Effects of Precedent Infection of Entomopathogenic Nematodes (Steinernematidae) on the Subsequent Invasion of Infective Juveniles

Effects of precedent infection of Steinernema carpocapsae and S. glaseri on subsequent nematode invasion were investigated at 25°C. Behavior of Galleria mellonella larvae infected through injection with ca. 50 S. carpocapsae infective juveniles did not change 6 hr post-infection, but invasion of both nematode species decreased by ca. 50%, compared to previously uninfected hosts. Precedent infection with S. glaseri promptly suppressed subsequent invasion of the two nematode species. Subsequent invasion was also significantly decreased by symbiotic Xenorhabdus bacteria: invasion rates to insects killed by X. nemato-philus injection, X. poinarii injection, and by hot water treatments were 1.8, 0, and 5.4% for S. carpocapsae and 1.8, 5.0 and 7.8% for S. glaseri, respectively, while invasion rates to the non-treated live insects were 16.5% for S. carpocapsae and 15.1% for S. glaseri. At the early stage of the subsequent invasion, the sex ratio (% male) of the invading S. carpocapsae in the hosts with and without the precedent infections were 5.3% and 12.3%, respectively. These results indicated that insects infected with a Steinernema/Xenorhabdus complex may emanate suppressive substance (s) against subsequent nematode invaders.