Adaptation of the entomopathogenic nematode, Steinernema carpocapsae, to insect food plant

We conducted assays to determine if characteristics of entomopathogenic nematodes vary in response to continued exposure to a particular insect/host plant combination, and whether selection results in changed performance on other insect/host plant combinations. Three isolates of Steinernema carpocapsae (Weiser) (Agriotos, Mexican, and Hybrid) were continuously cultured in corn-fed (Zea mays L.) or squash-fed (Cucurbita pepo L.) southern corn rootworm, Diabrotica undecimpunctata howardi Barber for 25 passages. The rootworm-selected nematodes were compared to the same isolates maintained on Galleria mellonella (L.) ("unselected"). There was no increase in the ability to kill rootworms in selected nematodes compared to unselected nematodes. Among rootworm-selected nematodes, there were changes based on the host plant on which the rootworms had fed. The ability of nematodes selected on corn-fed rootworms to kill corn-fed rootworms compared to squash-fed rootworms improved in the Mexican and Agriotos isolates. The squash-selected Mexican isolate lost virulence against rootworms. Virulence of the Hybrid isolate did not change in response to selection on rootworms or host plant/rootworm combination. In general, changes in infective juvenile (IJ) production were not related to number of passages through rootworms. Using computer-aided image analysis, we measured the effects of selection on length and neutral lipid density and area in IJ. IJ selected on corn-fed rootworms were longer than those selected on squash-fed rootworms or maintained in G. mellonella. Length of IJ was positively correlated with the area of stained neutral lipids. The Hybrid isolate had a greater area and density of stained lipids than the Agriotos or Mexican isolates. IJ from nematodes selected on corn-fed rootworms tended to have a greater area of stained lipids, but not lipid density, compared with IJ selected on squash-fed rootworms or maintained in G. mellonella. There was a pattern for an inverse relationship between numbers of IJ produced and measures of lipid content. This pattern is discussed with reference to trade-offs in life history characteristics. Our results suggest that some characteristics of entomopathogenic nematodes can respond to selection on insect host plant, but that changes are dependent on the nematode isolate and the particular host plant on which the insect feeds.