Early July is a good time to scout and evaluate cornfields for corn rootworm damage. This is especially true with the wet spring conditions many fields have experienced. Rootworm larvae feed on corn roots and potentially cause severe economic loss. 

The ideal time to scout for this insect pest is around 14 days after peak egg hatch. This is when corn rootworm larvae feeding will be fresh. When scouting for corn rootworm larvae, look for root pruning by digging and cleaning root balls in a bucket of water. The white larvae are less than a quarter-inch long and will float to the surface of the water.

When assessing corn roots for damage, pay attention to nodes 4 to 6. Each of these nodes has approximately 10 nodal roots. A severe corn rootworm larval infestation can destroy most of these nodes. 

Root pruning by larvae interferes with water and nutrient uptake, and potentially makes the plant unstable. Larval injury may also increase susceptibility to root and stalk rot fungi. For every node of roots pruned, an average of 15% yield loss can be expected.

Use root ratings to assess damage

The Iowa State node-injury scale ranges from 0 to 3 and is directly related to yield loss. If the average injury is above 0.5, rootworm management strategies should be adjusted. A rating above 0.5 would include finding feeding scars and having feeding on several roots, but not eaten to within 1.5 inches of the plant. Fields with continuous corn and areas with Bt performance issues are the highest priorities when scouting.

Monitoring for corn rootworm adults should also be considered to supplement root injury assessments. Corn rootworm adults generally are found in cornfields from early July through August. Adult rootworm numbers can be assessed to anticipate next year’s potential larval injury thresholds. 

One way to assess adult rootworm numbers is by use of sticky cards. Place at least four sticky cards throughout a field from late July to mid-August and replace the cards weekly. If the number of adult corn rootworms captured is greater than two per card per day, then the field should be managed for corn rootworm larva next year.

Methods to control corn rootworm

Agronomic practices can be used to help suppress corn rootworm numbers. Some of these sustainable management strategies are crop rotation, planting Bt corn or using soil-applied insecticides. 

Rotating fields out of corn production (for example, a corn-soybean rotation) remains an effective way to manage and break up the life cycle of corn rootworms. Planting pyramided Bt trait corn hybrids for control of rootworm larvae continues to be an effective management strategy as well.

Alternatively, using soil-applied insecticides instead of planting hybrids that contain the Bt trait is a strategy that may be used to help protect corn. Several products are available, and producers should consult with their chemical dealer or ISU Extension field agronomist. 

To successfully manage corn rootworms, a long-term strategy should be developed that includes various management practices used over multiple years.

Regardless of practices used to suppress corn rootworm (e.g., crop rotation, Bt rootworm corn or soil-applied insecticides), every cornfield should be scouted for corn rootworm root injury. Fields with continuous corn and areas with Bt performance issues are the highest priority for inspection.

This article has been adapted from two previous ICM articles written by ISU Extension entomologist Erin Hodgson. Offering additional information, they are:

Michel is an ISU Extension field agronomist covering southeast Iowa. Contact him at [email protected].