2025 was the sixth year of a coordinated effort to monitor corn rootworm adults across Iowa and the fifth year of combining this effort with the Regional Corn Rootworm Monitoring Network. The Hodgson Lab received funding from the Agricultural Stewardship Technical Committee to distribute traps to volunteer cooperators across corn-growing regions in the U.S. and Canada. The 2025 regional report is published, but this article summarizes the Iowa results in greater detail.
Trap site information
In 2025, data were reported to the regional network for 170 corn rootworm trapping sites. The Hodgson Lab coordinated 40 sites, and the Iowa Soybean Association contributed data for another 130 sites. All sites were corn in 2025; 69 sites were corn the previous year (continuous corn production), while 100 sites were previously soybean (corn-soybean rotation) (Figure 1).
Issues with corn rootworm prior to 2025 were reported for very few sites, but goosenecking/lodging and high beetle populations were reported for 15 sites. Suspected resistance to Bt was only reported at one site, and suspicion of resistance to crop rotation was not reported.
Despite the unknown history of corn rootworm issues in these fields, at least one management tactic was used specifically for corn rootworm at 100 sites, including 41 sites that were soybeans the previous year. Multiple management tactics for corn rootworm were used at half of those sites, with Bt hybrids + foliar insecticides being the most common (18 sites, seven of which were rotated), followed by Bt hybrids + soil-applied insecticides (15 sites) and Bt hybrids + soil-applied insecticides + foliar insecticides (11 sites).
Entomologists do not recommend using multiple tactics to manage corn rootworm, as research demonstrates that multiple management tactics could hasten resistance development (McColluch and Gassmann 2024), especially when Bt resistance is prevalent in the landscape. Crop rotation is still the most effective management tactic for corn rootworm, and using a single, effective tactic in continuous corn is more cost-effective.
Beetle counts
Corn rootworm adults are mobile, and populations vary across the landscape. It is difficult to know where beetles caught on traps originate, but research demonstrates that significant feeding injury can be expected the following year when at least two beetles/trap/day are captured on yellow sticky cards. This threshold determines whether alternative management should occur the following growing season. In 2025, only 6.5% of sites in Iowa reached the trapping threshold (Figure 2): eight of these sites were in continuous corn, while three were rotated. 11.6% of continuous corn sites reached threshold, while 3.0% of rotated sites did.
Dominant species during the peak week: >50% of total beetles caught
Overall, western corn rootworm (WCR) was the dominant species at 81 sites (47.6%), while northern corn rootworm (NCR) was the dominant species at 61 sites (35.9%; Figure 3). 28 sites did not have a dominant species, either because the same number of beetles were captured or no beetles were captured. WCR was the dominant species at 69.6% of the continuous corn sites (13.0% for NCR), while NCR was the dominant species at 52.0% of rotated sites (32.0% for WCR).
Species present at the site – tells us more about species composition
By the end of the trapping period, more than half of the sites had both WCR and NCR present (55.9%; Figure 4). Ten percent of sites reported no corn rootworm beetles, while 19.4% reported only WCR and 14.7% reported only NCR. At the continuous corn sites, it was most common to find both species (50.7%), followed by WCR alone (33.3%), no beetles (11.6%) and NCR alone (4.4%). At the rotated sites, it was most common to find both species (59.0%), followed by NCR alone (22.0%), WCR alone (10.0%), and no beetles (9.0%).
Remember, data collected from monitoring corn rootworm beetles with sticky traps can only be used to estimate the risk of significant root injury the following year. Because adults are highly mobile, sticky trap captures do not provide insight into root injury or yield loss during the current growing season, or provide confirmation of variant populations resistant to crop rotation. The best ways to assess those are:
- Estimate yield loss during the current season: Dig roots in July and early August and use the ISU 0-3 Node Injury Scale to assess root injury. Learn how to rate roots.
- Confirm variant populations: Do the “float test” in June to determine presence/absence of larvae and relative size, or dig roots in July and early August, as described above. Learn more about the float test.
Overall summary
The number of beetles captured on sticky traps across Iowa was much lower in 2025 than 2024, with fewer sites exceeding the trapping threshold of two beetles/trap/day. The percentage of sites that reported NCR as the dominant species during the peak week was similar in 2025 and 2024 (35.9% and 37.5%, respectively), after more than doubling from 2023 to 2024. Overall, a smaller percentage of sites reported NCR in 2025 than 2024 (70.6% vs. 85.5%), but a greater percentage reported only finding NCR in 2025 compared to 2024 (14.7% vs. 8.5%).
Participation in the trapping network is voluntary, and cooperators can choose any field of interest to monitor traps. Therefore, samples are not random and may not fully represent the population across the landscape. Additionally, corn rootworm populations vary widely across the landscape depending on field-specific management practices and fluctuate annually based on many factors. Although populations were generally lower in 2025 compared to previous years, many factors influence what populations might look like in 2026. We highly recommend planning to assess larval root injury and monitor adults.
What’s next?
Because we have a dataset that contains more than five years worth of data for corn rootworm adults in Iowa, several U.S. states, and Canada, we hope to provide a report of how beetle counts and species composition have changed over time and analyze any trends in the data. Keep an eye out for that report!
Regional corn rootworm adult monitoring network
- 2025 Regional Report
- 2024 Iowa Report
- Historical regional reports (2021-2024)
- Corn Rootworm Integrated Pest Management website
- Corn Rootworm Monitoring Network dashboard
References
McColluch and Gassmann, 2024. https://doi.org/10.1093/jee/toae149.
Thumbnail photo credit: Tomasz - stock.adobe.com
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