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Sweet corn growers beware of corn earworm; scout frequently

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Written Thursday, June 19, 2008  

Recent storms from the Gulf of Mexico have brought not only rains, but also corn earworm moths, said a Purdue University expert, and sweet corn growers need to be vigilant spraying to protect their crop.

"These corn earworm moths are here several months earlier than normal," said Rick Foster, Purdue Extension entomologist. "We are seeing counts that are much higher in our traps than typical and the ramification for that is damage to sweet corn - at a time we don't normally see it.

"Vegetable growers, particularly sweet corn growers, that have corn or crops in a vulnerable stage, which means green silks are present on the plant, need to be spraying now."

Foster said one producer in northern Indiana grows sweet corn for the July 4 market and he checked his crop a few days ago, only to find 50 percent of his corn ears infested by the pest.

"Growers typically don't spray for corn earworm until August," he said. "Generally when the corn earworm moths arrive there are a lot of crops that are very attractive for them to lay their eggs on and that's not the case now.

"This year, sweet corn that's silking will be like a magnet drawing in all the moths in an area. Even relatively low numbers of moths flying around can cause a lot of damage because they are all coming to one field to lay their eggs.

Corn earworm, also known as the tomato fruitworm or cotton bollworm, is a pest that can feed on a wide variety of crops. They typically feed on sweet corn, tomatoes and soybeans, but Foster expects to see them on some crops not typically associated with corn earworm like pumpkins, blueberries, pears or peaches. Other crops that earworm will feed on include cabbage, beans, melons, squash, peppers, many other vegetables, alfalfa, clover, grapes, small fruits, plums, roses, snap dragons, zinnias and other flowers.

Foster, who placed corn earworm pheromone traps around the state on June 9, didn't expect much and was surprised to find 178 moths in the trap by June 11.

"In most years, we would catch just a couple of moths, at most, during this time of year," he said. "Generally, we consider 10 moths per night to be the threshold for treatment and we are catching much more than that in these traps."

The numbers we caught are historically high and potentially very damaging; however the catches in most of the traps are starting to go down, he said.

When scouting the fields, growers should know that earworm larvae come in many different colors ranging from green and yellow to pink and orange with light and dark strips running lengthwise on the body. The head capsule is light brown and they grow to be nearly 1 to 1.5 inches long when mature. Corn earworm larvae usually enter corn ears through the tip, not through the side or shank like European corn borers and fall armyworms. They prefer to feed on the tips of corn ears, tomato fruit and bean pods.

Foster has traps around the state and cooperators report counts on a daily basis. The 2008 corn earworm pheromone trap catches are recorded online at http://extension.entm.purdue.edu/cornearworm/index.php . The blank columns are locations Foster hopes to have pheromone traps at in the near future.

For more information about corn earworm or sweet corn management with corn earworm, visit http://extension.entm.purdue.edu/publications/E-31.pdf . For additional information, contact Foster at (765) 494-9572 or rfoster@purdue.edu .

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