Virulence of entomopathogenic nematodes, Steinernema siamkayai and Heterorhabditis indica, in the control of Opisina arenosella Walker, and their impact on two natural enemies, Bracon brevicornis Wesmael and Euborellia annulipes Lucas

Abstract

Entomopathogenic nematodes (EPNs), specifically Steinernema siamkayai and Heterorhabditis indica, have been successfully used to control various soil-dwelling and above-ground lepidopteran insect pests. This study investigated the susceptibility of the coconut black-headed caterpillar Opisina arenosella, a serious pest of palm trees, to these two EPN species, as well as their impact on two natural insect enemies: the braconid wasp Bracon brevicornis and the ring-legged earwig Euborellia annulipes. Both EPN species demonstrated high efficacy against O. arenosella larvae, with mortality ranging from 60% to 100%, depending on the EPN concentrations within 72 h of exposure. Heterorhabditis indica and S. siamkayai exhibited median lethal concentration (LC₅₀) values of 9 and 11 infective juveniles (IJs)/larva, respectively. Additionally, the first-instar larvae and pupae of B. brevicornis were highly susceptible to both EPN species, with mortality of 89.6%–96.3% for larvae and 58.9%–67.2% for pupae within five days. In contrast, E. annulipes nymphs and adults showed lower mortality across both developmental stages. In addition, O. arenosella larvae exhibited 100% mortality when the two EPN species were applied alongside either B. brevicornis or E. annulipes. However, EPN applications significantly reduced B. brevicornis oviposition and larval development, leading to decreased adult emergence. Both EPN species exhibited low virulence against various developmental stages of E. annulipes, with mortality ranging from 3.3% to 26.7% after seven days. These findings highlight the potential of EPNs as effective biological control agents for O. arenosella larvae and illustrate the complex interactions between two EPN species and other insect natural enemies.