Using 2018 Corn Rootworm Beetle Counts to Help Evaluate the Risk of an Infestation for 2019


Trial Objective

  • ​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 corn rootworm populations in corn and soybean fields in 2018 to assist in risk evaluation for 2019.
  • This information may help guide decisions regarding management strategies including corn product selection.

Materials and Methods

  • ​One to four Pherocon® AM non-baited trapping sites were established at 1,499 field locations across the corn growing areas of  IA, IL, IN, OH, MI, WI, MN, ND, SD, NE, KS, and CO (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 corn rootworm, 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.

​2018 CRW Beetle Survey Data

  • ​— Populations were variable across the corn-growing area, which suggests that environment and management are factors in determining CRW pressure levels.
  • — 23% of corn fields had counts exceeding the threshold of 2 beetles/trap/day (Figure 2). 
  • — 7% of the corn fields were approaching threshold levels (Figure 2).
  • — Corn followed by (fb) corn had higher average maximum daily counts than 1st-year corn (4.99 vs. 1.43 beetles/ trap/day) (Table 1). 
  • — 17% of continuous corn fields exceeded the threshold (Figure 2). 
  • — Counts from soybean fields were low (0.34 beetles/trap/day) (Table 1). 
  • — The threshold was exceeded in 2% of all soybean fields sampled (Figure 2). 
  • — Counts of 0 were recorded in 28% and 35% of corn and soybean fields, respectively (Figure 2).​

2018 Data Interpolation (Figure 3a)

  • — 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 2019 risk potential, are potentially elevated in corn fields across eastern and southwest NE, northeast CO, western KS, northwest, central, and east central IA, southwest WI, northern IL, and western IN.
  • — Corn rootworm populations are estimated to be relatively low in many parts of ND, SD, MN 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 2017 vs. 2018 CRW Beetle Data (Figure 3b) 

  • ​— Absolute comparisons between 2017 and 2018 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; however, populations appear to have grown in some areas (e.g. northern IL) and dissipated in others (e.g. portions of SD).

What Does This Mean for Your Farm?

  • Corn rootworm are a notable 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 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 2019 season by selecting CRW Bacillus thuringiensis (B.t.)-protected corn products or soil-applied insecticides to protect your crop against the risk of CRW larvae damaging roots and reducing your yield potential. ​

1 Western corn rootworm. Diabrotica virgifera virgifera 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 Tinsley, 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. DOI:10.1111/j.1439-0418.2012.01736.x. Websites verified 11/9/18. 171106192900.