Virulence of Entomopathogenic Nematodes (Steinernema and Heterorhabditis spp.) Against Larvae of Spodoptera littoralis

Abstract

Entomopathogenic nematodes (EPNs) belonging to Steinernema and Heterorhabditis spp. are widely recognized as effective biological control agents against soil-dwelling insect pests, including the cotton leafworm Spodoptera littoralis. The present study evaluated the virulence of selected EPN isolates against larvae of S. littoralis under laboratory conditions. Infective juveniles of Steinernema and Heterorhabditis species were applied at different concentrations to early and late instar larvae, and mortality, time-to-death, and infection efficiency were recorded. Results indicated significant variability in virulence among nematode species and isolates. Heterorhabditis spp. generally exhibited higher pathogenicity compared to Steinernema spp., causing rapid larval mortality within 48–72 hours post-exposure. Early instar larvae were more susceptible than later instars, indicating stage-dependent variation in host resistance. Dose-dependent mortality responses were observed, with higher infective juvenile concentrations resulting in increased larval death rates and faster infection progression. Dissection of cadavers confirmed successful nematode penetration, multiplication, and symbiotic bacterial proliferation within the hemocoel, leading to septicemia and host death. Environmental conditions such as moderate temperature and high humidity enhanced nematode survival and infectivity. The study highlights the strong potential of EPNs as biological control agents for managing S. littoralis populations in agricultural systems. Differences in virulence among species emphasize the importance of isolate selection for effective field application. Overall, the findings demonstrate that both Steinernema and Heterorhabditis spp. can contribute significantly to integrated pest management programs, offering an eco-friendly alternative to chemical insecticides for controlling S. littoralis larvae in vegetable and field crops.