Checking for corn rootworm feeding and potential root injury should occur every year and in every cornfield. Both non-Bt and Bt-traited corn hybrids should be checked to determine if management strategies need to be changed for next year.
The optimum timing for scouting and evaluating cornfields is from mid-July through mid-August. Primary fields of concern and the fields in which to start are continuous cornfields and areas with Bt performance issues.
What about cornfields following soybeans in a crop rotation? These also should be evaluated to stay ahead of any potential concerns in the future.
Western corn rootworm (CRW) variants are present in Iowa but are localized in southeast and east-central counties. The western variants overcome crop rotation by laying eggs outside cornfields, primarily in soybeans.
Northern CRW uses a resistance strategy called extended diapause. Females will lay eggs in cornfields, but instead of hatching the following spring, they wait two to three years. Extended diapause is more common in western Iowa, but it can be found in central and eastern parts of the state as well.
It’s important to note that northern CRW does not lay eggs outside of cornfields.
Here are four ways to better manage CRW:
1. Track egg hatch. Soil temperature is the driving force behind CRW development. This is measured by growing degree days, and research has shown that 50% of egg hatch occurs between 684 and 767 accumulated degree days (with base 52 degree F soil).
Expect to see adults emerge seven to 10 days after reaching the peak. Some factors that can affect egg hatch include soil texture, snow cover or residue, and depth of egg placement. Males develop faster than females, and western CRW develops faster than northern CRW.
Corn rootworm egg hatch in Iowa typically occurs from late May to the middle of June, with an average peak hatching date of June 6 in central Iowa.
To help growers and agronomists, Erin Hodgson and Ashley Dean, entomologists with Iowa State University Extension and Outreach, write an Integrated Crop Management news article each year, keeping everyone updated on rootworm development and dates when egg hatch is expected to occur.
You also can monitor egg hatch for your area by going to the Iowa Environmental Mesonet. To create an accurate map, make sure to set the start date to Jan. 1 of the current year and the end date to the day you’re generating the degree day accumulation. Set the plot parameter to “soil growing degree days (base=52).”
You also can sign up for the Iowa Pest Alert Network to receive this information and other timely insect pest updates via text.
2. Examine corn roots for injury. The best time to assess rootworm feeding on corn roots is 10 to 14 days after peak egg hatch, or just after feeding is done — usually when you start to see adults in the area.
Use the zero to three node-injury scale developed by ISU entomologists to evaluate larval injury. Research has shown that every node of roots lost to larval rootworm feeding results in 15% to 17% yield reduction.
Look at nodes four to six (the three nodes closest to the soil surface). For each node, count the number of roots pruned back to within 1.5 inches of the soil line or stalk, and divide by the total number of roots on that node. Add the rating for all three nodes to achieve a final root rating.
- A rating of 0.25 is equal to the economic injury level.
- A rating above 0.5 is considered unacceptable by the EPA for pyramided transgenic hybrids (multiple belowground Bt traits or Bt plus RNAi).
- A rating above 1 is unacceptable for single-trait corn hybrids.
3. Use the float test. One of the methods that farmers can utilize to confirm the presence of corn rootworm larvae activity is the float test. In mid- to late June, take corn plants with surrounding soil from the field and place them in a bucket of water to loosen the soil. If there are larvae in the soil near the roots, they will float to the surface.
Watch this float test video to better understand how to do the float test and what information can be gathered from this method.
4. Monitor adult rootworm beetles. Another tool to assess CRW activity is the use of ISU coordinates the Corn Rootworm Trapping Network, which monitors, in part, adult corn rootworm activity across the state of Iowa. You can learn more about the network, how to use and set up sticky traps, and access summary results from previous years.
In some cases, adult beetle populations can be so overwhelming that they clip silks and threaten pollination. However, targeting adults can be challenging and expensive. Here are some guidelines to consider for adult rootworm management from Hodgson and Dean.
Summing up
Farmers should adjust their management approaches to keep larval feeding injury to below the economic injury level. The U.S. EPA uses the classification of greater-than-expected injury when root injury exceeds one node to single-trait Bt corn and 0.5 node to pyramided Bt corn.
When greater-than-expected injury occurs, it is reasonable to suspect that rootworm resistance to the Bt trait may be present, but only follow-up laboratory experiments can confirm the presence of resistance.
To learn more about corn rootworm identification, biology, scouting and management strategies, visit the Corn Rootworm IPM website.