Comparative pathogenicity of entomopathogenic nematodes, Heterorhabditis indica and Steinernema siamkayai, against younger instars of the fall armyworm, Spodoptera frugiperda

Summary We report the comparative efficacy of two native entomopathogenic nematodes, Heterorhabditis indica and Steinernema siamkayai , isolated from the same habitat where the fall armyworm, Spodoptera frugiperda , has increasingly become a major threat to maize crop in the north-central part of India. Heterorhabditis indica caused 100% mortality to the neonates (L1) and the second-stage larvae (L2) of S. frugiperda within 24 h of exposure across all the inoculum levels, ranging from 5 to 80 infective juveniles (IJ) larva −1 . Against the L1, the LD 50 of H. indica was 5.96 IJ larva −1 after 24 h, while S. siamkayai caused no mortality during the initial 36 h exposure. For L2, the LD 50 value for S. siamkayai was 37.93 IJ larva −1 after 24 h, nearly twice that of H. indica . Across all the treatments ranging from 10 to 50 IJ larva −1 , the L1 and L2 facilitated more than twice the rate of multiplication of H. indica as compared to S. siamkayai . The highest recovery of H. indica was 1111.8 ± 121.91 IJ L2 −1 , whereas S. siamkayai yielded only 527.5 ± 73.61 IJ L2 −1 at the highest inoculum level of 30 IJ L2 −1 , clearly suggesting the preference of the bacterial symbiont Photorhabdus luminescens over Xenorhabdus stockiae for the fall armyworm tissue. Overall, H. indica demonstrated greater specialisation in killing and efficiently reproducing on the younger instars of the fall armyworm, making it a more suitable candidate for commercial production and application in the local habitat.