Pinney Purdue Virtual Field Day: Protect against western corn rootworm

WANATAH, Ind. — Western corn rootworm damage is appearing in conventional, unprotected corn varieties this year.

Nationwide, around 85% of corn is a BT hybrid that protects plants from pests, said Christian Krupke, Purdue Extension entomologist.

Conventional corn, however, has nothing preventing rootworms from feeding on it.

“What we’ve seen is that you can get by doing (conventional corn) for a year or two, possibly even three, but at some point you’re going to have enough damage and enough rootworm feeding that you’re going to have corn that’s lodged in the field,” Krupke said.

“That’s what we’ve seen this year. With some of the heavier rains in the past few weeks, followed by some heavier winds, we’ve seen a lot of lodged corn.

“That’s one indicator that rootworm feeding has been heavy. By the time you see that, it’s too late to do anything.”

If you suspect you have rootworm feeding, it’s important to dig up corn roots and look for signs of larval feeding, Krupke said.

“As we stand here in early to mid-August, it’s a little bit late because there’s been some regrowth and you won’t see the feeding as well, but if we did this about a month ago it would be the perfect time,” he said.

Although it’s too late this year, farmers may want to consider inspecting roots next summer.

Checking for root damage is fairly straight forward.

Dig up the corn roots, chop the stalk off and rinse the root system in a five-gallon bucket of water to remove soil.

It’s a good idea to test multiple areas of the field, Krupke said.

“In an area with a lot of lodging, five or 10 plants would be a minimum, but more would be better,” he said.

“What you want to look for is, on some of the interior roots, signs of feeding and scarring due to insect feeding. There’s nothing else that would cause scars or feeding marks in a corn root. The root worm would be the only insect you would find causing that kind of damage.

“You might see a lot of fine roots and newer growth. This often happens after root damage to rootworm feeding if you have moist conditions, which we did.”

Grooves and dark brown, necrotic tissue are a sign that damage has scarred over.

“That root dig has to be done in a timely manner,” Krupke explained. “Mid-July to late July is the period that you’d see the damage. After that, you’ll see regrowth and you’ll have to do too much guesswork.”

Farmers have seen increased rootworm cases this year in conventional corn, which isn’t surprising, according to Krupke.

“What we want to focus on is what to do about it,” he said. “Your first option is rotating to another crop like soybeans.

“In a soybean field, assuming volunteer corn is controlled, you’ll have a lot of rootworm eggs hatch, they’ll look for food source, they’ll only find soybeans and they’ll die. That’s the most effective technique.”

In fields that are continuous corn, granular and liquid insecticides can help.

Learn more about western corn rootworm at https://extension.entm.purdue.edu/fieldcropsipm/insects/corn-rootworms.php.