Mild weather leads to fewer pests, unusual damage to crops

WEST LAFAYETTE, Ind. - Lower insect populations have led to minor crop damage this summer, but a Purdue Extension entomologist says farmers should still watch for anomalies when scouting.

Part of what has caused fewer insects has been the weather. Indiana temperatures have been mild, and rainfall has been timely for most of the growing season. Most importantly, the crops had good conditions for growth during the critical period just after planting.

"We've had a good summer for growing conditions," Christian Krupke said. "We've had rain, and plants just aren't as vulnerable."

Some insect pests that still have caused some damage to Indiana crops this summer are bean leaf beetles in soybeans and armyworms in corn.

Krupke has received reports of bean leaf beetle populations from all over the state, but the damage is more severe in northern counties. Some reports have described minor pod scarring, which isn't of much concern. Other reports, however, have included pods clipped from stems.

 "It's weird because you usually don't see the pods fall off," he said. "The beetles often feed on pods, but not on the peduncle, which connects the pod to the stem."

Bean leaf beetles are usually yellow, tan or red in color and about one-quarter inch long. Their distinguishing feature is a black triangular mark on the wing covers directly behind the neck area, or thorax.

If growers find bean leaf beetle damage in their fields, they should consider treating if the pods are still green, beetles are actively feeding and about 5-10 percent or more of the pods are damaged. More information on the bean leaf beetle can be found at http://extension.entm.purdue.edu/fieldcropsipm/insects/bean-leaf-beetle.php

Krupke said entomologists had anticipated more soybean aphid colonization by this time but have not seen many fields over the 250-aphid per plant threshold so far. Even so, he encouraged producers to continue scouting for the small, greenish-yellow pest.

One of the biggest surprises has been finding late-season armyworm damage to cornfields in northern Indiana. The pest doesn't pose much threat to corn yields this late in plant development, but it's unusual to see armyworm damage this late.

"It's a curiosity," Krupke said. "We worry about it in the spring when plants are tiny. When the plants are large, the worms are just feeding on the leaves so there's no damage on the important part - the ear."

In general, he said no control of late-season true armyworm is required in corn. Yield will not be affected by loss of lower leaves and armyworms do not feed on the ear.

With that said, Krupke emphasized that it is important to be sure the infestation is true armyworm and not fall armyworm. The latter usually feeds in the upper kernels of the corn ear in addition to the leaves.

More information about fall armyworm is available at http://extension.entm.purdue.edu/fieldcropsipm/insects/fall-armyworm.php

A full-grown true armyworm larva is dull-green to brown in color with stripes running the length of its body, while the adult armyworm is a pale brown moth with a white dot in the center of each forewing.

Farmers with insect damage can find more management recommendations on Purdue Extension's Field Crops Integrated Pest Management website at http://extension.entm.purdue.edu/fieldcropsipm/

Writer: Amanda Gee, 765-494-8402, agee@purdue.edu

Source: Christian Krupke, 765-494-4912, ckrupke@purdue.edu

Ag Communications: (765) 494-2722;
Keith Robinson, robins89@purdue.edu
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