Evolution of host search strategies in entomopathogenic nematodes.

There is interspecific variation in infective juvenile behavior within the entomopathogenic nematode genus Steinernema. This variation is consistent with use of different foraging strategies along a continuum between ambush and cruise foraging. To address questions about the evolution of foraging strategy, behavioral and morphological characters were mapped onto a phylogeny of Steinernema. Three species, all in the same clade, were classified as ambushers based on standing bout duration and host-finding ability. One clade of six species were all cruisers based on both host-finding and lack of standing behavior. All species in the ambusher clade had a high rate of jumping, all species in the cruiser clade had no jumping, and most intermediate foragers exhibited some level of jumping. Response to volatile and contact host cues was variable, even within a foraging strategy. Infective juveniles in the ambusher clade were all in the smallest size category, species in the cruiser clade were in the largest size categories, and intermediate foragers tended to be more intermediate in size. We hypothesize that the ancestral Steinernema species was an intermediate forager and that ambush and cruise foraging both evolved at least once in the genus.