Western and northern corn rootworm are major corn pests in Iowa and surrounding states (Photos 1 and 2). Farmers have seen several management changes, including the release of four Bt-rootworm traits to suppress corn rootworm larvae since 2003. Although both species are persistent pests, western corn rootworm is particularly adaptable. The Gassmann Lab at Iowa State University (ISU) has demonstrated western corn rootworm resistance to all Bt rootworm traits in Iowa. In order to prolong the effectiveness of Bt, farmers should monitor for corn rootworm and make management decisions based on larval injury to roots or adult activity. We recommend using the 0 to 3 node-injury scale developed at ISU to evaluate larval injury. To monitor for adults, follow the guidelines outlined below. Use Pherocon AM yellow sticky traps (unbaited). Set up sticky traps at corn silking and continue through the dent stage (i.e., mid-July through August). For areas in eastern counties with suspected rotation-resistant western corn rootworm, set up sticky traps at soybean flowering and continue through full seed set (i.e., mid-July through August).
Guidelines for sampling adult corn rootworm
1. For every 10-50 acres of corn or soybean, create two transects of 6 sticky traps/transect (Figure 1). Transects should be at least 330 feet apart. Use a flag or stake to mark the transects along the field edge.
2. Place the first trap at least 165 feet in from the field edge; place remaining traps within the same row and at least 165 feet apart.
3. For corn, attach the sticky trap directly to the developing ear. For soybean, attach the sticky trap to a post approximately 18 inches above the canopy.
4. Check traps weekly and count the number of western and northern corn rootworms. Replace sticky traps weekly and adjust trap height if needed.
5. For each sample, estimate the average number of western and northern corn rootworms per trap per day. For example, 123 total corn rootworms/12 traps/7 days = 1.46 rootworms/trap/day.
6. Action thresholds in corn are 2 corn rootworms/trap/day. Consider crop rotation if action thresholds are met. If planting corn again the following growing season, use a pyramided Bt trait with Cry34/35Ab1, or a soil-applied insecticide on non-rootworm Bt corn. Action thresholds in soybean are 1.5 western corn rootworms/trap/day. If action thresholds are met in soybean, consider using a pyramided Bt trait or a soil-applied insecticide on non-rootworm Bt corn the following season.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 1, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.