Impact of corn borer subject of study

Right now, both European and Southwestern corn borers are chewing up someone's Mississippi corn. A lesser evil, the European corn borer can be found around Leland and Batesville and in a few other pockets.

But Southwestern corn borers have been found all the way from Natchez to Canton to Corinth, says Don Parker, Mississippi Extension entomologist. “We're seeing more and more of them across the state. They're focused in the Delta area, but are spreading,” says Parker, who spoke at Monsanto's Center of Excellence Field Day in Yazoo City on Aug. 14.

Corn borer primer

The corn borer first arrives during the spring. As corn emerges, the first generation moves in. They lay eggs on the leaves, which look much like minnow scales. The larvae will hatch, feed on the leaves for a few days and then bore into the plant.

During the whorl stage of corn, it's very important for farmers to watch for borers, says Parker.

“Borers can cause dead heart. That means the larvae penetrates and kills the growing point of the plant, which then shuts down.

“I know people don't think this can happen in Mississippi, that the borer problem won't grow. But that isn't true. In 1995, I found a field of corn that was 60 percent dead heart.”

If you go out and see little shot holes and think it might be corn earworm, make sure it is. Don't assume anything, says Parker.

“Make sure you find larvae of one kind or another. Southwestern corn borer feeding can mimic earworm. Be careful.”

The second generation of borers moves into the corn near tasseling. Whenever the plant has an ear, you'll start finding eggs plus or minus three leaves from the ear. Parker says that's where 95 percent of the borers lay their eggs.

“When I go scouting for borers, I generally grab two leaves on the plant. First, I check the ear leaf and go one up. On the next plant, I grab the ear leaf and go one down. I'm not sampling all the leaves, but I'm doing enough to get a percentage idea.”

When the eggs hatch, the worms feed on the leaf a bit and then move behind the leaf sheath. They stay there for eight to 10 days and then bore into the plant.

The trick is killing them before they get inside the plant. Timing is extremely critical because you've got to get the insecticide out before they bore into a place chemicals can't reach.

Yieldgard offers a potential way of controlling them without having to worry about the application problem. However, because of refuge requirements, farmers still have to plant conventional varieties.

“That means whether they want to or not, farmers will have to decide whether to spray a foliar application. As mentioned, timing is very critical.”

Once the pest bores into a plant, it hollows out the center. Nutrients are then cut off and the plant can't get food to the ear.

“Corn borers can damage the ear, too. They'll bore into the shank. When they do that, the ears often fall to the ground.”

The third generation used to be considered the most damaging, says Parker. That's not the case anymore because farmers often plant early enough to get the crop out before the third generation causes lodging.

However, if a farmer has late corn, the third generation will bore all the way down into the base of the plant.

“They get below ground level. Once they've done that, they come back above the soil a couple of inches and girdle the stem. They build a roof and then wait for the wind. When a breeze comes, the plants will break over.”

Corn borers overwinter as larvae in the base of the plant. If you're suspicious about having borers, now is a good time to check, says Parker. If you just hip back on them or let them be, “they'll have a good time overwintering and hit you hard during the first infestation next season.”


Parker says he's been asked questions about pheromone traps and how they help control corn borers. “The traps help monitor populations and give us a read on when moths are flying and how many there are. When we start seeing moths is when we start looking for eggs. We use the traps to know when to start scouting.”

Scouting for corn borers is fairly hard, says Catchot. If there's not a fairly big flight, you might be stumbling around in a field for a while before finding eggs.

“And if you've never had traps on your land before, things can become alarming quickly, especially late season. That's when moth numbers can reach phenomenal levels.”

Parker says he's experienced that firsthand. “One year, I had a trap in Morgan City, Miss. In three days there were over 3,000 moths in it.

“This year, a farmer called me worried because he had caught 56 corn borer moths in a trap. I told him that number was still low and to wait for higher numbers before getting too worried,” says Parker.


So how important is it to control Southwestern corn borer in your cornfields?

“A study we're conducting is looking at that using Bt corn along with its parent lines of non-Bt. That's very important because we want to make sure the yield responses we're measuring are accurate — that any disparity is due to insects and not the difference between Bt and non-Bt,” says Parker.

The large plot test is in 11 locations across Mississippi. Parker and colleagues are still collecting data, but preliminary research shows that, across all locations, they've found no Southwestern borers in the Bt varieties.

“In the non-Bt varieties, averaged across locations, we averaged between 67 and 42 percent infestation. Of those fields infested, the tunnel length is averaging between 9 inches and 14 inches.

“For example, at the Shelby, Miss., location, 78 percent of the non-Bt plants were infested. The average tunnel length within the plants is 17 inches. There's some serious damage going on.”

Yields have ranged between 183 and 194 bushels. So far, that means no statistical difference is obvious between the two systems.

This study is still in the early stages, says Monsanto's Angus Catchot. “When we start getting more data from these 11 locations, we'll probably label each as light, moderate, or heavy with borer pressure. Then we can see what the yield data says from each.

“In the end, what we're hoping to do is see what economic damage Southwestern corn borer does in Mississippi's corn crop. This is valuable information for growers,” says Catchot.

Varieties and spraying

Up until recently, there haven't been any good Roundup Ready corn varieties, says Catchot. However that's changing.

“We have several varieties that are yielding and looking good. One is Dekalb's DK 687 RR and another Roundup Ready is DK 6410 (which is in Mississippi State University trials this year). Terral also has some good Roundup Ready varieties.”

Catchot says there are three basic systems with which to grow Roundup Ready in the state. One is a total post program — Roundup only. You're allowed 1.6 pints of Roundup UltraMax, which is equivalent to the 2-quart rate of Ultra.

“You can apply Roundup on Roundup Ready corn anytime from planting to V-8 stage or 30 inches tall. That's a pretty wide window,” says Catchot.

The second system is a pre-herbicide tank-mixed with Roundup and applied post. “For example, you might go with atrazine and Roundup UltraMax as a delayed pre-somewhere before the 12-inch stage.”

The third program is a half rate of pre-material applied at planting. That will still leave you with two shots of Roundup later.

“Those three programs tend to evolve according to what works best in a particular field. I've seen situations where one shot of Roundup is all we need. That isn't normal, though. What will likely happen is as farmers plant more Roundup Ready corn, they'll end up tailoring the three options to their own operations,” says Catchot.

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