Deposition on field corn silks of Gemstar® and Entrust® applied at high and low spray rates for control of corn earworm

By

Bradley K. Fritz, Juan D. Lopez, Jr., Daniel E. Martin, W. Clint Hoffmann, Yubin Lan

Contact: Dr. Juan Lopez    E-mail:  juan.lopez@ars.usda.gov

Summary:

Corn earworm is a major pest of sweet corn, especially when grown organically.  Aerial application of insecticides is important for both conventionally- and organically-grown sweet corn production as sweet corn is frequently irrigated to assure return on investment given the high production costs.  Aerial insecticide application costs can be minimized through use of reduced spray rates if insecticide efficacy can be maintained at the lower spray rates.  The objectives of the study were to characterize deposition on field corn silks when applied at 5 gpa (with VMDs at 230 and 400 μm) and 10 gpa (with VMD at 400 μm) spray rates.  Applications of  the bioinsecticide, Gemstar®, and the insecticide, Entrust®, which are both approved for use in organic production, were made over three different fields.  The amount of spray material deposited on individual silks for each treatment was determined.  Deposition of spray material on the silks was very similar across all application treatments.  At the 5 gpa rate, the lower droplet size sprays generally resulted in less deposition than the other treatments.  Efficacy determinations were attempted by counting the number of larvae of different sizes per ear before and after treatment applications.  Heavy reinfestations of ear worms negated much of the efficacy work, but did point out the potential need for timely follow-up treatments.  Aspects of the results for future research and implications for aerial applications for corn earworm control on sweet corn will be discussed.    

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