Sex ratio and the infection process in entomopathogenic nematodes: are males the colonizing Sex?

In laboratory assays, male infective juveniles of some species of entomopathogenic nematodes in the genus Steinernema differ from females in their tendency to disperse and in their responses to the volatile cues emitted by parasitized and nonparasitized insects. These differences suggest that male infective juveniles might locate and establish in insect hosts before females, and that infection by males might render hosts suitable for nematode development and more attractive to females: "the male colonization hypothesis." We tested this hypothesis in laboratory experiments in which larvae of the greater wax moth, Galleria mellonella (L.), were exposed to infective juveniles of S. glaseri (Steiner) (NC strain) in sand columns for various periods of time. The hosts were dissected to determine the sex ratio of the adult nematodes that became established. We found that infective juveniles entered hosts over periods of up to at least 14 h, and that sex ratios varied among experiments. However, there were no temporal differences in colonization by males and females during the infection process: the proportion of males in host cadavers was not related to exposure time or to the total number of nematodes in host cadavers. This result is inconsistent with the male colonization hypothesis. We conclude that differential colonization of hosts over time by males and females either does not occur in S. glaseri, or occurs only rarely or under conditions differing from ours, or occurs to such a small extent that it is difficult to detect. In S. glaseri, male infective juveniles tend to emerge from host cadavers before females. This might give males an advantage over females in locating new hosts following a natural emergence. Nonetheless, to date, males have not been shown to colonize hosts before females in any steinernematid species. Copyright 1998 Academic Press.