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Late-Season Corn Earworm Alert: Focus on Sweet Corn

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

As noted in mid-August, corn earworm moth flights usually do not increase substantially until late summer, and throughout September. Consequently, only the latest maturing hybrids, or late-planted sweet corn is more vulnerable to egg-lay and larval infestations in ear tips.

This past week, our trap network cooperators observed a substantial increase in moth counts in south-central and southeastern Minnesota (Fig. 1, Blue Earth, Owatonna, Rosemount). Moth numbers at these locations are much higher than our standard “action threshold” of  >5 moths per night for at least two consecutive nights (measured via pheromone traps).

When this threshold is reached and silking sweet corn (fresh/green silks) is present, it will be highly attractive to female moths for egg-lay. Although CEW cannot overwinter in the upper Midwest, the moths readily migrate north each summer/fall, from the southern U.S. on wind currents.
Figure 1. Corn earworm moth flight increase for southern Minnesota, Sept. 1, 2019.
For conventional growers, once moth catch increases, protective insecticides should be applied approximately every 5 days during warm weather (highs >85), to 7 days with the cooler temperatures (highs in 70s) we have recently experienced. Insecticide groups should be rotated between the pyrethroid (e.g, Warrior) and diamide (e.g., Coragen) groups as much as possible, to minimize the risk of resistance development. The goal of a CEW management plan is to minimize larval infestations >5% per field, as shown in Fig. 2.
Figure 2. Corn earworm larvae and frass infestation in mature sweet corn ear; note color variation, which is common within the species.
For more information on management of CEW in sweet corn, see the VegEdge UMN pest profile on CEW.

The moth flight page should also be viewed for recent updates.

Organic growers can rely to some extent on naturally occurring generalist predators, common in Minnesota, primarily the minute pirate bug (Orius spp.), the multicolored Asian lady beetle (Harmonia axyridis) and other lady beetle spp. Also, Dipel, a non-genetically engineered form of Bacillus thuringiensis (Bt, natural bacterium) may still be available in some areas of the state, and can be effective if applied every 3-5 days.

Historically, the European corn borer (ECB) was the most consistent insect pest of Minnesota sweet corn. However, due to the ongoing area-wide suppression effect, correlated with the use of Bt field corn in MN and the Midwest region, ECB infestations have remained quite low in sweet corn. This year at our research plots at Rosemount (UMORE Park), we have found only 3 ECB larvae in 600 ears sampled to date, where the plots reflect multiple planting dates of sweet corn hybrids. This information along with historically low ECB moth flights indicate the suppression effect is still active in Minnesota, which is a benefit to sweet corn growers.

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