Top Three Caterpillar Sweet Corn Pests
May 9, 2012 
With all of the abnormalities going into this growing season, sweet corn growers have to wonder what to expect with insects. One thing is certain: caterpillars, more specifically the European corn borer, the corn earworm, and the fall armyworm, are the ones to watch out for.
Galen Dively, Extension specialist at the University of Maryland, breaks down the situation, explaining that all three of these pests invade the ear, making them difficult to control with insecticides. If the pests aren’t properly controlled, ear quality is affected, jeopardizing the crop’s marketability.
“If they were just feeding on the plants, like aphids or leafhoppers,” he explains, “they wouldn’t be affecting the marketable part of the plant. These worms get in the ear, though, and cause direct damage, taking you off grade. No one wants to open the ear and have a worm in there.”
Preemptive Measures
To avoid the consumer’s horror of opening an ear of corn to the warm welcome of a happily feeding worm, growers have a few options. Two of the three pests, the armyworm and the corn borer, give growers a slightly longer window of time to act. Laying their eggs on the leaves as opposed to directly in the silk (like the corn earworm), these pests move around a bit before they enter the ear. The earworm, however, enters the husk in a matter of hours after hatching, thus making it important that growers spray in a timely manner.
In order to halt an invasion, there are a few steps that growers can take. The first step on Dively’s list is choosing a variety with tight tip coverage. “The ear architecture is very important as it makes it more difficult for insects to invade the ear,” he explains.
The second step is to adjust planting dates accordingly, which can help growers avoid a high-pressure pest situation. Planting early means pests might not arrive at the optimal feeding time, and planting late could mean trouble as the worms may be out in full force.
A very late planting, however, may be to the grower’s advantage. “Really late” Dively specifies, “would mean corn coming off [plants] in the middle of October, which is the latest that sweet corn can be harvested.” The idea behind planting so late in the season is that nights will be cooler, resulting in a reduction in moth activity.
Gathering Information
Paying attention to planting dates, though, doesn’t mean growers are relieved of their scouting duties. To determine a spray schedule, Dively emphasizes that growers must make their decisions based on the information they gather from the field, not the time of year it is.
“This is important,” he stresses, “because you could save a lot of money in a light pressure year by spraying less, and make yourself a lot of money in a heavy pressure year by spraying more and saving your crops.”
For example, he says that the number of moths caught in pheromone traps can tell growers exactly how often they should spray. He also suggests the use of light traps for the same purpose.
In addition, he says many states have pheromone networks and data available online through pest management programs that can help growers determine when they need to take action. For those who opt to use their own traps to collect data, Dively encourages them to contribute their specific information to these websites so all growers in the area will have up-to-date information. One site Dively recommends is www.pestwatch.psu.edu, which is handled by Shelby Fleischer, an entomologist at Penn
State University.
Immediate Action
The information gleaned from scouting dictates when spraying should occur. “Some years, growers need to treat even before the crop silks — before fall armyworm can get in there,” says Dively. “Once corn starts to silk, and you have moth activity that justifies a treatment, you have to get in there and spray regularly. Once the worm gets in the husk, there is nothing that can be done.”
Explaining that not all corn silks at the same time, and that it can take three to four days for all of the ears to silk and be pollinated, Dively says that growers have to spray enough to cover new emerging silks, so when the worms hatch and feed on the silk tissue, the insecticide residue kills them. “Because there is continuous exposure of fresh silk,” he says, “you have to have a tight spraying schedule.”
What is being sprayed is just as important as when it is sprayed, says Dively. Pyrethroids are effective, but because they are so widely used, growers have been facing some resistance issues. By using products with different modes of action, or premixes, the incidence of resistance can be reduced.
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