Corn rootworm (CRW) is a pest that, if left unmanaged, can cause economic damage in most of the Corn Belt. Damage can result from root feeding while they are in larval form and from adult beetles clipping silks during pollination.

Beetles are attracted to and feed on pollen and corn silks. Females lay egg clusters at the base of the plant. The female will return to feed on silks and pollen until frost kills off both sexes.

Eggs have a period of suspended development called diapause that prevents hatching during the winter or during dry periods when energy sources are scarce. After dormancy, when soil temperatures reach above 52° F, eggs will begin to develop. Rootworm larvae hatch after 680 to 770 GDU accumulation in the spring.

TIP: When you start seeing lightning bugs at night, corn rootworm larvae feeding usually begins.

After the larvae emerge, they go through three instars defined by the length of the larva. NCRW and WCRW usually require four to eight weeks to finish the third instar. In SCRW, it only takes two to three weeks to develop through the third instar.

Most economic damage to corn occurs in the root systems during the larval stage.

Root damage due to larval feeding may cause plants to show signs of nutrient or drought stress. In severe cases, the plant will remain short and enter reproductive growth early. Pollen shed and silk elongation take massive amounts of water, and the reproductive efficiency of corn suffers, ultimately reducing yield. After the third instar the larvae go through pupation, then adult CRW beetles emerge from the soils.

Adults feed on the silks of corn plants, and can reduce pollination, resulting in reduced kernel set and lower yield. The corn plant will continue to push silks out until the kernel is pollinated for up to about 10 days. Treatment of adult CRW beetles is usually not an effective method for reducing CRW populations, but severe cases of silk clipping occasionally warrant control. Silks clipped back to 0.5 in. or less with less than 50% pollination complete is cause for treatment.

These are multiple strategies to assess how your rootworm control program is currently working and whether or not to make changes.

Dig up an individual corn plant, leaving the soil around the root intact

Raise and lower the plant in the water to loosen and remove soil

Leave the roots in the bucket for two to three hours

Larvae will float to the water surface. Observe the instar stage to project the immediate threat to the crop

Dig up a plant and remove the soil from the roots

Nodal roots develop in rings around the stalk — one at each node. Evaluate how many nodal and crown roots have been damaged or trimmed back by feeding within 1.5 in. of the crown. If all of the roots at one node have been pruned, assign the first digit of the score a one; if two nodes are pruned, a two

If only a fraction of the roots are damaged, use a decimal

A score of 1.5 means that one set of nodal roots is completely destroyed, as well as half of another

Feeding Assessment Scale
(Rating and Damage Description)

1.0 | One full node of roots has been pruned

1.75 | One full node as been pruned with 75% of second node pruned

“Pruned” is defined by roots that have been chewed on to within 1.5 in. of the crown/stalk

Corn rootworm causes the largest economic loss by an insect to corn growers in the Midwest. Corn growers concerned with rootworm damage should consider the following management.

Plant corn hybrids with pyramided root worm traits. Using hybrids with multiple effective traits will reduce inseason damage to corn. In heavy pressure areas, hybrids that have the Rnai trait technology should be considered.

Consider adult beetle control measures. Insecticide applications to control adults in both corn and soybean fields( western variant) will reduce larve load the following season.

Rotate away from host crops. Cucumbers, melons, and pumpkins are all host to corn rootworm.

In high pressure areas stacking control options of rootworm traited hybrids and soil applied insecticides may be warranted. Be sure to read insecticide labels to make sure the product is an effective option for controlling corn rootworm.

Knowledge is key. Performing rootworm surveys is crucial for success. Know what species of rootworm is present in your field, and preform root digs and floats to measure pressure.