One of the more persistent and economically important field crop pests is corn rootworm. In Iowa, there are two species of concern: western corn rootworm and northern corn rootworm.

While corn rootworms tend to be more of an issue with continuous corn production, all corn in Iowa is at risk for infestation. Scouting for these pests allows the following:

In Iowa, rootworm egg hatch typically begins the end of May and continues through mid-July. As eggs hatch in soil, larvae make their way to corn roots where they start feeding. Their feeding can eventually cause root pruning that interferes with water and nutrient uptake. Additionally, severe pruning can make plants more vulnerable to lodging.

By late June and early July, adult corn rootworm beetles start to emerge in fields and will feed on pollen, green corn silks and leaves. Silk clipping is the major concern with adults, as that can interfere with pollination. The female rootworm beetles lay eggs later in the summer, which overwinter in soil and hatch the next year to produce the root-feeding larvae.

Scout for rootworm injury

An ideal time to get out and scout for larval feeding on corn roots is about 10 to 14 days after peak egg hatch while the injury is still fresh. Use accumulated degree days to determine when peak egg hatch occurs in your area. ADD is similar to tracking heat units for corn growth and development. Peak egg hatch occurs between 684 to 767 ADD of soil temperatures using the base temperature of 52 degrees F.

Go to the Iowa State University Agronomy Mesonet website to generate a map showing ADD for corn rootworm egg hatch. Set the start date to Jan. 1, 2020, use the current date for the end date, and make sure the plot parameter is set to “soil growing degree days (base = 52).”

To assess corn roots for the presence of larvae and evaluate for rootworm feeding, dig up at least one corn plant from 10 random locations in a field. When digging up a plant, a flat shovel or spade works well, and you’ll want to leave about a 6- to 7-inch circle of soil around the corn plant to preserve as much of the roots as possible. Cutting the cornstalk off above the roots can make it easier to handle and work with the plants.

Check for larvae

Once the plants are collected, check for the presence of larvae by either one of these two ways

To assess the corn roots, use the ISU 0 to 3 Node-Injury Scale where:

With this scale, damage in between complete nodes being pruned back is noted as a percentage of the node that is missing. For example, a root assessment of 1.25 would mean that one complete node is missing, and another quarter of a node has been pruned back. Each node pruned back due to rootworm feeding can result in 15% yield loss.

Make management decisions

While there are no specific management recommendations based on the number of larvae found on plant roots, this does provide insight on rootworm density, especially if the field was previously planted to soybeans. It can also raise awareness on how big the larvae are, how much more injury they will do, and how rootworm management tactics are working.

This should be used in conjunction with the ISU 0-3 Node-Injury Scale. If the average root injury is 0.5 on the 0-to-3 node-injury scale, it is suggested you adjust your corn rootworm management strategy. This may include crop rotation, using a soil applied insecticide with a non-corn rootworm Bt-traited hybrid, or rotating the traits being used.

Monitor adult rootworm beetles

Another way to monitor corn rootworm activity in a field is the use of sticky traps (cards) to collect adults. The window for monitoring adult corn rootworms in a field is from when the corn starts silking until it reaches the dent or R5 stage, which is typically mid-July through August.

Check the moth traps weekly to determine the number of rootworms per trap per day. Count the number of western and northern corn rootworm adults separately. After counting, replace with a new sticky trap. Also be checking for potential silk clipping that could interfere with corn pollination.

The beetle counts from the sticky cards help to guide future management decisions. In corn, if you have two or more corn rootworm beetles per trap per day, you have reached the action threshold. In soybeans the action threshold is 1.5 WCR beetles per trap per day. If action thresholds are met, consider planting a corn hybrid with a pyramided Bt trait (with Cry34/35Ab1 as one of the traits) in that field the next year. Or use a soil-applied insecticide on a non-rootworm Bt corn hybrid the following growing season.

Also, if the action threshold is met in corn, consider crop rotation. Plant soybeans or another non-corn crop in that field the next year.  Regarding silk clipping, applying a foliar insecticide may be warranted if you have five or more beetles per plant, silks have been clipped to less than a half-inch from the ear tip, and pollination is not complete.

Sometimes corn rootworm populations develop resistance to the methods used to try to control this insect pest. If you suspect you have resistance to a corn rootworm Bt trait or a soil-applied insecticide, reach out to your local ISU Extension field agronomist.

Resources to manage pest

For more information on managing corn rootworm here are three helpful resources:

Vittetoe is an ISU Extension field agronomist in east central Iowa. Contact her at [email protected].