Delays in the planting season have corn and soybean fields at varying growth stages across the Midwest. Some may wonder what this means for pest control.

Typically, late-season bug attacks can be thwarted simply because of the maturity of a crop. However, small plants later in the season cannot hold up under this increased pressure.

Kevin Rice, University of Missouri Extension entomologist, says farmers need to be aware of what pests are out there. He finds one new pest creating problems across the heartland and a few others causing more damage this year.

Soybean concerns

The thistle caterpillar is flying in from the south. This caterpillar overwinters in Mexico and flies thousands of miles to establish populations. Rice says there may be an uptick in numbers because of the air stream movement and storms this spring.

This caterpillar is brown and black in color with yellow striping on both sides. It has spiny hairs on its body that gives it a prickly appearance. It measures 1¼-inch long. As an adult, the thistle caterpillar morphs into the butterfly known as the painted lady.

These pests tend to feed along soybean field edges. Economic thresholds that require treatment are 30% at vegetative stage and 20% in reproduction or post-bloom. Once a thistle caterpillar hits chrysalis, the damage is done, so look for a cocoon.

STRIPPING YIELD: Soybeans usually can withstand an attack from the Japanese beetle. However, with delayed planting, the plants are young and susceptible.

One regular soybean offender is causing damage to reach economic thresholds quicker.

Rice says traps are capturing Japanese beetles by the hundreds in fields. Emergence will continue to peak for the next three weeks.

Although the Japanese beetle doesn’t typically cause much yield damage in soybeans, that may not be the case this year. The economic threshold is 30% leaf defoliation at vegetative stage. Because soybean fields were planted late, beans are a lot shorter, and leaf damage may be quicker with the same number of beetles. Farmers should consider increasing chemical applications or treat early to control beetles.

Corn problems

Typically, late-planted cornfields are ones hardest hit by armyworm.

“Like an invading army, the armyworm shows up overnight, marches across one field after another, causing devastation,” Rice says.

STOP THE INVASION: Left unchecked, armyworms can stay in a corn plant, eating the tassel and then the corn ear.  

Farmers should walk fields and take note of any corn plants with holes in the leaves, as this could be an indication of armyworm feeding. Left unnoticed or untreated, larvae stay in the corn plant and continue to feed until young corn ears are developed. Once young corn ears are available, armyworms can feed on corn ears leading to no grain production on that stalk.

Whether in corn or soybean fields, Rice says farmers should scout often — preferably weekly — to assess insect problems. Then contact a county Extension agent or company agronomist to discuss treatment options.