Rates of Seed Treatment Insecticides and Control of Stewart's Wilt in Sweet Corn

Neonicotinoid insecticides applied as seed treatments reduce the incidence of Stewart's wilt. The objectives of this study were to examine the efficacy of different rates of seed treatment insecticides to control Stewart's wilt on susceptible sweet corn hybrids and to compare the economic value of Stewart's wilt control in sweet corn grown for processing and fresh market. Clothianidin (Poncho), imidacloprid (Gaucho), and thiamethoxam (Cruiser) applied to seed at rates ranging from 0.125 to 1.25 mg a.i. per kernel were evaluated in 11 field trials in Illinois and Delaware from 2000 to 2003. Incidence of Stewart's wilt was significantly lower when seed was treated with insecticides than when plants were grown from nontreated seed in all but one trial. The level of control usually was between 50 and 90%. Small but statistically significant differences in incidence of systemically infected plants occurred among rates of insecticides in all trials except those in 2001. Usually, incidence of systemic Stewart's wilt was lower when higher rates of insecticides were applied; however, increasing the rate of insecticides from 0.125 mg a.i. to 1.25 mg a.i. per kernel had a relatively small effect on the level of Stewart's wilt control compared with the difference between treated and nontreated sweet corn seed. Based on a regression analysis, the lowest rates of the insecticides provided 64 to 72% control. The level of control increased about 1.85% with each additional 0.1 mg a.i. of insecticide per kernel from 0.125 mg a.i. to 1.25 mg a.i. Clothianidin provided an 8 or 9% higher level of control than thiamethoxam or imidacloprid at the same rate. Recommendations for application of seed treatment insecticides to processing and fresh market sweet corn differed somewhat due to substantial differences in the value of the crops. Based on estimated costs of $6 to $12 per 0.4 ha for the seed treatments, the economic break even point (i.e., cost of control = value from control) occurred in the range of 3 to 6% Stewart's wilt incidence for processing sweet corn valued at $325 per 0.4 ha and at about 1% Stewart's wilt incidence for fresh market sweet corn valued at $1,625 per 0.4 ha. Relatively small differences in levels of control conferred by commercially available rates of clothianidin (0.25 mg a.i. per kernel) and thiamethoxam (0.125 mg a.i. per kernel) were of little consequence in processing sweet corn but had considerable economic value in fresh market sweet corn.