Overwintering behavior of the entomopathogenic nematodes Steinernema scarabaei and Heterorhabditis bacteriophora and their white grub hosts

Entomopathogenic nematodes (EPNs) are obligate pathogens known to naturally persist in many habitats. Because survival is a fundamental component of persistence, we investigated whether vertical movement and other avoidance behaviors (i.e., in‐host survival and latent infection), previously speculated as viable survival mechanisms, are exhibited during the cooler months in a temperate turfgrass habitat. The vertical distribution of populations of two EPN species, Steinernema scarabaei Stock & Koppenhöfer (Rhabditida: Steinernematidae) and Heterorhabditis bacteriophora Poinar (Rhabditida: Heterorhabditidae), and two important hosts of these EPN species, the white grub species Popillia japonica Newman and Anomala orientalis Waterhouse (both Coleoptera: Scarabaeidae), were regularly monitored in turf plots from October through April in two consecutive years. Entomopathogenic nematode vertical distribution showed limited changes for H. bacteriophora but none for S. scarabaei. Recovery of H. bacteriophora showed a strong and consistent decline at 0–4 cm depth in the 1st year and a weaker decline at 4–10 cm in the 1st year and at 0–4 cm in the 2nd year. Due to high variability in the data, it was not possible to determine whether the decline in the upper soil layers was due to downward migration or attrition of infective juvenile nematodes. The decline occurred mostly during the first half of the season before the soil froze to any significant extent. The vertical distribution of both white grub species changed with temperature during fall and spring, but not during winter. Overwintering infective juveniles were only recovered in the soil. There was no evidence for successful in‐host survival or latent infection by the nematodes in endemic white grub populations.