Increased Efficacy of Entomopathogenic Nematode-Insecticide Combinations Against Holotrichia oblita (Coleoptera: Scarabaeidae).

Combinations of entomopathogenic nematodes Steinernema longicaudum X-7 and Heterorhabditis bacteriophora H06 with chlorantraniliprole, diflubenzuron, and imidacloprid insecticides at different rates for the control of the white grub, Holotrichia oblita Faldermann (Coleoptera: Scarabaeidae), were evaluated both in the laboratory and in peanut fields. In the laboratory, the combinations had a synergistic or additive effect on the second-instar larvae of H. oblita and caused faster mortality than one nematode species or insecticide alone. Heterorhabditis bacteriophora-chlorantraniliprole and H. bacteriophora-imidacloprid showed synergistic effects on the larvae. When higher concentrations of the insecticides were combined with nematodes, the stronger synergistic effects were found. In peanut fields, S. longicaudum-imidacloprid, H. bacteriophora-imidacloprid, or H. bacteriophora-chlorantraniliprole also showed synergistic effects against the larvae. The three nematode-insecticide combinations produced similar percentage reductions of the grub larvae and less percentages of injured legumes, compared with the chlorpyrifos treatment. Cost-benefit analysis showed that H. bacteriophora 5.0 × 103 infective juveniles (IJs) per plant (equal to 7.5 × 108 IJ ha-1) combined with imidacloprid at the recommended concentration is a practical strategy for the practitioner to manage the white grubs in the peanut production.