Dig Corn Roots Now to Check for Injury

Refresher Course on How to Dig for Corn Rootworm

Root pruning is indication corn rootworm larvae were at work earlier in the season and additional management steps will be needed in coming years. (DTN photo by Gil Gullickson)

It's hot. It's dry. And the last place farmers likely want to be is in a corn field, shovel in hand, digging up corn plants.

Still, corn rootworm doesn't sleep during sizzling summers.

"When it comes to corn rootworm, take a proactive approach," said Safeer Hassan, Bayer corn systems manager. "What you do this season can affect what happens [with corn rootworm infestations] next season."

Safeer and other Bayer scientists and agronomists held a root-dig demonstration in late July at Bayer's Atlantic, Iowa, facility. They pointed out that root digs particularly help farmers assess future corn rootworm infestations in corn-on-corn situations. That's because the odds of future corn rootworm infestations in corn-on-corn exceed those of rotated corn.

However, first-year corn doesn't get a pass. Northern corn rootworm can attack first-year corn in areas of extended diapause, where damage-causing larvae hatch in fields planted to soybeans or a non-host crop the previous year. The western corn rootworm variant can also infest first-year corn. This variant migrates from corn fields to feed on pollen, flowers and foliage in neighboring fields planted to soybeans or other crops. Eggs laid by the variant hatch the following year, and the resulting larvae can damage roots if corn is planted in that field. Volunteer corn can also attract egg-laying females.

The optimum time to do a root dig is pre-tassel or at tasseling. Wait too long, and roots may regrow and mask rootworm injury, said Edwin Benkert, Bayer seed and trait technology development representative.

Chances are your fields are past that tasseling stage this year, but there's still plenty of reason to dig. This time of the year, rootworm management should be focused on how to manage rootworms in the next corn crop or to prioritize harvest on fields with compromised roots.


First, target areas of lodged or goosenecked corn for root digs, added Benkert. Goosenecking results when plants have root damage and lodge, but are still following corn's ability to grow straight, he said.

Once in the field away from the edge, select 10 plants at random and dig straight down 4 to 6 inches away from the plant. Avoid digging at an angle, as this could cut injured roots, said Benkert.

"Sometimes, you have to dig on both sides to get it [the plant] out," Benkert explained. "You don't want to hear the snapping sound of roots breaking off in the soil."

Once popped out of the ground, Benkert advises cutting away the plant's top portion with clippers from the root mass for handling ease.

"Gently knock the soil off with your boot or the ground," he added. "The key is to remove some soil so there is not as much to wash off later. But you don't want to break off any roots in doing so."

Next, soak the roots in a five-gallon bucket of water for at least 15 to 20 minutes and, if needed, overnight. This further removes soil as does subsequently washing them with a garden hose, Benkert said.


Roots are now ready to check for damage. "When corn rootworm feeds on roots, we call it pruning," Benkert said. Pruned roots exhibit a brown necrotic color. In severe cases, corn rootworm can prune roots back to the stalk.

These researchers use an Iowa State University 0-3 damage rating scale that's based on roots attached to nodes (circular rings) around the stalk.

"With the 0-3 sale, we use 1.5 inches [of feeding] as our criteria on nodes 4-6 inches [long]," Benkert says. "We count feeding that is 1.5 inches or less from the base of the stalk," Benkert said. "Count the number of roots on each node that meet that 1.5-inch criteria and divide by the total number of roots on each node. Do this for roots of all 10 plants. The average number will be between 0 and 3. Zero equals a healthy root system, with no feeding. A 3 rating has all nodes with (root) feeding at 1.5 inches or less."

Even a 1 rating has serious consequences, as it corresponds to a 15% to 17% yield loss, Benkert said.

It's also advisable to sample other areas of a field, even if damage is not apparent. "Just because you don't see a problem doesn't mean you don't have a problem," said Andrew Penney, a Bayer technical agronomist based in Ames, Iowa. "Last year was wetter, and when you have more soil moisture, you may not know you have a problem unless you have a wind event."


Safeer advises corn farmers to consult with their crop advisers about changes in next year's management tactics for next year with ratings of 0.5 and above. Examples may be planting a pyramided trait package if none was planted this year or rotating to a non-host crop such as soybeans.

"There is no silver bullet for corn rootworm control," he said. "It's a combination, and not just one solution."

"The root digs we've done have been interesting," said Josh Carlson, who farms with his father Jim and brother Ben near Gowrie, Iowa. "You can go out to some fields thinking there is some root feeding and there's none. But in other fields, you find out you will need to rotate to soybeans or try a different trait next year."

Confused about what hybrids contain in the way of traits? The Handy Bt Trait Table helps sort through the corn trait maze. Find it here: https://www.texasinsects.org/…

For a DTN article about using The Handy Bt Trait Table go to:


For a recent blog post on northern and western corn rootworm go to: https://blog-crop-news.extension.umn.edu/…