Temperature Effects on the Growth and Virulence of Steinernema feltiae Strains and Heterorhabditis heliothidis.

Temperature influences the successful application of steinernematids and heterorhabditids as biological control agents against soil dwelling insects (1,3). It affects also the maturation and growth rates of Steinernema feltiae strains on meridic diets (10), nematode reproduction (12), growth in Galleria mellonella (4) and Hylobius pales (12), dauerjuvenile infection potential and emergence time (11), migratory activity (3), sex ratios (4), and insect death rates (12). Studies of temperature effects on Heterorhabditis spp. are scarce. However, larvae of Choristoneura fumiferana parasitized by H. heliothidis died more slowly when incubated at 17 C than at 24 C (8). Adapting S. feltiae strains and H. heliothidis to selected soil temperatures would likely improve control of soil dwelling insects. Burman and Pye (3) acclimated S. feltiae dauer juveniles to soil temperatures of 15-25 C by culturing the nematodes at those temperatures, but the acclimation was transient. Our objectives were to test the potential of selecting for temperature adapted strains of S. feltiae and H. heliothidis by serial passaging the nematodes through G. mellonella larvae at specific temperatures and by prolonged culture of the nematodes on semi-defined diet in vitro at selected temperatures. Tempera ture adaptation was assessed by determining the yield of nematodes on artificial diet, monitoring changes in the concentration of dauerjuveniles required to kill 50% (LC50) of the larvae,