2014 Sweet Corn Pest Season Begins!

European Corn Borer Moths Active, Larvae Feeding in Early Corn

The 2014 University of Maine Cooperative Extension Integrated Pest Management (IPM) program for sweet corn is underway. More than twenty volunteer farms are serving as pest monitoring and demonstration sites, with fields in North Berwick, Wells, Dayton, Cape Elizabeth, New Gloucester, Poland Spring, Auburn, Lewiston, Sabattus, Nobleboro, Warren, Monmouth, Wales, Wayne, Oxford, Farmington, Levant, Stillwater, Garland and East Corinth. Pheromone traps have been set up at these farms to monitor the adult (moth) stages of European corn borer, corn earworm and fall armyworm, and we have begun scouting the fields for feeding injury by insect larvae.

European Corn Borer Scouting, photo by David Handley

SITUATION

Farmers with well-drained fields were able to get some early plantings going in spite of a rather cool, wet start to the season. Early seedings started under clear plastic or fabric row covers are now in the pre-tassel to tassel stage, with one southern field already starting to show some silk. In spite of the wet conditions during the spring, some fields are starting to get a bit dry, so the recent rain was quite welcome. Warmer weather predicted for next week should speed up plant development. European corn borer moths have been active in some fields, as evidenced by some light feeding damage we have seen in some pre-tassel fields this week.

European corn borer:  We have set up pheromone traps for European corn borer this week, and we’ll start reporting on moth activity in the next issue. We found some feeding injury and very small larvae in pre-tassel fields this week, indicating that moths are now laying eggs on the undersides of corn leaves. The egg masses are small and look like overlapping fish scales. European corn borer overwinters in Maine, and 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 provides a good estimate of the total amount of injury in a field.

European Corn Borer Moth, photo by David Handley

European Corn Borer Damage on Pre-tassel, photo by David Handley

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 the later stages 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 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®. We found corn borer feeding injury in pre-tassel fields in Sabattus, Farmington, Livermore Falls, New Gloucester and Nobleboro this week; but only the field in Nobleboro exceeded the injury threshold. 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:  Pheromone traps are now 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.

Corn Earworm Moth, photo by David Handley

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