Experts warn against possible earworm pest damage

Published 8:45 pm Thursday, August 17, 2017

As soybean farmers get ready for harvest this fall, experts are also warning them to keep an eye out for earworms.

Corn earworms are native to the majority of North America, and while they usually feed on corn, other crops and produce are also at risk, including cotton. Most recently, North Carolina farmers have noticed higher numbers of the pest in soybean crops.

“Defoliation, leaf feeding of soybean leaves by insects, is the most commonly observed type of soybean insect injury,” a report from the University of Nebraska–Lincoln Institute of Agriculture and Natural Resources states. “Soybean defoliation is one of the most readily observed types of insect injury to soybean and can occur from emergence to harvest.”

Beaufort County is one of the top five counties that grow soybean acres in North Carolina. For this reason, continued defoliation could lead to substantial losses to the crop if allowed to get out of hand, said Dr. Dominic Reisig, associate professor and Cooperative Extension specialist at N.C. State University.

“I spoke with one of the largest growers in (Beaufort County) this morning who said that populations were spotty, but definitely around in many fields. Where they were present, populations were really heavy and required spraying,” Reisig said. “Our biggest problem is usually with damage to pods and seeds within the pods. Of course, growers harvest those seeds in the pods, so they need to be protected.”

(University of Nebraska-Lincoln)

When it comes to spraying for earworms, though, the correct plan of action requires farmers to use caution in assessing the situation.

Before the soy plant blooms, the Cooperative Extension sets the threshold at 30-percent defoliation before spraying is recommended to avoid yield losses. The threshold after blooming is 15 percent. These thresholds are set to help farmers manage the cost of spraying in comparison to possible economic loss caused by the pests.

As for after blooming, a study funded by the North Carolina Soybean Producers Association showed no definite correlation between the number of earworms in this stage and the eventual crop yield.

“My recommendation is to only treat earworm in blooming soybeans if they are present at the podding threshold levels and if the plants are stressed,” Reisig wrote on the blog. “If the plants are not stressed, they will compensate for earworm feeding even at high populations, perhaps by retaining flowers, creating more seeds per pod, or filling out heavier pods.”

To help combat the potential damage caused by earworms in other ways, farmers have begun to plant narrower rows of soybeans and plant the crop earlier, Reisig said.

In some cases, though, spraying is still necessary to prevent yield loss, and thus financial loss, he said.

“On average, corn earworm is the most expensive insect pest, in terms of damage and cost of control, to soybeans and widespread insect pest of soybeans in Beaufort County and the state,” Reisig said. “As a rough estimate based on last year’s losses and what we’re seeing this year, from soybeans alone, losses and cost of control might be around $1.5 million in the county for this year.”