Sublethal effects of broflanilide and isocycloseram on Spodoptera littoralis and associated natural enemies

Abstract

The cotton leafworm, Spodoptera littoralis, is a major agricultural pest, particularly in Mediterranean and African regions, due to its broad host range and its ability to develop resistance to multiple insecticides. In this study, we evaluated the toxicity of the novel neurotoxic insecticides broflanilide and isocycloseram on a field population of S. littoralis, as well as the effects of their sublethal concentrations (LC15 and LC30) on life history parameters. Sublethal concentrations of both insecticides significantly impacted larval and pupal development, prolonging larval durations and reducing pupation percentages at both concentrations. Additionally, we investigated the compatibility of these insecticides with the entomopathogenic nematodes (EPNs), Steinernema feltiae and Heterorhabditis bacteriophora, as well as the egg parasitoids Trichogramma cacoeciae and T. evanescens. S. feltiae exhibited high tolerance to the registered field rates of both insecticides (broflanilide: 20 mg L-1; isocycloseram: 30 mg L-1), with survival rates exceeding 86% and infectivity of 88.33%. In contrast, H. bacteriophora was more sensitive, particularly to broflanilide, with survival rates of 69.67-78.17% and infectivity of 65.00-81.67%. While both insecticides caused 100% mortality of adult T. cacoeciae and T. evanescens, high emergence rates were observed when parasitized eggs were treated, despite lower parasitism rates in the F1 generation. The results showed that S. feltiae was compatible with both insecticides, whereas H. bacteriophora and the two tested Trichogramma species were adversely affected, which should be considered when designing Integrated Pest Management (IPM) strategies.