Temporal distribution of heliothines in corn-cotton cropping systems of the Mississippi Delta and relationships to yield and population growth

A 4-year study (1995--1998) was conducted in Leflore County, Mississippi to estimate the temporal distribution of Helicoverpa zea (Boddie) and Heliothis virescens (F.) on wild hosts and cultivated crops, and to quantify the effects of transgenic corn and cotton plants expressing endotoxin protein of Bacillus thuringiensis on heliothine densities and crop yield. Transgenic BT and non-BT corn (Zea mays L.) varieties evaluated during 1996 through 1998 included varieties derived from Event 176 (Maximizer 457) and Event BT 11 (Northrup King 7590BT and 7639BT). Results show a significant reduction in the number of southwestern corn borer (Diatraea grandiosella Dyar) larvae observed in the BT varieties and a reduction in the percentage of plants damaged during the whorl stage of crop development. Few differences were detected in the number of corn earworm (H. zea) larvae on BT and non-BT varieties once plants began silking.;Studies comparing insect management strategies during 1995 through 1997 were utilized to explore the use of in-season cotton plant mapping and post-season yield mapping to evaluate differences between management strategy treatments. Yield mapping was utilized to group harvestable cohorts of fruit and plant mapping was utilized to approximate the week during the growing season in which the cohort of fruit began development. Results show that comparable yield can be obtained with conventional varieties managed by guidelines set by Mississippi Cooperative Extension Service as compared to transgenic BT varieties although costs associated with crop protection is generally higher for the non-transgenic varieties.;Estimates for the temporal distribution of heliothines on Geranium dissectum L., Trifolium incarnatum L., Abutilon theophrasti Medic., Lonicera japonica Thunb., Paulownia elongata L., Zea mays L., Sorghum vulgare Pers., Glycine max L., and Gossypium hirsutum L. suggest that during most of the crop growing season in the study area (Leflore Co., MS) heliothine larvae were predominantly in crops and not wild hosts. These data have important implications for resistance management strategies for heliothines. They provide valuable estimates to substantiate the need for crop-refuge areas for heliothines during the crop-growing season. A spreadsheet model was utilized to demonstrate the selection exerted on heliothine populations if more area were planted to transgenic BT corn and/or cotton based on heliothine estimates from wild hosts and agronomic crops.