by Ric Bessin, Kentucky Pest Newsletter 01/09/2006

Dr. Rick Weinzierl reported last week in the Illinois Fruit and Vegetable News that there appear to be pyrethroid resistant corn earworm moths in Illinois, Indiana, Wisconsin, and Minnesota. There has been a dramatic decrease in control observed in university conducted efficacy trials with pyrethroid insecticides in sweet corn. They report that some growers report unacceptable levels of 'wormy ears' with later harvests even though spray schedules have been tight. The pyrethroids used on sweet corn include Pounce, Capture, Mustang Max, Warroir, Proaxis, Baythroid, and Asana.

Corn earworm is a pest of a number of different field and vegetable crops, but here in Kentucky it is a serious pest of sweet corn, tomatoes, soybeans, and field corn. In sweet corn, pyrethoid insecticides have been used extensively for control to prevent ear infestations. Lost of effectiveness of this class of chemistry is of serious concern as there are alternatives that are as economical and effective. While the resistance issues have been identified to our north, this is a concern for Kentucky growers as corn earworm does not overwinter in these states. It migrates from areas in the south to reinfest these states each year. Problems encountered in these states are likely to occur here as well.

What should Kentucky sweet corn growers do in 2006? Growers need to consider all of their alternatives. In many parts of the state, pyrethroid insecticides may continue to perform very well. Growers should continue to monitor corn earworm with pheromone traps and maintain flight records.

Resistance is not the only reason why sprayed ears may be found wormy at harvest. Spray timing and coverage are factors that growers must continuously monitor. Initial sprays for corn earworm should be applied as soon as fresh silks become visible. Additional sprays are generally needed at 2 to 5 day intervals until the silks become dry, with the number of days between sprays dependent on pheromone trap counts. After the silks dry, no additional sprays are needed for corn earworm. Spray equipment should be inspected to ensure that the best possible coverage of spray is provided. Sprays should be directed to the ear zone of the plant using hollow cone nozzles. Drop nozzles on either side of the row are recommended. Early planted sweet corn typically has much less corn earworm pressure than later plantings. In Kentucky, sweet corn harvested after early harvest will likely encounter more intense earworm attack.

Bt sweet corn can be used to reduce earworm damage, particularly with later plantings. Bt sweet corn does not provide 100% control, but is greatly reduces the reliance on insecticides alone. Studies have shown 85 to 95 % control with these Bt lines. For 2006, additional sweet corn types will be available with Bt protection.

Insecticide alternatives to the pyrethroids for sweet corn are limited, but include Lannate, Larvin, Sevin, and Spintor. Growers will need to watch their crop carefully in 2006 to continually evaluate the levels of control they are obtaining with pyrethroid sprays.