Lethal and Sublethal Effects of Three Microbial Biocontrol Agents on Spodoptera litura and Its Natural Predator Rhynocoris kumarii

Abstract

Entomopathogenic microbes such as Spodoptera litura nucleopolyhedrovirus (SpltNPV), Metarhizium anisopliae, and Pseudomonas fluorescens are biological agents used for the control of multiple arthropod pests. The objective of this study was to assess their effects on the biological parameters of Spodoptera litura (Lepidoptera: Noctuidae) larvae, and its natural reduviid predator Rhynocoris kumarii (Hemiptera: Reduviidae) under laboratory conditions. Results suggested that P. fluorescens reduced the food consumption index, relative growth rate, approximate digestibility, the efficiency of conversion of ingested food, and the efficiency of conversion of digested food of S. litura third instar larvae compared to prey infected with M. anisopliae and SpltNPV. Both SpltNPV and M. anisopliae caused similar mortality of S. litura life stages after 96 h of observation. To observe the effect of an infected prey diet on predator behavior, infected S. litura larvae were offered to the third, fourth, and fifth instar nymphs of R. kumarii, and their prey handling time, predation rate (number/day/predator), developmental period, and the survival rate was recorded. When the life stages of R. kumarii were offered entomopathogen-infected S. litura larvae, their predation rate was comparable to or higher than the untreated control. The juvenile predator, after feeding on P. fluorescens-infected S. litura larvae, had a significantly longer developmental period (2–4 days) compared to those fed on larvae infected with other microbial control agents. However, feeding on P. fluorescens alone did not affect the predator nymphal survival rate or the adult sex ratio. Although three entomopathogens had some degree of effect on the biological parameters of R. kumarii, the outcome of this study suggests that integration of reduviids with the tested entomopathogens are a compatible and potentially effective strategy for the management of S. litura populations. However promising, this combined strategy needs to be tested under field conditions to confirm the laboratory findings.