Sweet Corn IPM Newsletter- No. 1- June 25, 2021
Sweet Corn IPM Newsletter- No. 1- June 25, 2021
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2021 SWEET CORN PEST SEASON BEGINS!
European Corn Borer, Corn Earworm May Be Active in Early Fields
The 2021 University of Maine Cooperative Extension Integrated Pest Management (IPM) program for sweet corn is getting underway. We continue to follow pandemic protocols, as per University policy, which requires more time and resources in order for our scouts to carry out their work safely. As a result, we have had to reduce the number of sites we’ll visit regularly. We will set up pheromone traps to monitor the adult (moth) stages of European corn borer, corn earworm and fall armyworm, and scout the fields for feeding injury by insect larvae.
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Dry-warm conditions allowed for early seeding in many fields this spring, and most plantings are off to a good start. However, many sites are now under drought conditions, and fields that are not irrigated may be under stress, which could affect pollination and ear size. Corn insect monitoring in New York and New Hampshire has already begun, and their sites are reporting early captures of European corn borer and corn earworm. This suggests that activity will begin soon in Maine, and growers with early corn at pre-tassel or beyond should initiate scouting for the presence of these pests.
European corn borer:
Pheromone traps for moths are set up in the grassy borders around cornfields. New York and New Hampshire sites are now reporting corn borer flights, and growing degree day models suggest that this flight will be peaking soon. Larval feeding damage will likely soon be found in whorl to pre-tassel corn. To monitor corn borer feeding, we examine 100 corn plants in each field, typically twenty plants in a row at five different locations. This provides a good estimate of the total amount of injury in a field.
In the early stages, European corn borer feeding damage looks like small “pinholes” in the leaves. Whorl stage corn only needs to be sprayed if fresh feeding injury is found on 30% or more of the plants scouted in a field. Once the corn reaches the pre-tassel stage, the control threshold is lowered to 15% because larvae feeding at this stage are more likely to move into the ears. On the tassels, feeding damage first appears as chewing and brown waste found in the small florets. After the tassel has emerged from the whorl, the larvae chew into the stalk just below it, often causing the tassel to fall over. Sprays during the pre-tassel stage, when both moths and larvae are present, target the larvae before they can move into the protection of the stalks and ears. Good spray coverage of the entire plant provides the most effective kill of larvae as they move from one part of the plant to another. Rotating the type of insecticide used also improves control. Materials registered for controlling European corn borer include Bacillus thuringiensis products (XenTari®, Dipel DF®), Avaunt®, Coragen®, Warrior II®, Lannate®, Baythroid®, Asana®, Radiant®, Delta Gold®, Mustang®, Sevin XLR® and Intrepid®. Newly hatched European corn borer larvae are very small and translucent with shiny black heads. They emerge from small egg masses that look like a tiny clump of overlapping fish scales on the undersides of corn leaves. European corn borer overwinters in Maine, and is usually the first pest to become a significant problem.
Growers should start scouting whorl to pre-tassel stage corn for feeding injury now. Once corn reaches the silk stage, sprays may be based on the number of corn borer moths caught in pheromone traps rather than feeding injury. European corn borer moths will lay eggs on flag leaves of silking corn, and the larvae can move into the ears without leaving visible feeding injury that would be noticed when scouting. If more than five moths are caught during a week in a field with silking corn, a spray is recommended. Varieties of corn genetically modified to produce the Bt toxin (e.g. Bt corn, Attribute® varieties), should not need to be sprayed to control European corn borer.
We are setting up pheromone traps around the state for corn earworm moths. Corn earworm generally appears in Maine in early-mid July, but the actual date varies greatly. New York sites are reporting corn earworm moths in pheromone traps this week, so it appears they may be off to an early start. The arrival of this pest is only a concern for fields with corn in the silk stage. Fields not yet in silk do not need to be protected from corn earworm. When corn earworm moths are caught at a site, all silking corn in the fields should be protected with a spray. These moths lay eggs on the fresh silks, and the larvae move directly into the ears of corn. When corn earworm moths cannot find silking corn to deposit their eggs on, they may lay eggs on the leaves or tassels of younger corn. The larvae will feed on the foliage and tassels, similar to armyworm, until the ears become available. This damage should be counted, along with any borer or armyworm damage, to determine if a spray is warranted.
This is usually the last serious corn insect pest to arrive in Maine. The moths fly in from southern overwintering sites, and tend to lay their eggs on the youngest corn available. The young larvae chew large, ragged holes in the leaves, and may bore into developing ears. Larvae can also move into the ears through the silk channel, behaving similarly to corn earworm.Pheromone trap catches will indicate if there is a threat to silking corn. However, corn is often on a spray program for corn earworm when fall armyworm is present, and both insects should be controlled. There have been no reports of fall armyworm yet in New England.
Squash vine borer moths were caught in pheromone traps in Massachusetts and New Hampshire this week, indicating that egg laying will begin soon. Squash vine borer moths are black and orange and resemble wasps. They lay their eggs at the base of squash plants. The larvae bore into the base of the plants, causing vines to wilt and collapse. Growers with squash should be on the lookout for vine borer symptoms and protect plants if moths or damage are seen. See the 2020-2021 New England Vegetable Management Guide for control options.
Potato leafhopper alert: potato leafhopper is becoming active in vegetable and berry fields. These small, bullet-shaped insects feed on plant sap from the undersides of leaves, causing the leaves to become curled, stunted and yellow-streaked. Beans are often the first crop to show symptoms, but other crops are also susceptible, including potatoes, peppers, eggplant, strawberries and raspberries. To scout for leafhoppers, brush the leaves of the plants with your hand. The small, whitish adults can be seen flying off the plant. Look for small, light green leafhopper nymphs on the underside of injured leaves. They are about 1/16 inch long. When touched, they will crawl sideways in a crab-like manner. Control options for potato leafhoppers are listed in the New England Vegetable Management Guide.
To get the most accurate information about the pest situation on your farm you should monitor the fields yourself on a regular basis. Pheromone traps and lures are available that can give you an accurate, early warning of the arrival of all of the major insect pests. Traps and lures can be purchased from pest management supply companies such as Gempler’s (1.800.382.8473) or Great Lakes IPM (517.268.5693). You may also want to download a copy of our fact sheet Managing Insect Pests of Sweet Corn which is available on our website.
David T. Handley
Vegetable and Small Fruit Specialist
Highmoor Farm Pest Mgmt. Unit
P.O. Box 179 17 Godfrey Drive
52 U.S. Route 202 Orono, ME 04473
Monmouth, ME 04259 1.800.287.0279
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