Using 2019 Corn Rootworm Beetle Counts to Help Evaluate the Risk of an Infestation for 2020


  • Monitoring of corn rootworm (CRW) beetle numbers in current corn and soybean fields can be used to help assess the potential risk of a CRW larval infestation reaching economic damage levels in corn fields during the next growing season. 

  • The objective of this study was to measure adult CRW populations in corn and soybean fields in 2019 to assist in risk evaluation for 2020. 

  • This information may help guide decisions regarding management strategies including corn product selection. 



Location 1442 fields  Planting Date Various
Soil Type Drained or well drained Harvest Date Various
Previous Crop See Figure 1 Potential Yield (bu/acre) 110-250
Tillage Type Various Seeding Rate (seeds/acre) Various


  • One to four Pherocon® AM non-baited trapping sites were established at 1442 field locations across the corn-growing areas of IA, IL, IN, OH, MI, WI, MN, ND, SD, NE, KS, CO, and MO (Figure 1).

  • The trapping sites were installed in the interiors of corn and soybean fields that encompassed a variety of crop and management histories. Soybean fields were sampled in parts of the corn-growing area to assess the potential risk associated with the variant western CRW, which is known to lay eggs in soybean fields.

  • The Pherocon® AM traps were refreshed at 5- to 10-day intervals for 2-8 consecutive weeks through CRW adult emergence, mating, and egg laying phases (late July through late September).

  • Following each sampling interval, the counts of adult northern and western CRW beetles were recorded and used to calculate the average number of CRW beetles/trap/day by field.

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



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

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

2019 CRW Beetle Survey Data

  • CRW populations were variable across the corn-growing area, which suggests that environment and management are factors in determining CRW pressure. 

  • 11.2% of corn fields had counts exceeding the economic threshold of 2 beetles/trap/day.

  • 6% of the corn fields were approaching threshold levels.

  • Corn followed by (fb) corn had higher average maximum daily counts than first-year corn (1.57 vs. 0.33 beetles/trap/day) (Table 1). 

  • 18% of continuous corn fields exceeded the economic threshold while less than 3% of first-year corn fields exceeded the threshold (Figure 2).

  • Counts from soybean fields were low where no adults were captured in 36% of the fields and just less than 2% of the fields exceeded the threshold. 

  • Counts of 0 were recorded in 35% of corn.


2019 Data Interpolation 

  • Point data were 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, CRW populations, and consequently 2020 risk potential, are potentially elevated in corn fields in central and southwest NE, northeast CO, northwestern KS, northwest, central, and east central IA, southwest WI, northern IL, southcentral and central MN, and southeastern ND (Figure 3). 

  • Corn rootworm populations are estimated to be relatively low in many parts of ND, SD, MN, IN, and central IL; however, localized hot spots can be found every year.

  • CRW beetle presence in soybean fields was found to be above the threshold in a small area in north central IL and southern WI.


Comparison of 2018 vs. 2019 CRW Beetle Data (Figure 3a and b).

  • Absolute comparisons between 2018 and 2019 populations should be made with limited confidence due to differences in sampling intensity and distribution. However, trends may still be reliably identified.  

  • Areas with large populations (i.e. “hot spots”) are generally consistent from year to year.


  • Corn rootworm is a persistent threat to yield and profit potential, making it a pest that cannot be ignored. University research has demonstrated that even a moderate level of CRW feeding can cause yield losses averaging 15% with losses of 45% or more being possible.3

  • In the absence of site-specific data, local/regional 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 CRW infestation potential. Use this information to help prepare for the 2020 season by selecting CRW Bacillus thuringiensis (B.t.)-protected corn products or soil-applied insecticides to help protect your crop against the risk of CRW larvae damaging roots and reducing your yield potential.


1Western corn rootworm. Diabrotica virgifera virgifera LeConte. Extension & Outreach. Department of Crop Sciences. University of Illinois.

2Hodgson, E. and Gassmann, A. 2016. Guidelines for using sticky traps to assess corn rootworm activity. Integrated Crop Management. Iowa State University.

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. Journal of Applied Entomology.



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