Sweet Corn IPM Newsletter No. 1- June 26, 2020

Sweet Corn IPM Newsletter No. 1- June 26, 2020

Sweet Corn

Click on photos to enlarge.


European Corn Borer, Corn Earworm May Be Active in Early Fields

The 2020 University of Maine Cooperative Extension Integrated Pest Management (IPM) program for sweet corn is getting underway. New protocols, to protect against COVID-19 infection, will require more time and resources in order for our scouts to carry out their work safely.  As a result, we will have to reduce the number of sites we visit regularly this season. 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. We’ll share the information we collect along with management recommendations through this newsletter. It will also be posted on the Highmoor Farm web page blog Sweet Corn IPM Newsletter. If you would prefer to receive this newsletter via e-mail, send an e-mail message to caitlin.ramsay@maine.edu.


Early seeded corn got off to a slow start due to very cool, wet conditions this spring.  A couple of late frosts damaged corn that was up in late May, and now many fields are experiencing high temperatures and drought conditions, which could affect pollination and ear size.  Corn insect monitoring in Massachusetts 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:

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

Pheromone traps for moths are set up in the grassy borders around cornfields. Both Massachusetts and New Hampshire sites are now reporting early corn borer flights, although still in moderate numbers.  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 on more mature corn plants 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.


Corn Earworm:

Corn Earworm Larvae
Corn Earworm Larvae; photo by David Handley

We are setting up pheromone traps around the state for corn earworm moths. Corn earworm generally appears in Maine in early July, but the actual date varies greatly. New Hampshire and Massachusetts sites have captured 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, the damage should be accounted for, along with any borer or armyworm damage, to determine if a spray is warranted.

Fall Armyworm:

Fall Armyworm Moths
Fall Armyworm Moths (female right, male left); photo by James Dill

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

Three Squash Vine Borer Moths
Three Squash Vine Borer Moths; photo by Lindsey Ridlon

moths were caught in pheromone traps in Massachusetts and New Hampshire this week. Counts were above the spray threshold of 5 moths have already occurred, suggesting that this pest is now a significant threat to squash and pumpkins. 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 now 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.


Do-It-Yourself IPM:

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 at 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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