Growers in the Midwest considering another corn-on-corn year may wish to consult their local agronomist or university extension specialist before planting. According to the 2022 Regional Corn Rootworm Monitoring Network Summary issued by the Corn Rootworm IPM Regional Working Group, corn rootworm populations are anticipated to be high again in areas of Iowa, Wisconsin, Minnesota, Michigan, Nebraska, Illinois, North Dakota, South Dakota, Nebraska and Kansas. These states are at an elevated risk as they produce a lot of corn-on-corn acres, and findings show that rootworm is becoming more resistant to the Bt trait.
The report highlights, “Western corn rootworm (WCR) and northern corn rootworm (NCR) continue to dominate as economic pests throughout the U.S. and Canada … Recently, high corn rootworm populations and the development of resistance to Bt corn hybrids in some areas has sparked greater interest in scouting and alternative management of corn rootworms.”
Stine® agronomists are on heightened alert as they have monitored the rootworm crisis in their regions and continue to spread word of the potential for yield loss if not properly managed. According to the Crop Adviser Institute, rootworms cause more than $1 billion in damage and control costs annually in the U.S..
“With each passing year, we see the rootworm population increasing throughout the Corn Belt,” says Tom Larson, Stine’s director of agronomy. “Unfortunately, as we know well, these pests can devastate a crop. If you haven’t started planting yet, it’s not too late to consider a switch to soybeans on fields that experienced high rootworm pressure last year. If you’ve already planted corn-on-corn this year, talk to your local agronomist or extension expert to discuss what can be done in-season to slow the spread.”
2022 Findings
According to the 2022 results, 38% of sites tested exceeded the trapping threshold. The report notes, “For corn, the trapping threshold is 2 beetles/trap/day, regardless of species. For soybean, the trapping threshold is 1.5 western corn rootworm/trap/day; no soybean sites exceeded the threshold. These trapping thresholds indicate that the grower should consider switching up management practices the following year, because adult populations indicate that egg-laying in the field will likely result in severe larval injury the following year if corn is planted.”
Other report findings concluded western corn rootworm is the most dominant species throughout the Corn Belt but populations of northern rootworm are increasing, and fields with a history of corn rootworm are likely to have continuous issues with populations in the future. Of the sites that exceeded the trapping threshold, at least 59% were in continuous corn.
The most common issues reported in infected fields included goosenecking/lodging, high beetle populations or a combination of the two. Another reported issue was resistance to the Bt trait, which is problematic as growers have relied on the trait to suppress populations in difficult fields for several years.
“These findings align with what experts have been warning growers about,” says Larson. “Corn rootworms are not to be taken lightly. If it’s a problem one year, it will be a problem the next.”
Scouting tips
There are two types of corn rootworm prevalent in the Midwest — Western and Northern. Both are capable of laying eggs that can overwinter in the soil, so early-season scouting should include digging below the soil to check the roots of the plant. Later in the season, as they transition from larvae to adult rootworm beetles, they can be found directly on the plant as they feed off the leaves and silks as adults. This can greatly affect pollination during silking.
“Early in the planting season, you’ll want to scout for the rootworm larvae by digging the soil around the corn plant,” says Larson. “Iowa State University Extension and Outreach has some great guidance for sampling, including the ‘float test,’ which involves filling a bucket of water and placing the soil and roots directly into the bucket so the larvae float to the top. This can help you detect any root damage to the plant.”
Iowa State also has an interactive node injury scale, which can help growers decipher the level of damage caused by corn rootworm larvae.
Later in the growing season as plants near silking and pollination, growers should be on the lookout for adult rootworm beetles. Western corn rootworms — the most commonly reported — are typically yellow or green and have black striping on their wings. Northern corn rootworms are light green or brown with no distinguishable marks. There are also Southern corn rootworm beetles that aren’t as common in the Midwest. These are yellow and green and have black spots on their backs.
“If you’re scouting for adult beetles, you’ll want to consider the time of day,” says Larson. “Research shows beetles are more active in the morning or late afternoon as they feed on silks and leaves, so it’s best to scout during their most active times.”
In addition to scouting, growers can employ sticky traps, which can help determine the amount of infestation. Sticky traps were one of the methods used by the Corn Rootworm IPM Regional Working Group for their 2022 findings report.
Mitigation
“It’s clear that back-to-back corn-on-corn is an instigator when it comes to adding to the corn rootworm problem,” says Larson. “Crop rotation will continue to be critical in controlling populations.”
If crop rotation isn’t a viable strategy this year, growers need to prioritize scouting throughout the growing season. If rootworm larvae or beetles are detected, the next step would be to examine the level of infestation. From there, having a conversation with an agronomist or local extension expert can help determine next steps. One in-season strategy they might recommend is a soil- or foliar-applied insecticide. While it’s not the end-all strategy, it can help control the population throughout the important pollination stage.
“The biggest action growers can take is to plan ahead,” says Larson. “What you can do in season is minimal, so growers need to spend time discussing crop rotation and what traits make the most sense to plant next year. We know Bt traits are showing some signs of resistance, but there are likely other options growers haven’t discussed with their salesperson. Stine has an entire lineup of Agrisure Duracade® brand corn, for example, which has proven effective in mitigating corn rootworm. It’s definitely worth the conversation.”
One consideration to keep top of mind when discussing future strategies, according to the 2022 report, “Extension professionals do not recommend using multiple tactics to manage corn rootworm as research demonstrates no yield benefit, no reduction of larval feeding injury, and no reduction in adult emergence when both a Bt-rootworm hybrid and soil-applied insecticide are used. Furthermore, multiple management tactics could hasten resistance development.”
For more tips on preparing your fields for corn rootworm infestation this year and next, contact your local sales rep. Stine can help you maximize the potential of each and every acre by keeping corn rootworms at bay.