Using 2017 Corn Rootworm Beetle Counts to Help Evaluate the Risk of an Infestation for 2018



  • The 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 infestation reaching economic damage levels in corn fields during the next growing season.
  • Use of this information may help guide decisions regarding management strategies including corn product selection.

Figure 1. Location of 2017 corn rootworm beetle (CRW) monitoring fields by crop (top) and characterization of 2017 sampled fields by present crop and previous crop with average maximum daily captures for western (WCR) and northern (NCR) CRW beetles (bottom).


  • The objective of this project was to measure adult corn rootworm population levels in corn and soybean fields in 2017 to assist in risk evaluation for 2018.
  • The objective of the trial was to evaluate the visual response of corn products planted in neutral pH (5.8 to 7.5) soils and high pH (7.6+) soil conditions. Improved product characterization allows for better product placement to help maximize yield potential.
Location Soil Previous Crop Tillage Type Planting Date Harvest Date Potential Yield/Acre Planting Rate/Acre
1495 Fields Drained or Well Drained See Figure 1 Conventional Various Various 110-250 bu/acre 28-36,000 seeds/acre


  • One to four Pherocon® AM non-baited trapping sites were established at 1495 field locations across the corn growing areas of IA, IL, IN, OH, MI, WI, MN, ND, SD, NE, KS, MO, and CO (Figure 1, Top).
  • The trapping sites were installed in the interiors of corn and soybean fields that encompassed a variety of crop and management histories (Figure 1, Bottom).
  • The Pherocon® AM traps were refreshed at 5-10 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 (NCRW) and western (WCRW) 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.



Figure 2. (Top Left) Corn rootworm (CRW) beetle monitoring locations for 2017 categorized by peak beetle counts observed during sampling and (Right) peak beetle emergence frequency categories (beetles/trap/day) observed in 2017 CRW monitoring fields by crop and rotation.


Figure 3. 2017 Interpolated peak beetle captures across the Corn Belt in corn (Top Left) and in soybean (Top Right). Comparison of 2016 (Bottom Left) and 2017 (Bottom Right) CRW beetle captures in corn. (Circled areas represent similar areas of higher CRW populations in 2016 and 2017).

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

  • Less than 2 beetles indicate a low risk of ED.
  • Greater than 1 beetle suggests a low risk for ED but could indicate populations are increasing.
  • Greater than 2 beetles indicate ED probability is likely if control measures are not used.
  • Control measures include CRW Bacillus thuringiensis (B.t.) protected corn products or soil-applied insecticides.
  • Greater than 5 beetles indicate ED is very likely and populations are expected to be very high.

2017 CRW Beetle Survey Data (Figure 2, Left and Right).

  • Populations were variable across the corn area (Figure 2, Left).
  • Suggests that environment and management are factors in determining CRW pressure levels.
  • 19% of corn fields had counts exceeding the threshold of 2 beetles/trap/day.
  • 11% of the corn fields were approaching threshold levels (Figure 2, Right).
  • Corn fb corn had higher avg. max. daily counts than 1st-year corn (4.7 vs. 0.74 beetles/trap/day (Figure 1, Table).
  • 39% of continuous corn fields exceeded the threshold (Figure 2, Right).
  • Counts from soybean fields in IL and E IA were low (0.42 beetles/trap/day) (Figure 1, Table).
  • The threshold was exceeded in 5% of all soybean fields sampled (Figure 2, Right).
  • Counts of 0 were recorded in 14% and 38% of corn and soybean fields, respectively (Figure 2, Right).
  • 2017 Data Interpolation (Figure 3, Top Right and Left).
  • 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 systematic grid applied to the estimation area.
  • On a broad scale, CRW populations, and consequently, risk potential is elevated in corn fields across E and SW NE, NE CO, W KS, SE SD, as well as NW, C, and EC IA.
  • Corn rootworm populations continue to be relatively low in many parts of ND, MO, IL, and S WI; however, localized hot spots can be found every year.
  • Notable CRW beetle presence in soybean fields was isolated to small areas in NC IL and NE IA.

Comparison of 2016 vs. 2017 CRW Beetle Data (Figure 3, Bottom).

  • Absolute comparisons between 2016 and 2017 populations should be made with low confidence due to large differences in sampling intensity and distribution. However, trends may still be reliably identified.
  • Areas with large populations (i.e. "hot spots") are consistent from year to year. Populations appear to have grown in some areas (e.g. IA) while dissipating in others (e.g. portions of IL and S WI).


  • Corn rootworms pose a significant threat to yield and profit, 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 up to 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 2018 season by selecting B.t. protected corn products to protect your risk of CRW larvae damaging roots the following year.


1 Western corn rootworm. Diabrotica virgiferavirgifera LeConte. Extension & Outreach. Department of Crop Sciences. University of Illinois.; 2 Hodgson, E. and Gassmann, A. 2016. Guidelines for using sticky traps to assess corn rootworm activity. Integrated Crop Management. Iowa State University.; 3 Evaluating corn rootworm risk and economic impact. 2017. Agronomic Spotlight. Monsanto Company. Websites verified 11/9/17. 171106192900

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