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Time to scout corn rootworm

This year's late start for the corn crop means common pests are tardy in their arrival and potential damage. That's no more true than with corn rootworm, which could be starting to cause damage in fields now. That makes it critical to start scouting for root damage, entomologists say.

Rootworm larvae are present in fields now, and adults could start causing economic damage in fields that are silking, say Iowa State university entomologists Erin Hodgson and Aaron Gassman in a university report.

Larvae root damage is the biggest concern right now and calls for closest monitoring. Where you have lodging corn plants, you may have rootworm larvae present.

"Evaluate root injury to better understand the efficacy of your management program. Monitoring over several years will help establish a historical record of how larvae respond to management tactics," according to Hodgson and Gassmann. "One common outcome of severe larval feeding is lodging of corn plants. However, it is important to confirm that feeding from corn rootworm was the cause of lodging and that it did not result from other factors such as severe winds."

Iowa State University has a scale for evaluating corn rootworm damage based on the number of root nodes that have been pared back by the pest larvae. Ranging from 0 to 3, the scale reflects the number of nodes harmed, and anything above 0.25 (approximately 3 roots) reflects economic damage.

"The Iowa State University root-injury scale is linear and directly related to plant lodging and yield loss. Root injury that exceeds 0.25 is likely causing economic loss," Hodgson and Gassmann say. "For Bt hybrids, any injury ratings that exceed 1.0 would be considered unexpected."

If you think you're facing potential adult corn rootworm damage, look for a couple of things. First, check your leaves. You might see some scarring, a sign that adult cutworms are present. But the real damage is inflicted on the silks, where the adults will congregate and interrupt pollination.

"Corn rootworm will feed on leaves and cause some scarring; however, this does not cause economic loss. Adults can cause yield loss if emergence occurs when corn is silking, and therefore, this is a critical time period to scout fields," according to Hodgson and Gassmann. "Adults are strongly attracted to silks and will mass on plants to feed and mate. Adults that trim silks during pollen shed will interfere with optimal pollination."

A couple of conditions can add to the potential for rootworm damage, the entomologists say. First, if your corn's under some drought stress, those plants won't be able to withstand as many adult rootworms, especially when they're pollilnating. A plant under ideal conditions can tolerate up to 15 adult corn rootworm beetles while one under drought stress can only take on five.

And, those fields that were planted later in the spring are more susceptible to economic rootworm damage than those planted earlier.

"Late-planted fields or late-flowering hybrids are generally attractive to adult corn rootworm. Silks will still be developing in these fields when older fields have brown or drying silks," according to Hodgson and Gassmann. "Adults may migrate and aggregate in these later-maturing fields."

If you've got an infestation on your hands, consider a quick foliar insecticide treatment. "With adult corn rootworm becoming active during silking this year, we highly recommend keeping an eye on fields in order to protect yield. A foliar insecticide may be warranted if there are five or more beetles per plant, silks have been clipped to less than one-half inch of the ear tip, and pollination is not complete," say Hodgson and Gassmann. "Also take into consideration other insects that may be feeding on the silks at the same time (e.g., Japanese beetle)."

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