Entomopathogenic nematodes for control of the cotton cutworm Spodoptera litura in marigolds

Abstract

AbstractSpodoptera litura Fabricius (Lepidoptera: Noctuidae) is a major pest of marigolds in Thailand. It causes severe damage to flower quantity and quality. The most common control method is synthetic insecticides, although this method struggles to control hidden larvae inside flowers or buds and underground pupae. Entomopathogenic nematodes (EPNs) are widely used as biocontrol agents against many foliage and cryptic insects. The virulence of two commercial EPNs strains, Steinernema siamkayai (Stock, Somsook and Reid) and S. carpocapsae (Weiser), isolated in Thailand, were assessed against different developmental stages of S. litura. The younger larvae of first and third instar were more susceptible to S. siamkayai with the LD50 of 15.84 and 40.34 dauer juveniles (DJs) per host, whereas S. carpocapsae caused higher mortality in older larvae. Against the fifth instar larvae and pupae, the LD50 ranged between 54.37 and 607.97 DJs/host. Field experiments compared the effect of EPNs with farmer's practice (insecticides). EPNs treatments with S. siamkayai and S. carpocapsae significantly reduced the percentage of damaged flowers to 2.53 ± 1.2% and 2.97 ± 1.5%, respectively, compared with insecticide treatment at 4.21 ± 2.1% in the first trial. In trial 2, the insecticide treatment had the highest percentage of damaged flowers of 5.02 ± 5.7%. The benefit–cost ratio (BCR) of EPNs treatments ranged from 2.42 to 2.53 in the first trial and 1.24–1.50 in the second trial and was superior to insecticide treatments with 2.21 and 1.16 in the two trials.