Co-existence at a cost: The outcome of competition between Steinernema and Heterorhabditis within the same insect host.

Competition within a single insect between the two genera of entomopathogenic nematodes, Heterorhabditis and Steinernema, was explored using H. downesi, H. bacteriophora, S. carpocapsae, and S. feltiae. Insect larvae were infected with one genus (primary invader) and then exposed to the other genus (secondary invader) after 24, 48 or 72 h. We hypothesised that (1) Steinernema and Heterorhabditis will not both reproduce within the same insect (2) reproductive success of the secondary invader will depend on identity of the primary invader and length of occupation and (3) nematodes will avoid insects offering poor reproductive potential. Most larvae yielded infective juveniles (IJs) of only one species, however in 5% of infected larvae IJs of both genera emerged. The primary invader was more likely to reproduce, but this depended on the species combination and time between infections. Dissection of insect cadavers showed reduced rates of invasion, survival and/or development of the secondary invader, which could have contributed to reproductive failure. Unexpectedly, H. downesi was more likely to invade Steinernema-infected insects than live controls, and the invasion rate was higher the more Steinernema individuals present, suggesting possible kin-selected behaviour. In an agar-plate assay, EPN-infected cadavers (except H. downesi) were at least as attractive as live insects to IJs of the other genus. There was no correlation for any species between the attractiveness of hosts and their suitability for reproduction, but for Steinernema species there was a good correlation between invasion rate and reproduction, indicating that discrimination against unsuitable hosts is based on contact rather than distance cues.