With the introduction of the Boll Weevil Eradication Program (BWEP) in Georgia and the adoption of the use of transgenic Bt cotton, cotton insect control has taken on a new dimension. Growers are seeing a new spectrum of insect pests beginning to emerge as a major economic factor in cotton production. With fewer insecticide sprays being applied to Bt cotton, questions have been raised regarding insect thresholds, and economic impact. Should over sprays be used on Bt cotton? Will yield be increased with the use of over sprays? Is one type of insecticide superior to another for late season, non-traditional insect populations?

This study was designed to determine the effects of over sprays on Bt cotton. The field study was conducted in Dodge and Worth Counties, Georgia in 1997 and 1998 and in Dooly County, Georgia in 1998. Single replicate large strip plots were planted with transgenic Bt cotton. Four treatments were used in the trial: Untreated, methyl parathion (Penncap M), Tracer, and pyrethroid (Karate and Baythroid). Two treatments were made in each plot ten to twenty-one days apart beginning in late July and ending in mid-August. This time frame coincides with traditional dry down of corn in Georgia. Three insect pest were targeted for this study: bollworm, fall armyworm, and stink bug. Plots were scouted weekly for four to six weeks after the initial treatment. Insect populations were noted and bolls were collected to determine the percent damaged bolls due to stink bugs. Lint yield was determined at harvest.

Insect populations generally increased in the absence of insecticide sprays. Bt cotton worked well to control tobacco budworm and performed well against bollworm. However, low levels of bollworm and fall armyworm were present. The pyrethroid treatment significantly reduced the bollworm population when compared to the untreated plot. In addition, Tracer and the pyrethroid treatments numerically reduced the fall armyworm populations when compared to the untreated plots. Though the stink bug populations did not exceed the one per six feet of row treatment level, the use of methyl parathion and pyrethroid treatments significantly reduced the stink bug level when compared to the Tracer and the untreated plots. In addition, the percent damaged bolls was reduced where stink bug control insecticides were used. Even though the threshold of one per six feet of row was not reached, greater than twenty percent damage was experienced where control was not effected. Yield increases were observed in the three treated plots, with the pyrethroid showing a significant yield increase over the other three treaments.

The use of pyrethroid sprays on BT cotton decreased bollworm, fall armyworm and stink bug populations and would be the preferred treatment for late season insect control. Cotton yields increased when insecticide treatments were used on Bt cotton with pyrethroid sprays providing a significantly higher yield when compared to the other treatments. The use of scouting to determine the necessity for insecticide sprays remains the number one method of determining environmentally safe and effective insecticide scheduling. More study is needed relating to stink bug thresholds and treatment triggers.