Horizontal Transmission of Helicoverpa Nucleopolyhedrosis Virus (NPV) in Soybean Fields Infested with Corn Earworm

Helicoverpa NPV (HearNPV), a viral biocontrol agent, is utilized in foliar insecticidal applications to control corn earworm, Helicoverpa zea, populations. This virus relies on both initial contact and horizontal transmission to infect present larvae. Several well documented routes of horizontal transmission have been previously observed, including transmission via predators, parasitoids, other larvae, and viral replication within initially infected larvae. Horizontal transmission is an important aspect of HearNPV when considering it as an effective insecticide, however; only lab studies have sought to quantify the horizontal transmission. The objective of this study was to quantify the horizontal transmission of HearNPV when applied to soybean fields infested with corn earworm.

                Viral horizontal transmission was evaluated by spraying a 50’ by 50’ area with HearNPV, and then taking 3 samples within zones of distance including 0, 0-25, 25-50, 50-100, and 100-200 feet from the application. Samples were taken before, 3, 7, 14, and 21 days after application. Polymerase chain reaction (PCR) was conducted to determine the presence of HearNPV within each sample. HearNPV was only found in the area initially sprayed 3 days after application, but by 7 days it was found 200 feet from the initial application. Fourteen days after the application the HearNPV was only found in three samples: 1 at 0 feet, 1 at 50 feet, and 1 at 100 feet. At 21 days after no virus was detected. This data suggests that horizontal transmission of HearNPV peaks 7 days after application and dissipates by 21 days.