Corn Earworm Alert for Minnesota Sweet Corn, Tomatoes, and Green Beans

Bill Hutchison, Eric Burkness & Suzanne Wold-Burkness
Minnesota Extension IPM Program, Dept. of Entomology, UMN, St. Paul campus

This past week, our trapping network detected the first significant increase in corn earworm (CEW), Helicoverpa zea, moth flights at for two locations: Blue Earth (MN/IA border), and Owatonna.

In previous years, Blue Earth is often the first location to experience the first increase in moth flights, and this year’s catch is right on schedule for early August. Moth flight numbers averaged over 12 per night at Blue Earth (as of 8/13), and over 5 per night at Owatonna. Our action threshold to initiate sprays for CEW in sweet corn is when trap catches average more than 5 moths per trap per night for 2 consecutive nights, and sweet corn is between 15 to 50% silk (for the first spray).

Corn earworm, reflecting a diverse color variation, and damage to sweet corn.

Subsequent sprays should be based on continued moth flight activity with sprays applied approximately every 5-7 days prior to harvest (see below). As noted previously, CEW is one of our economic pests that cannot overwinter in MN but must migrate into the state each year to colonize crops. Sweet corn and field corn are the most preferred crops, but in the Midwest region, tomatoes and green beans are also readily attacked by CEW if sweet corn is not available. Growers should also realize the CEW moths can easily enter greenhouses and high-tunnels (e.g., for tomato), when the sides are left open for ventilation.

While the species cannot overwinter in Minnesota, it migrates from southern states and moth flights have been present, again at low numbers, across the state in June and July: (for information read VegEdge: https://www.vegedge.umn.edu/moth-data/cew-info).

CEW larvae can range in color (Fig. 1). CEW larvae do not typically bore into the ear like corn borer, but if not managed, the slightly larger larvae (>1 inch when fully grown) will consume 10-15 kernels per larva during larval development. European corn borer (ECB) moth flights have been quite low this year (< 2 moths/wk) for all southern Minnesota locations.

Managing Corn Earworm

CEW moths also lay most of their eggs directly on fresh silks once the eggs hatch (4-6 days in summer), the larvae quickly enter the ear tip, where insecticides are no longer effective. Therefore, scouting is typically not feasible, and insecticide spray timing is often based on moth counts from pheromone traps (e.g., ‘Scentry’ or ‘Hartstack’ trap, with ‘Hercon’ lures, recommended; see resources, below) while fresh silks are present.

The goal is to protect the newly emerging silks and ear, with protective insecticide applications, to prevent larvae from entering the ear. Although some CEW resistance to the synthetic pyrethroids has been observed, recent results were promising. Based on efficacy data from 2018, we found that medium to high rates of pyrethroids (Warrior, Mustang Maxx) worked well. The pyrethroids should also be rotated with one of the diamide class products (e.g., Coragen) to minimize the risk of further resistance in CEW. Please use best practices when applying insecticides in order to minimize their off-target impact on pollinators and beneficial insects.

Other Treatment Options

We also recently completed research confirming that Minute Pirate Bug (Orius spp.) is commonly present in MN sweet corn (particularly on silk tissue), and is an effective predator of CEW eggs laid on silks. However, Orius will be more active in Bt sweet corn fields, where less insecticide is used.