Insecticide Efficacy Against Corn Earworm in Sweet Corn, 2021b

Corn earworm (CEW) is the primary pest of sweet corn in the mid-Atlantic. Since 2008, pyrethroid resistance has been documented through adult vial bioassay, small plot research trials, and grower experiences of damage incidence when using pyrethroids alone. The objectives of this trial were to determine whether there are efficacy differences among the pyrethroids, evaluate different pyrethroid tank mixes, and evaluate different spray programs. Sweet corn var ‘Overland’ was planted at the Carvel Research and Education Center in Georgetown, DE (Sussex County) on 15 July with a seeding rate of 24,000 seed per acre. Plots consisted of three 20-ft rows on 30-inch centers and arranged in a CRB design with 12 treatments and 4 replicates. 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 was added to every treatment at a rate of 1 pt per 100 gal spray volume. Two treatments were initiated approximately seven days prior to silk (designated A, B), all others were initiated at 25% silk. Treatments were applied on 28 August (A), 1 September (B), 4 September, 7 September, 10 September, 13 September, 16 September, and 19 September (1–6). On 24 September, 25 ears from each plot were harvested and graded for Lepidopteran damage. Ears were categorized as clean, processing (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 a large feeding area and an exit hole in the side of the husk, as well as fall armyworm, which comprised 1.5% of the worm complex. Data were analyzed with ANOVA, and means separated according to Tukey–Kramer HSD.

All treatments reduced worm counts and ear damage (Table 1). Among tested pyrethroids, a three-way combination resulted in greater ear protection than Warrior II alone, all other pyrethroids and pyrethroid mixtures were intermediate. Besiege alternated with Baythroid XL performed slightly better than Besiege applications first followed by Baythroid XL applications. Besiege alternated with Baythroid XL provided the greatest protection, followed by all other multimode of action spray programs. Spray programs that were initiated prior to silking did not perform quite as well as those initiated at silking, but each had a diamide component application during silk.¹

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