It could be worse than usual this year
Insects have already caused an unusual amount of damage to some crops this year. And it's not over yet. Be prepared for soybean pod feeding.
"Start checking soybean fields during the last half of August and continue through early September," recommends University of Illinois entomologist Mike Gray. Bean leaf beetles, grasshoppers and stinkbugs all are able to cause yield loss in soybean fields during the pod-fill stage.
"We run into some pod feeding every year," reports Dave Harms, owner of Crop Pro-Tech, Inc., a crop consulting firm headquartered at Bloomington, IL. "Generally, 10% of the fields get near the threshold level for treatment, and we end up spraying 1-2% of the fields. However, this year we expect more damage than usual because the first generation of bean leaf beetles was very high."
Harms says bean leaf beetles, in his experience, typically do the most damage to pods. Stinkbugs are the next most destructive, and grasshoppers can be very harmful in a dry year.
Bean leaf beetles have been a pod-feeding annoyance in northern Iowa in recent years, although they've usually been below threshold levels, says crop consultant Hal Tucker of Storm Lake. But Tucker cautions that they could be more of a problem this year.
Gray says bean leaf beetles normally begin feeding on pods after the leaves start to lose their green color. The beetles scrape the green tissue off the pods but don't chew through the pod walls. However, scars on the pods leave an opening for excessive moisture and secondary pathogens. Seeds can become discolored and moldy.
Grasshoppers are even more aggressive. They often chew directly through the pod and bite the beans or eat entire seeds, Gray notes.
If more than 5-10% of the pods are injured by bean leaf beetles (and there are 10 or more beetles per foot of row and leaves are green) or grasshoppers, an insecticide application may be warranted, says Gray.
Insecticides labeled for both pests include Asana XL, dimethoate, Lorsban 4E, Penncap-M, Sevin XLR Plus, and Warrior T or IE. Asana XL, Penncap-M, and Warrior T or IE are restricted-use products and can be applied only by a certified applicator.
Chewing insects like bean leaf beetles and grasshoppers find pods less attractive as they turn yellow to yellow-brown, says Purdue University entomologist John Obermeyer.
Stinkbugs pierce rather than chew. They suck juice from the pods.
"The injury is difficult to assess because their mouth parts leave no obvious feeding scars," Gray explains.
However, the feeding can be quite damaging, says Obermeyer. If the damage occurs early, beans may abort. Older pods may contain discolored or malformed seeds.
Because stinkbugs can continue to damage the beans well after leaves turn yellow and pods turn yellow-brown, the fields need to be scouted through that period.
Both Gray and Obermeyer say treatment may be warranted if there is an average of one or more stinkbugs per foot of row during pod fill. Insecticides labeled are Penncap-M and Warrior T or IE.