Synergized toxicity exhibited by indigenous entomopathogenic fungal strains, plant extracts and synthetic insecticides against fall armyworm Spodoptera frugiperda (J.E. Smith) under laboratory and semi-field conditions

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a destructive transboundary lepidopterous pest affecting more than 78 countries worldwide by infesting a wide array of agronomic and horticultural crops such as maize and cabbage. In a previous study, we demonstrated the synergized toxicity of three local plant extracts ( Rhazya stricta Decne. , Sophora mollis (Royle) Baker and Withania somnifera (L.) Dunal) with synthetic insecticide chlorantraniliprole against 3rd instar larvae of S. frugiperda . Here we evaluated the combined efficacy of indigenous entomopathogenic fungi (EPF) ( Beauveria bassiana (Balsam) Vuillemin and Metarhizium anisopliae (Metschn.) Sorokin) along with above-mentioned plant extracts and chlorantraniliprole against 3rd instar larvae of S. frugiperda under laboratory and semi-field conditions. Toxicity bioassays with two EPF revealed that the highest concentration (1.0 × 10 9 conidia per mL) of B. bassiana caused maximum larval mortality (76%) at 10th day post-treatment, followed by M. anisopliae , causing 70% larval mortality at the same concentration. In vitro evaluation of six binary combinations of LC 33 s of EPF ( B. bassiana and M. anisopliae ) and plant extracts revealed a synergized toxicity (83–87%) of four combinations comprising S. mollis and W. somnifera with B. bassiana and M. anisopliae against 3rd instar S. frugiperda larvae. Furthermore, these insecticidal combinations including chlorantraniliprole (@ half of the label-recommended dose) exhibited as well a significant synergized toxicity ( i.e. 37.75% at 1st day to 99.25% at 7th day post-treatment) under semi-field conditions, suggesting their further investigation under field against S. frugiperda and on its natural enemies.