Developmental stage affects the interaction of Steinernema carpocapsae and abamectin for the control of Phthorimaea operculella (Lepidoptera, Gelechidae)

Abstract

Laboratory bioassays were conducted to evaluate the interactions between an entomopathogenic nematode (EPN), Steinernema carpocapsae (Nematoda: Steinernematidae), and the insecticide abamectin against the Potato tuber moth (PTM), Phthorimaea operculella, one of the key insect pests of potatoes (Solanum tuberosum). Steinernema carpocapsae was highly virulent against pre-pupae but significantly less virulent against pupae with respective LC50 values of 10.5 and 102.1 infective juvenile nematodes (IJs) per individual. Abamectin was similarly toxic to pre-pupae and pupae with respective LC50 values of 25.3 and 28.0 ppm. However, abamectin had detrimental effects on S. carpocapsae IJs causing up to 70% mortality when they were exposed at 1000 ppm for 24 h. PTM life stage had a strong effect on insect mortality when exposed to combinations of S. carpocapsae and abamectin. Against pre-pupae, combinations mostly had a synergistic effect on mortality with the exceptions being additive; against pupae, combinations mostly had an antagonistic effect on mortality with the exceptions being additive. The synergistic interaction in pre-pupae was the strongest and the antagonistic interaction in pupae the weakest when the combinations included the lowest abamectin rate (LC10) which can be explained by the more limited negative effect of the lower abamectin rate on the IJs. The synergistic interaction and the negative effect of higher abamectin rates on it were confirmed in a greenhouse experiment on potato plants. Combinations of abamectin and S. carpocapsae have potential for the control PTM and could help reduce to usage of synthetic insecticides in PTM management.