Infection of Entomopathogenic Nematodes Steinernema glaseri and Heterorhabditis bacteriophora against Popillia japonica (Coleoptera: Scarabaeidae) Larvae

Abstract

Defense mechanisms of Japanese beetle larvae, Popillia japonica, and their effectiveness against entomopathogenic nematodes were investigated in the laboratory. Behavioral grooming by P. japonica larvae was effective in reducing invasion of both Steinernema glaseri and Heterorhabditis bacteriophora. Forty percent of H. bacteriophora infective juveniles were inactivated or killed in the host gut fluid within 8 hr, whereas 11% of S. glaseri were killed. Host melanotic encapsulation, induced by injection of 20 H. bacteriophora infective juveniles, caused 75% nematode mortality. S. glaseri was less susceptible to the host immune response, which caused only 12% mortality of injected nematodes. In a sand column assay, S. glaseri was found in the host hemocoel 8 hr earlier than H. bacteriophora. However, Photorhabdus luminescens, the bacterium of H. bacteriophora, was detected 16 hr earlier in the host's hemolymph than S. glaseri's bacterium, Xenorhabdus poinarii. Despite the differences in effectiveness of host defenses, S. glaseri and H. bacteriophora caused similar host mortality in the sand column assay. We demonstrated that the two nematode species use different strategies for invasion of P. japonica larvae. S. glaseri, which tolerated P. japonica gut fluid and avoided the host immune response, has a high initial rate of penetration and survival; H. bacteriophora, which exhibited poor tolerance to the host gut fluid and induced a strong host immune response, releases its bacteria during or soon after penetration to kill the host.