Insecticide susceptibility vis-à-vis molecular variations in geographical populations of fall armyworm, Spodoptera frugiperda (J.E. smith) in India

In an emergency response to the introduction, subsequent detection and rapid spread of the invasive insect pest fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) in the country, Government of India offered ad -hoc approval for few pesticide molecules namely, emamectin benzoate, spinetoram, chlorantraniliprole, novaluron, thiodicarb, and λ-cyhalothrin for FAW management in corn crop across the country. Five major maize (corn) growing geographical areas (i.e., Bihar, Delhi, Karnataka, Punjab and Tamil Nadu) were selected during the main crop season of 2020 (Oct-Nov), and sampled for the target-insect populations. The insect populations were lab-reared on maize leaves (15–20 days old); the F 1 generation insects (third instar, 25–30 mg/larva) were subjected to bioassay to determine susceptibility levels of FAW against ad-hoc recommended insecticides. The previously reported target-site molecular variations in the genes ace , encoding acetylcholinesterase (AChE), and vgsc , encoding voltage-gated sodium channel were analyzed. Among the five test-populations, Bihar test-population recorded least susceptibility to all the test-pesticides, whereas the South Indian populations (Karnataka, Tamil Nadu) were found most susceptible. North Indian and South Indian test-insect populations formed two distinct groups in terms of susceptibility levels, speculatively on account of prevailing climatic factors. Being the population with least ace mutation frequency, but with the higher resistance ratio for all the test-pesticides, Bihar insect population implies a bigger role of broad range detoxification machinery than the narrow scope of target site insensitivity. Though, resistance has not developed to the recommended insecticides by FAW, except the case of low-medium resistance development; which is better explained due to behavioural avoidance of synthetic pyrethroid (λ-cyhalothrin). However, there is no room for complacency. Resistance-monitoring tools such as location/region-specific determination of discriminating diagnostic concentrations/doses for FAW in recommended insecticides are to be devised at the earliest.