Compatibility of entomopathogenic nematodes (Nematoda: Rhabditida) and the biocide, spinosad for mitigation of the armyworm, Spodoptera litura (F.) (Lepidoptera: Noctuidae)

Abstract

Two entomopathogenic nematodes (EPNs) Heterorhabditis indica and Steinernema carpocapsae were tested against the armyworm, Spodoptera litura (F.) (Lepidoptera: Noctuidae). Also, their compatibility with spinosad (Tracer® 240SC) was assessed under laboratory conditions. In study I, compatibility test was performed to evaluate the viability of nematodes at different concentrations of spinosad. None of the concentrations was observed as toxic against both EPNs. The highest concentration (800 ppm) of the spinosad showed 14% mortality in H. indica juveniles and 13% in S. carpocapsae 48 h post-treatment. In study II, the spinosad and the EPN mixtures showed that both EPNs enhanced the efficacy of spinosad against S. litura larvae. The highest mortality rates (80.0 and 76.6%) in the second instar larvae indicated a synergistic effect (χ2 = 39.69 and 7.02) 48 h post-treatment, with infective juveniles (IJs) of S. carpocapsae and H. indica, respectively, with the highest concentration of spinosad. In the case of the third instar larvae, a synergistic effect (χ2 = 16.73) was observed in H. indica and the highest concentration of spinosad, while other treatments showed additive efficacy of the EPNs. After 96 h, all treatments exhibited synergistic effect, except when H. indica was applied at the lowest concentration of spinosad. However, these combination treatments need to be tested further with EPNs under field conditions to confirm their efficacy.