Biological control of the potato cutworm Agrotis deprivata (Lepidoptera: Noctuidae) with Steinernema feltiae LR (Nematoda: Steinernematidae): influence of the temperature, host developmental stage, and application mode on its survival and infectivity

Abstract The native entomopathogenic nematode (EPN) Steinernema feltiae isolate Lican Ray (LR) was studied against the potato cutworm Agrotis deprivata (Walker). Under laboratory conditions sterile soil at field capacity was inoculated with a suspension of 25 IJS/cm2 of and then exposed to four temperatures (10, 16, 22 and 28°C) for 30 days in dark incubators. Every five days survival of the IJSs and its infectivity on larvae L6 of Galleria mellonella was evaluated, observing that both parameters were affected in soils exposed to temperatures superior than 22°C. Then, a second experiment evaluated infectivity and recycling capacity of this EPN on L4 and L6 larvae and pupae of the cutworm Agrotis deprivata exposed to 16, 22 and 28°C during six days. While mortality of pupae reached 62.5 ± 20.2%, in larvae this value was 100%, regardless of temperature. At 28°C, despite the high level of mortality in larvae, EPN could not emerge from the cadaver in White traps. In a third experiment under semifield conditions S. feltiae LR were applied in potted sterile soil with five maize plants using three application systems: (1) within a G. mellonella cadaver buried 5 cm deep; (2) in aqueous suspension buried 5 cm deep; and (3) in aqueous suspension on the soil surface. Additionally a control treatment with water, a synthetic insecticide and a biological insecticide were applied. Two days after the inoculation an L6 A. deprivata L6 was put on the soil and allowed to bury itself naturally. Also a G. mellonella larva inside a modified Eppendorf vial containing soil was buried 5 cm deep. After 6 days damage on the plants by A. deprivata and survival of insects were evaluated. Mortality level of the cutworm when IJSs were applied in aqueous suspension was equivalent to the treatment with the synthetic insecticide. When applied within the cadaver, mortality level was equivalent to that caused by both commercial insecticides, but death of larvae took longer than when applied in any of the aqueous suspensions. This leaves a period where larvae feed on the plants causing a higher damage level, which must be taken into account when evaluating the success of the EPN as a pathogenic insecticide against A. deprivata.