Ames, IA (August 19, 2024) - Corn earworm is a pest that causes direct injury to corn ears. Corn earworms do not survive winters above the 40th parallel but some migrate north to the Corn Belt every year. Economic injury in field corn is uncommon due to the high adoption rate of Bt traits in Iowa (e.g. Cry1Ab, Cry1F, Cry1A.105+Cry2Ab2, Vip3A). However, non-Bt sweet corn and popcorn is at risk every growing season, and the financial impact in these specialty crops is especially important.
In 2024, two pheromone trapping projects in Iowa (one by Ashley Dean and the second by Craig Able at the USDA) confirmed adult captures in May. This is earlier than usual in Iowa, which resulted in early-planted sweet corn being infested with corn earworm when it is typically not at risk. Many commercial and residential growers noticed unprecedented infestations and injury from corn earworm larvae this year.
Pyrethroids are the most common insecticides used to protect ears. There are pyrethroid-resistant corn earworm populations confirmed in sweet corn in the southern U.S. and mid-Atlantic region for more than a decade. Unsurprisingly, there is anecdotal evidence of pyrethroid resistance in Iowa following frequent exposures in sweet corn.
In July, collections of corn earworm larvae were made at two commercial farms in Iowa following reports of corn earworm injury after foliar insecticide applications. Brad Coates at the USDA performed genetic screening for pyrethroid resistance (gene cyp337b3). One farm, which had applied pyrethroids several times in 2024, had 100% pyrethroid-resistant individuals while the second farm, which had not applied a pyrethroid in 2024, had 57% pyrethroid-resistant individuals. This mechanism of resistance has not been confirmed for corn earworm before, and therefore a significant finding.
With this documentation of pyrethroid resistance, we strongly recommend alternating the mode of action used for corn earworm in sweet corn and popcorn. We’ve created a downloadable table for reference in sweet corn in Iowa.
According to the Midwest Vegetable Guide (pages 289-291), treatment is justified when moths are being caught in traps while green silks are present. In general, the higher the moth catches, the shorter the interval between sprays. If fewer than 5 moths are being caught per night, a five-day spray interval should be adequate. As moth catches approach 50 to 100 per night, a two- to three-day spray interval would be more appropriate. If no field corn in the area is silking, moths will lay eggs primarily on silking sweet corn. In this situation, use a threshold of 1-3 moths per trap per night. Stop treating for corn earworms when 90% of the silks are brown. Use high volume and pressure (30 psi), and direct nozzles to the ears. Corn ear pests are difficult to suppress and some susceptible survivors are expected.
Authors: Erin Hodgson, professor; Ashley Dean, agriculture and natural resources extension specialist II; Integrated Crop Management News, and Iowa State University Extension and Outreach.