This 2-yr on-farm study was designed to evaluate the ability of grain sorghum, Sorghum bicolor (L.) Moench, to serve as a trap crop for the corn earworm, Helicoverpa zea (Boddie), by attracting corn earworm females into the sorghum as they emerged from cornfields. Three plots of sorghum trap crops and three equally sized plots of cotton, Gossypium hirsutum L., trap crops were planted in a strip between a commercial cornfield and a commercial cotton field. The cotton field adjacent to the trap crop plots was divided into field cotton plots associated with the sorghum trap crop plots and field cotton plots associated with the cotton trap crop plots. Three commercial cotton fields adjacent to corn, but without trap crops, also were sampled. The number of corn earworm eggs per plant and the percentage of plants with corn earworm eggs was higher in the sorghum trap crop plots than in the cotton trap crop plots for both years, demonstrating that corn earworm females preferred to oviposit in the grain sorghum over cotton. A higher percentage of plants with corn earworm eggs was found in cotton in control fields compared with fields with trap crops, indicating that the grain sorghum trap crop was not the source of corn earworm. An economic threshold of 5% corn earworm young (first and second instars) was exceeded more times for cotton in control fields compared with cotton in fields with trap crops. Thus, for two seasons the grain sorghum trap crops helped reduce the need for insecticide applications for this pest. Percentage of parasitization by the egg parasitoid Trichogramma pretiosum Riley and the number per plant of the predator Orius insidiosus (Say) were higher in the sorghum trap crop plots than the cotton plots. However, the grain sorghum trap crop plots were not sinks for these natural enemies. We conclude that grain sorghum could serve as an effective trap crop for corn earworm in cotton.
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