the egg hatching. The hatching rate increased when larger numbers of host larvae were present. Second instar mosquito larvae induced significantly higher hatching rates than any other stages. These findings explain how S. spiculatus synchronizes its life cycle with its host life cycle and population dynamics to increase its fitness when the natural habitat is constantly covered by water. Direct exposure of the nematode eggs to host larvae resulted in consumption of as many as 20 eggs per host. The eggs consumed caused 0–70% host mortality depending on the number consumed, which indicated an infection path other than cuticle penetration although it may represent a rare situation in nature. The result of host cue assays showed that the combination of chemical cues and physical vibration induced the highest egg hatching, which may increase the chance of host availability after hatching. However, once hatched, the nematodes ignored vibrations and used only chemical cues for host location. These findings suggest that eggs hatch synchronously with the most susceptible mosquito stage and with peak mosquito larval density.