Insecticidal activity of Heligen (HearNPV) against Helicoverpa zea: Leaf-dip bioassays to characterize time-to-feeding cessation and time-to-kill by larval instar

Helicoverpa zea (AKA soybean pod worm, corn earworm, cotton bollworm, and tomato fruit worm) and other species in the subfamily Heliothinae are severe pests worldwide. Heligen (HearNPV) is an effective nucleopolyhedrovirus for the control of the Heliothinae pest complex. This highly selective insecticide belongs to the new IRAC mode-of-action group 31 and may play an important role in managing resistance to chemical insecticides across a wide range of crops. In this study, soybean leaf dip bioassays were conducted with the objective of measuring the time to feeding cessation and the time to death by larval instar of H. zea after ingestion of treated leaves. Leaf disks were dipped into treatment solutions representing relevant field rates (1.0-1.3-1.6 oz/10 gal) and a water-only check. Leaf discs were allowed to air dry for 1h, placed in petri dishes each containing 1 larva, and maintained in an incubator at 28°C and 14L: 10D. After 24h, treated leaf discs were replaced with untreated discs every day until all larvae in each HearNPV treatment died. After each 24h period, experimental units were evaluated for percent leaf defoliation and larval mortality.  At all rates tested, Heligen acted faster (4 days) on early instars with minimal foliar damage (90% reduction in foliar damage vs. UTC) and slower on older instars (5-7 days) with increased foliar damage (32-57% reduction in foliar damage vs. UTC). These results substantiate Heligen label recommendations to target field applications at early instars for increased mortality and minimal foliar damage.