Mortality of the corn earworm (Lepidoptera: Noctuidae) on sorghum panicles in Georgia

A 3-yr on-farm study was designed to evaluate grain sorghum, Sorghum bicolor (L.) Moench, as a trap crop for the corn earworm, Helicoverpa zea (Boddie) in cotton. The goal of this paper was to construct partial life tables for H. zea on panicles of the sorghum. Abundance of H. zea per ha on sorghum panicles varied among years and planting dates, but the heaviest infestation of H. zea eggs occurred when this pest was dispersing from a corn (Zea mays L.) field into the sorghum trap crop located between the corn field and a cotton field. Percentage total real mortality (rx) for H. zea on sorghum was very high, exceeding 99% for each year and planting date. Percentage real mortality for H. zea was highest for eggs with 1st-instars in second place. Percentage total apparent mortality (qx) was high (70-85%) for corn earworm eggs and 3rd, 4th and 5th instars and moderately high (45-50%) for 1st and 2nd instars. Trichogramma pretiosum Riley and Orius insidiosus (Say) were the predominant beneficial insects of H. zea in the sorghum traps. Other predators observed feeding on H. zea included the big-eyed bug, Geocoris punctipes (Say), spined soldier bug, Podisus maculiventris (Say), green lynx spider, Peucetia viridans (Hentz) and lady beetles (mainly Hippodamia convergens Guérin-Méneville). Percentage parasitization by T. pretiosum increased linearly over the three sorghum planting dates in both 2001 and 2002 strongly suggesting that the sorghum trap crop served as a refuge and nursery for this parasitoid throughout the growing season. The number of O. insidiosus per plant was highest for the first planting date compared to the other two planting dates. Congregation of O. insidiosus in this first planting date of sorghum could have been a response to localized aggregation of H. zea in the trap crop. In conclusion, conservation of natural enemies in sorghum resulted in the trap crop becoming a sink for H. zea, preventing populations of the pest from increasing in the sorghum trap crop and then dispersing into cotton.