Penetration of Steinernematid nematodes (Nematoda: Steinernematidae) into Japanese beetle larvae, Popillia japonica (Coleoptera: Scarabaeidae)

Penetration of infective juveniles of Steinernema glaseri and Steinernema carpocapsae into Japanese beetle larvae, Popillia japonica, was compared using external exposure and injection methods. Steinernematids entered the grubs through the mouth and anus, but not through the spiracles or cuticle. For all treatments, S. glaseri had higher levels of penetration than S. carpocapsae. Penetration of the gut by orally injected nematodes began 4-6 hr postinoculation. All regions of the gut were penetrated, with more than 82% of all penetrating nematodes invading the host hemocoel via the midgut. Gastric cecae were important sites for nematode penetration. Penetration sites of anally injected nematodes were the rectal sac of the hindgut. Penetration rates of isolated guts and that in vivo did not differ. Defecation by P. japonica was rapid, with orally injected nematodes appearing in the feces in 4-8 hr. Our results indicate that compared with S. glaseri, S. carpocapsae is poorly adapted to parasitize Japanese beetle larvae.