Overwintering behavior of the entomopathogenic nematodes Steinernema scarabaei and Heterorhabditis bacteriophora (Rhabditida: Steinernematidae and Heterorhabditidae) and their white grub hosts (Coleoptera: Scarabaeidae)

Entomopathogenic nematodes (EPN) Steinernema scarabaei and Heterorhabditis bacteriophora, species endemic to New Jersey turfgrass habitats, have great potential as biological control agents of various white grub pest species. EPN have potential for long-term white grub suppression, but to more reliably achieve this, a better understanding is necessary of the nematodes' survival mechanisms expresses during harsh seasonal conditions. Infective juveniles (IJ) may employ similar vertical migratory patterns as other soil-inhabiting animals during harsh seasonal conditions. To determine the existence of vertical relocation as an overwintering mechanism we investigated the vertical distribution of S. scarabaei and H. bacteriophora relative to (1) fluctuating soil temperature and (2) changing vertical position of two white grub hosts, Japanese Beetle (Popillia japonica) and oriental beetle ( Anomala orientalis), during the late fall, winter and early spring season. The vertical distributions of white grub and IJ populations were monitored every 14-18 days in established turf plots (3-4 years old) from mid-October to the first week of May in two consecutive years (2006-2007 and 2007-2008). The vertical distribution of S. scarabaei, H. bacteriophora, and Steinernema carpocapsae did not change from the fall to the spring season. Soil temperature did not appear to influence IJ vertical distribution. The vertical distribution of both white grub species changed with temperature during fall and spring but not in winter. Overwintering S. scarabaei and H. bacteriophora IJs were only recovered in the soil. There was no evidence for successful in-host survival or latent infection in endemic white grub populations. Although the relationship between temperature and vertical distribution for EPN and their white grub hosts differed, a considerable degree of spatial and temporal overlap was observed between respective EPN and host populations. Overwintering EPN appear to survive primarily or exclusively as IJs in the soil and presumably employ various physiological and biochemical mechanisms as survival strategies during seasonally cold and freezing conditions.