Insecticide Efficacy Against Corn Earworm in Sweet Corn, 2019

There is a well-documented decline in pyrethroid and Bt trait efficacy for corn earworm (CEW) in sweet corn in recent years. There were two objectives of this study: evaluate pyrethroid efficacy and pyrethroid rotations in sweet corn and to compare insecticide efficacy in Bt-traited sweet corn. Sweet corn var ‘Obsession’ and ‘Obsession II’ was planted on 24 Jun at the Carvel Research and Education Center in Georgetown, DE at a seeding rate of 24,000 seeds per acre. All treatments were applied using a CO₂-pressurized backpack sprayer powering a single-row boom equipped with two D2 tips and #25 cores spaced 4 inches apart and offset from each other at an angle. The boom was calibrated to deliver 40 GPA at 38 PSI and was held sideways to apply nozzle output directly to the ear. The NIS adjuvant Induce (Helena Chemical Co., Collierville, TN) was added to every treatment at a rate of 1 pt per 100 gal spray volume. The first block of ‘Obsession’ consisted of 3 row plots 25-ft long arranged in an RCBD with eight treatments and four replicates. The third row was chopped out immediately prior to silking. Treatments were applied on 8 Aug, 11 Aug, 14 Aug, 17 Aug, 20 Aug, and 23 Aug. The second block of sweet corn was planted adjacent to the first and consisted of eight rows of ‘Obsession’ and eight rows of ‘Obsession II’; plots in each were 2, 25-ft rows without guard rows, three treatments, and four replicates. Treatments were applied on 8 Aug, 11 Aug, 14 Aug, 17 Aug, 21 Aug, and 23 Aug.

Treatments were applied according to university recommendations based upon nearby pheromone trap capture. Twenty-five ears from each plot were harvested on 28-Aug and graded for Lepidopteran damage. Ears with damage were categorized as clean ears, processing ears (clean ears plus ears with tip damage <1″ from the end) or damaged ears (ears with worm damage >1″ from the end). CEW were categorized as small, medium, and large. Total CEW includes worms that completed development and exited the corn, as evidenced by large feeding area and an exit hole in the side of the husk.

All treatments reduced the number of earworms per 25 ears compared to the untreated check. Avaunt eVo plots had an intermediate number of worms between the untreated and all other treatments. Baythroid XL resulted in more clean ears than other pyrethroids and was equivalent to a Besiege/Warrior rotation. The ‘IPM’ treatment resulted in intermediate protection between the best treatments and pyrethroids but was better than Coragen alone (Table 1).

In the second block, there were no differences between treatments among the two varieties in terms of the number of total worms or ear damage. The Besiege/Warrior rotation provided better protection than Warrior alone. There was no impact of the Bt trait on treatment efficacy (Table 2). No phytotoxicity was observed following any treatment.¹

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