Using 2025 Corn Rootworm Beetle Counts to Assess the Risk of Economic Injury in 2026

TRIAL OBJECTIVE

• The objective of this study was to measure corn rootworm beetle populations in corn and soybean fields in 2025 to help evaluate the potential risk for corn rootworm larval feeding in 2026 corn fields.
• The monitoring of rootworm beetle numbers in current corn and soybean fields can be used to help assess the potential risk of rootworm larval infestations reaching economically damaging levels in the following corn crop.
• The data may help guide corn rootworm larval management decisions, including corn product selection, for the next corn crop.

RESEARCH SITE DETAILS

• One to four Pherocon® AM non-baited trapping sites were established in 2025 at 555 field locations across the corn-growing areas of CO, IA, IL, IN, KS, MI, MN, MO, NE, ND, SD, WI, and PA (Table 1 and Figure 1).

• The trapping sites were placed in the interiors of corn and soybean fields that encompassed a variety of crop and management histories (Table 1 and Figure 2). One soybean field was sampled in the corn-growing area to assess the potential risk associated with the western corn rootworm variant, which is known to lay eggs in soybean fields.

• The Pherocon® AM traps were changed at 5- to 10-day intervals for two to six consecutive weeks through corn rootworm beetle emergence, mating, and egg laying phases (late July through late September).
• Following each sampling interval, counts for northern and western corn rootworm beetles were recorded and used to calculate the average number of beetles per trap per day (beetles/trap/day) by field (Table 2).

Table 2. Summary of field sampling and corn rootworm beetle captures in 2025.

• At the end of the collective sampling period, the highest average capture value for each field was determined and the data were used in further analysis.

UNDERSTANDING THE RESULTS

Categories for rootworm beetle counts are based on action thresholds (beetles/trap/day) suggested by Extension entomologists at the University of Illinois and Iowa State University and provide the economic injury potential for the following season.^1,2

• Less than 2 beetles/trap/day indicate a relatively low risk of economic injury.
• Greater than 1 beetle/trap/day suggests a low risk for economic injury but could indicate populations are increasing.
• Greater than 2 beetles/trap/day indicate the probability for economic injury is likely if control measues are not used.
• Greater than 5 beetles/trap/day indicate that economic injury is very likely and populations are expected to be very high the following year.

2025 Corn Rootworm Beetle Survey Data

• Corn rootworm beetle populations varied by crop rotation (Figure 2 and Table 2).
• Beetle populations were variable across the corn-growing area, which suggests that environment and management practices affect beetle pressure (Figure 3).
• 31% of the corn fields sampled in 2025 had counts exceeding the economic threshold of 2 beetles/trap/day, which was 2% higher than 2024 fields (29%), 22% less than 2023 and 2022 fields (53%), and 7% less than 2021 fields (38%) (Table 3).
• 46% of the continuous corn fields sampled in 2025 were above the economic threshold, which was up 3% from 2024 (43%), down 25% from 2023 (71%), down 28% from 2022 (74%), and down 6% from 2021 (52%) (Table 3).
• 17% of the first-year corn fields in 2025 were above the economic threshold, which was 1% higher than 2024 (16%), 3% higher than 2023 (14%), up 7% from 2022 (10%), and equal to 2021 (17%) (Table 3).

Table 3. Percent beetle counts above economic threshold of two beetles/trap/day.

2025 Data Interpolation

• Point data was interpolated to estimate populations and relative risk at the landscape level.
• To account for variations in sampling density and distribution, interpolations were based on average maximum values calculated within a systematic grid applied to the estimation area.
• On a broad scale, rootworm larval populations—and consequently 2026 risk potential—are possibly elevated in corn fields in eastern and southeastern WI, northern IL, south central IL, western IA, eastern, southeastern, and southwestern NE, and northern CO (Figure 3 and Figure 4, right).
• In comparison to the 2024 rootworm larval forecast (Figure 4, left), the risk for 2026 (Figure 4, right) appears to be reduced in North and South Dakota but expanded in areas of IL, NE, IA, and WI.
• Larval populations in 2026 are estimated to be relatively low in many areas of ND, SD, MN, NE, KS, MO, IL, IN, MI, and PA; however, localized hot spots can be found every year (Figure 3 and Figure 4, right).

KEY LEARNINGS

• Corn root injury from corn rootworm larvae is a persistent and annual threat to yield and profit potential, making rootworm a pest that cannot be ignored. University research has demonstrated that even a moderate level of rootworm larval feeding can cause yield losses averaging 15%, with losses of 45% or more being possible.³
• In the absence of site-specific data, local and regional corn rootworm beetle surveys may provide insight at the landscape level and can be used to make informed decisions regarding management and product selection decisions.
• Beetle numbers and infestation geographies change. Continue to monitor present and historical data to gain information regarding rootworm larval potential. This information can be used to help prepare for the 2026 season and the selection of corn rootworm Bacillus thuringiensis (B.t.)-protected corn products or soil-applied insecticides to help protect your crop against the risk of rootworm larvae damaging roots and reducing yield potential.

Sources

¹University of Illinois Urbana-Champaign. Western corn rootworm. Department of Crop Sciences. http://extension.cropsciences.illinois.edu/fieldcrops/insects/western_corn_rootworm/
²Hodgson, E. and Gassmann, A.J. 2016. Guidelines for using sticky traps to assess corn rootworm activity. Integrated Crop Management. Iowa State University. https://cornrootworm.extension.iastate.edu/basics/scouting-guidelines
³Tinsley, N.A., Estes, R.E., and Gray, M.E. 2012. Validation of a nested error component model to estimate damage caused by corn rootworm larvae. Department of Crop Sciences, University of Illinois, Urbana, IL. Journal of Applied Entomology 137(3): 161-169. https://doi.org/10.1111/j.1439-0418.2012.01736.x