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

The control of the potato cutworm Agrotis deprivata (Walker) by the native entomopathogenic nematode (EPN) Steinernema feltiae isolate Lican Ray (LR) was evaluated. Under laboratory conditions, sterile soil at field capacity was inoculated with a suspension of 25 infective juveniles (IJs)/cm(2) and then exposed to four temperatures (10, 16, 22 and 28 degrees C) for 30 days in the dark. Every five days, the survival of the IJs and their infectivity on larvae L6 of Galleria mellonella was evaluated, and both parameters were affected in soils exposed to temperatures higher than 22 degrees C. Then, a second experiment evaluated the infectivity and recycling capacity of these EPNs on L4 and L6 larvae and pupae of A. deprivata at 16, 22, and 28 degrees C for six days. While mortality of pupae reached 62.5 +/- 20.2%, in larvae, this value was 100%, regardless of temperature. At 28 degrees C, despite the high level of mortality in larvae, EPNs could not emerge from the cadaver in White traps. In a third experiment under semi-field conditions, IJs of S. feltiae LR were applied in potted sterile soil with five maize plants using three application techniques: (1) within a G. mellonella cadaver buried 5 cm deep; (2) in an aqueous suspension buried 5 cm deep; and (3) in an aqueous suspension on the soil surface. The experiment also included (4) a control treatment with water, (5) a synthetic insecticide, and (6) a biological insecticide. 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 perforated 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. The mortality level of the cutworm when IJs were applied in an aqueous suspension was equivalent to that on the treatment with the synthetic insecticide. When applied within the cadaver, mortality level was equivalent to that caused by both commercial insecticides; however, the death of larvae took longer than when applied in any of the aqueous suspensions. This result suggests a time lag 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.