Susceptibility of the fall armyworm, Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae), larvae to un-irradiated and gamma-irradiated entomopathogenic nematodes

Background The fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) is an endemic destructive pest for several cultivars in America and recently in Africa and Asia. Due to the development of pesticide resistance as well as environmental contamination, chemical control of the fall armyworm is ineffective. Alternatively, entomopathogenic nematodes (EPNs) provide a successful biological control tool sustainably. This study was designed to estimate the virulence of 2 isolates (Steinernema carpocapsae (All) and Heterorhabditis bacteriophora (HP88)) on 3rd and 5th larval instars of FAW under laboratory conditions. As well, the effect of gamma radiation (with 2 Gy) on the nematodes' pathogenicity was studied. Results The results revealed that S. frugiperda larvae were sensitive to the 2 tested nematodes which were more apparent to S. carpocapsae. The mortality rates presented a significant elevation with the increase in un-irradiated and irradiated nematode concentrations. The highest recorded mortality for the 3rd and 5th larval instars was 100% after 3 and 4 days of treatment at concentration (80 IJs/ml) irradiated S. carpocapsae and the recorded death rate for un-irradiated S. carpocapsae was 72.2 and 77.8% for the two treated larval instars, respectively, after 4 days of the treatment with the same concentration. However, H. bacteriophora caused mortality of 88.9 and 61.1% at irradiated concentration (80 IJs/ml) and 66.7 and 50% at un-irradiated concentration (80 IJs/ml) for the 3rd and 5th larval instars, respectively, after 6 days of treatment. Based on the LC50 values, the 3rd instar larvae was more susceptible than the 5th instar larvae. In addition, juveniles' irradiation increased their virulence. Conclusions Laboratory studies indicated that S. carpocapsae had a high potency among S. frugiperda larvae, especially the irradiated juveniles. Therefore, they have the potential to be developed as a biological control agent for S. frugiperda after further field studies.

Automated summary

The study demonstrated high potency of S. carpocapsae in controlling S. frugiperda larvae, especially with irradiated juveniles. The mortality rates increased significantly with higher concentrations of un-irradiated and irradiated nematodes.