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Sweet Corn IPM Newsletter No. 1 – June 15, 2012

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Sweet Corn

For full page print version, please see link at the bottom.  Click on photos to enlarge.


European Corn Borer Moths Active, Larvae Feeding in Early Corn

The 2012 University of Maine Cooperative Extension Integrated Pest Management (IPM) program for sweet corn is underway.  More than twenty volunteer farms will be serving as pest monitoring and demonstration sites this year, with fields in North Berwick, Wells, Hollis, Cape Elizabeth, New Gloucester, Poland Spring, Auburn, Lewiston, Sabattus, Dresden, Jefferson, Nobleboro, Warren, Monmouth, Wayne, Oxford, Farmington, Levant, Stillwater, Garland and East Corinth. Pheromone traps are being set up at these farms to monitor the adult (moth) stages of European corn borer, corn earworm and fall armyworm, and we will be scouting these fields for feeding injury by insect larvae.  We will share the information we collect at these sites and management recommendations every week during the season through this newsletter and blog.

European Corn Borer Scouting

European Corn Borer Scouting, photo by David Handley

A spell of warm, dry conditions in late April and early May allowed farmers in southern Maine to begin planting corn early through plastic mulch or under row covers.  Some of this corn is now in the late whorl to pre-tassel stage.  However, the more recent cool, damp conditions has kept development of later plantings very slow, varying from just a few leaves to early whorl stage.  We expect that the relatively mild winter may result in the early emergence of some of the major corn pests, including European corn borer and corn earworm.

European Corn Borer Moth

European Corn Borer Moth, photo by David Handley

European corn borer:  We are just starting to find European corn borer moths in the pheromone traps around the state, but activity will pick up as warmer evening temperatures arrive.  These moths will be laying eggs on the undersides of corn leaves.  The egg masses are small and look like overlapping fish scales.  European corn borer is the only one of the three major insect pests of corn that can successfully overwinter in Maine, and it is usually the first pest to become a significant problem.  To monitor corn borer, we scout 100 corn plants in each field, examining twenty plants in a row at five different locations.  This sample provides a reasonably accurate 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.  Corn in the whorl stage 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%.  This is because larvae feeding on the later stages are more likely to move into the ears of the plant.  On the tassels, feeding damage first appears as chewing and brown waste found in the small florets.  After the tassel has emerged from the stalk, 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, reduce the opportunity for larvae to move into the stalks and ears of the plant.  Once the larvae are in the stalks they are protected from sprays.  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®, Lannate®, Baythroid®, Asana®, Radiant®, Delta Gold®,Mustang® Sevin XLR® and Larvin®.  When corn reaches the silk stage, sprays may be based on the number of corn borer moths caught in pheromone traps rather than just 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 any visible feeding injury that would be noticed when scouting.  Therefore, if more than five moths are caught during a week in a field with silking corn, a spray will be recommended.  Any growers growing varieties of corn genetically modified to produce the Bt toxin (e.g. Bt corn, Attribute® varieties), should not need to spray to control European corn borer.  Very little feeding damage has been found so far on late whorl to pre-tassel corn, and no field has been over the recommended spray threshold.  We can expect more injury to be showing up soon as more eggs begin to hatch.

European Corn Borer Damage

European Corn Borer Damage, photo by David Handley

Corn earworm:  Pheromone traps are being set up around the state to monitor the arrival of corn earworm.  Corn earworm generally appears in Maine in early July, but the actual date varies greatly.  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 start being 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 of younger corn.  The larvae will feed on the foliage and tassels, similar to armyworm, until the ears become available.  When larvae are found feeding on younger corn, the damage is accounted for, along with any borer or armyworm damage, to determine if a spray is warranted.  We will be watching our traps closely to keep you informed as the season progresses.

Corn Earworm Moth

Corn Earworm Moth, photo by David Handley


Harstack Trap

Harstack Trap, photo by David Handley

Fall armyworm:  This is usually the last serious corn insect pest to arrive in Maine.  The moths must fly in from southern over-wintering sites, and tend to lay their eggs on the youngest corn available.  When the larvae hatch, they chew large, ragged holes in the leaves, and may bore into developing ears.  Larvae may 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 will usually be on a spray program for corn earworm when fall armyworm is present, and both insects would be controlled.

Male Fall Armyworm Moth

Male Fall Armyworm Moth, photo by David Handley

Common armyworm alert:  Common armyworm is often found chewing on early corn.  Like fall armyworm, this caterpillar chews large holes in young corn.  The larvae are light brown with yellow and black stripes running along the body.  This insect is usually only present early in the season and the corn can often outgrow the injury.  However, heavy infestations can occur and may require control.  This week heavy injury to field corn plantings by common armyworm has been seen in the Unity area, and states in southern New England have also been reporting heavy common army worm feeding in both field corn and hay crops.  All of the products listed for European corn borer control (above) are also registered for common armyworm.

Common Army Worm

Common Army Worm, photo by Purdue University

Common stalk borer: This pest can be a problem early in the season, but usually only around the edges of fields.  The injury is similar to European corn borer, but the feeding holes are larger, usually showing as four or five small holes running across the width of a leaf.  The larvae are purple colored with white stripes.  If high numbers of stalk borer are found in pre-tassel stage corn within the field (not just along the edges), include the injury with corn borer to determine if control is needed.  Injury found in whorl stage corn is typically not a concern because these larvae will leave the plant before the ears emerge.

Common Stalk Borer Damage

Common Stalk Borer Damage, photo by David Handley

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).

To learn more about IPM scouting techniques, insect identification and control thresholds, order the fact sheet Managing Insect Pests of Sweet Corn available from the University of Maine Cooperative Extension.  Color pictures are provided to help with insect identification, and a chart with spray thresholds is supplied to post near your sprayer for easy reference.  You can download a copy from our website at: or call the Pest Management Office at 1.800.287.0279.

Sweet Corn IPM Weekly Scouting Summary

Location CEW Moths ECB Moths FAW Moths %ECB Damage Recommendations / Comments
Cape Elizabeth I 0 0 0 No spray recommended
Cape Elizabeth II 0 2 0 0% No spray recommended
New Gloucester 0 7 0 0% No spray recommended
North Berwick 0 0 0 1% No spray recommended
Poland Spring 0 5 0 11% No spray recommended
Wells I 0 1 0 1% No spray recommended
Wells II 0 4 0 0% No spray recommended

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:


David T. Handley
Vegetable & Small Fruit Specialist

Highmoor Farm                      Pest Management Office
P.O. Box179                          491 College Ave
Monmouth,ME 04259            Orono,ME 04473
207.933.2100                        1.800.287.0279

Published and distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914, by the University of Maine Cooperative Extension, the Land Grant University of the State of Maine and the U.S. Department of Agriculture cooperating.  Cooperative Extension and other agencies of the U.S.D.A. provide equal opportunities in programs and employment.

Where brand names or company names are used it is for the reader’s information.  No endorsement is implied nor is any discrimination intended against other products with similar ingredients.  Always consult product labels for rates, application instructions and safety precautions.  Users of these products assume all associated risks.


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