Assessment of Insecticide Compatibility with Metarhizium anisopliae for Fall Armyworm (Spodoptera frugiperda J.E. Smith) Management in Maize

Abstract

The fall armyworm (Spodoptera frugiperda) is an invasive pest of maize, which are kept under control by mostly chemical insecticides. The environmental impact and potential for resistance development due to continuous use of insecticides has resulted in the search for alternatives for the management of FAW. Metarhizium anisopliae, an entomopathogenic fungus (EPF), is a promising biological control agent that has been recorded effective against several lepidopteram pests including S. frugiperda. An array of insecticides have been recommended for the management of FAW, while simultaneous studies pertaining to evaluation of EPF are made to target the pest through non chemical approaches. However, the compatibility between insecticides and the entomopathogenic fungus M. anisopliae has not been studied at large. The present study focuses on the compatibility of insecticides with EPF in the management of FAW. Six insecticides, representing different chemical classes, were tested at their field recommended doses for their inhibitory effects on M. anisopliae fungal colony growth, at various concentrations (50%, 75%, and recommended concentrations). Mycelial growth inhibition was measured at 7, 14, and 21 days after inoculation (DAI). Results showed that most insecticides exhibited varying degrees of compatibility with M. anisopliae, with growth inhibition ranging from 6.25% to 51.88% at 7 DAI. The lowest growth inhibition occurred with Emamectin benzoate 5 SG and Novaluron 5.25 + Emamectin benzoate 0.9 SC, showing minimal suppression of fungal growth. In contrast, Spinetoram 11.7 SC and Thiodicarb 75 WP caused significant inhibition at higher concentrations, with the latter showing up to 51.88% inhibition at 7 DAI. At 14 and 21 DAI, the growth inhibition generally decreased across treatments, with Chlorantraniliprole 9.3 + Lambda-cyhalothrin 4.6 ZC and Emamectin benzoate 5 SG demonstrating the best compatibility. Overall, the results suggest that most insecticides tested were compatible with M. anisopliae, with Spinetoram and Thiodicarb being exceptions, particularly at higher concentrations. This study provides valuable insights into selecting insecticide-fungus combinations for integrated pest management.