Sweet Corn IPM Newsletter No. 3 — July 14, 2023


Fall Armyworm Moths and Larvae Found in Several Fields

Early planted corn is now silking in much of southern Maine, while early-mid season corn is between whorl and pre-tassel. Many fields are showing signs of nitrogen and/or zinc deficiency, which is a result of the excessive rains leaching these nutrients out of the soil, while the heat encourages rapid plant growth. This makes it difficult for plant nutrient uptake to keep up with demand. There were fewer moths in traps this week, but silking corn is still at risk in most fields.

European Corn Borer Larvae on Pre-tassel Stage Corn
European Corn Borer Larvae on Pre-tassel Stage Corn; photo by David Handley

European corn borer: Larval feeding injury was found in over half of the sites visited this week. Fields in Wells and Biddeford were over the control threshold of 15% in pre-tassel stage corn. We have only caught one corn borer moth in our traps, but we expect to see more in the coming weeks, which may present a threat to any silking corn. Moths will lay eggs on flag leaves of silking corn, and the larvae can move into the ears without leaving visible feeding injury. Therefore, if more than five moths are caught in our traps during a week where there is silking corn, a spray will be recommended.

Corn earworm: The number of corn earworm caught this week was down from last week, but will still require protection in any silking corn for most locations. Fields not yet in silk do not need to be protected from corn earworm. We have seen some earworm feeding on whorl to pre-tassel plants, which will happen when silking corn is not yet available. This damage is counted, along with any borer or armyworm damage, to determine if a pre-silking spray is warranted. A 4-day spray interval for silking corn was recommended for one site in Wells. A 6-day spray interval was recommended for Auburn, Cape Elizabeth, Dayton, Farmington, Gray, Lewiston, and one site in Wells.

Fall Armyworm on Corn Leaf
Fall Armyworm on Corn Leaf; photo by David Handley

Fall armyworm: Moths were caught in pheromone traps at two sites, Auburn and Cape Elizabeth, this week, but not over threshold for silking corn. If more than 3 moths are caught in pheromone traps in a field with silking corn, a spray would be recommended. Some larval feeding damage has been found in pre-tassel corn. This is combined with any European corn borer injury found to determine if the 15% injury threshold has been reached. Sprays were recommended for fields in Wells and Biddeford, but only because injury from corn borer was also present.

Squash vine borer: We have been catching squash vine borers in pheromone traps around the state. Growers with squash should be on the lookout for vine borer symptoms and protect plants if moths or damage are seen. See the New England Vegetable Management Guide for control options.

Cucurbit downy mildew alert: With ongoing rainy weather there is now a high risk for cucurbit downy mildew throughout southern Maine and beyond. The strain being found primarily infects cucumber and cantaloupes. Producers of these crops should apply protectant fungicides prior to any future storms. These are listed in the New England Vegetable Management Guide . Chlorothalonil or copper products are protectant fungicides that most growers probably already have on-hand. It may be worth switching to fungicides with specific activity against CDM. They can also be found in the New England Vegetable Management Guide. Monitor your crops regularly, and keep an eye out for symptoms of CDM. Excellent photos and more information about this disease can be found on Cornell’s vegetable resources page.


David T. Handley
Vegetable & Small Fruit Specialist

Highmoor Farm
P.O. Box 179
52 U.S. Route 202
Monmouth, ME 04259

UMaine Extension Diagnostic
Research Lab, Pest Mgmt. Unit
17 Godfrey Drive
Orono, ME 04473

Sweet Corn IPM Weekly Scouting Summary

Location CEW Moths ECB Moths FAW Moths %Feeding Damage Recommendations / Comments
Auburn 3 0 2 8% 6-day interval recommended for silking corn
Biddeford 1 0 0 20% One spray for ECB feeding on pre-tassel corn
Bowdoinham 0 0 0 2% No spray recommended
Cape Elizabeth 2 1 2 6% 6-day interval recommended for silking corn
Dayton 3 0 0 0% 6-day interval recommended for silking corn
Farmington 6 0 0 3% 6-day interval recommended for silking corn
Gray 2 0 0 0% 6-day interval recommended for silking corn
Lewiston 2 0 0 0% 6-day interval recommended for silking corn
Oxford 1 0 0 0% No spray recommended
Sabattus 3 0 0 0% 6-day interval recommended for silking corn
Wayne 0 0 0 2% No spray recommended
Wells I 2 0 0 17% 6-day interval recommended for silking corn
Wells II 14 0 0 2% 4-day spray interval for silking corn

CEW: Corn earworm (Only fresh silking corn should be sprayed for this insect.)
ECB: European corn borer
FAW: Fall armyworm

Corn Earworm Spray Thresholds for Pheromone Traps

Moths caught per week Moths caught per night Spray Interval
0.0 to 1.4 0.0 to 0.2 No spray
1.5 to 3.5 0.3 to 0.5 Spray every 6 days
3.6 to 7.0 0.6 to 1.0 Spray every 5 days
7.1 to 91 1.1 to 13.0 Spray every 4 days
More than 91 More than 13 Spray every 3 days

Thresholds apply only to corn with exposed fresh silk. Lengthen spray intervals by one day if maximum daily temperature is less than 80°F.

European Corn Borer Thresholds

Whorl stage: 30% or more of plants scouted show injury.
Pre-tassel-silk: 15% or more of plants scouted show injury.
Silk: 5 or more moths caught in pheromone traps in one week.

IPM Web Pages:
Explore Integrated Pest Management in Maine (UMaine)
Integrated Pest Management Data Visualization Tool (Penn State)
Integrated Pest Management (UMass Amherst)

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