2024 marked the fifth year of a coordinated effort to monitor corn rootworm adults across Iowa and the fourth year of combining this effort with the Regional Corn Rootworm Monitoring Network. The Hodgson Lab receives funding from the Agricultural Biotechnology Stewardship Technical Committee to distribute traps to farmers across corn-growing regions in the U.S. and Canada. The 2024 Regional Corn Rootworm Monitoring Network Summary is now available and this article summarizes the Iowa results.
Trap Site Information
In 2024, 165 corn rootworm trapping sites were reported to the regional network. Iowa State University coordinated 31 sites and the Iowa Soybean Association contributed data for the other 134 sites. All sites were in corn fields in 2024; 87 sites were corn the previous year (continuous corn production), while 78 sites were previously soybean (corn-soybean rotation).
Each cooperator was asked to report the history of corn rootworm issues at each site and the management tactics used for corn rootworm during the current growing season. Most sites had an unknown history of corn rootworm issues (139 sites), but nine sites reported no history of issues, and eight sites reported that the only problem was high beetle populations. The most commonly reported management tactic used alone was a Bt hybrid (32 sites), followed by a foliar-applied insecticide (19 sites). Soil-applied insecticides and high-rate seed treatments were not commonly reported as a sole management tactic for corn rootworm (8 sites). 59 sites reported using more than one management tactic for corn rootworm throughout the growing season (see Figure 1).
Beetle Counts and Species Present
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. Across Iowa, only 23% of sites reached this threshold in 2024 (38 sites; Figure 2). Of those sites, 21 (55%) were in continuous corn production.
A primary goal of the regional network is to monitor species composition over time. Western corn rootworm is the prominent species in most corn fields; however, we hear anecdotal evidence of fields dominated by northern corn rootworms. Across all sites in 2024, western corn rootworm was the dominant species at 90 sites (55%), and northern corn rootworm was dominant at 62 sites (37.5%; Figure 3). At the continuous corn sites, western corn rootworm was the dominant species at 48 sites (80%). At the rotated sites, northern corn rootworm was the dominant species at 53 sites (50%).
By the end of the trapping period, most sites had both species on sticky traps (127 sites; 77%); however, 19 and 14 sites reported capturing only western corn rootworm or northern corn rootworm, respectively (Figure 4). The continuous corn sites primarily reported both species (49 sites; 82%), but 15% of continuous corn sites reported only capturing western corn rootworm. The rotated sites also primarily reported both species (78 sites; 74%), but northern corn rootworm and western corn rootworm alone were reported at 13 and 10 sites, respectively.
Overall Summary
The number of beetles captured on sticky traps across Iowa was lower in 2024 compared to 2023, with fewer sites exceeding the trapping threshold of two beetles/trap/day. The percentage of sites that reported northern corn rootworm as the dominant species during the peak week more than doubled in 2024 compared to 2023 (37.5% and 15%, respectively). Similarly, compared to 2023 data, more sites reported capturing northern corn rootworm on traps at some point in the growing season, either alone (8.5% vs. 3.8%) or in combination with western corn rootworm (77% vs. 47.5%).
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 2024 compared to previous years, many factors influence what populations might look like in 2025. We highly recommend planning to assess larval root injury and monitor adults.
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- ANR Crops Team Leader