- 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 and soybean fields during the next growing season.
- Use of this information may help guide decisions regarding management strategies including corn and soybean product selection.
- The objective of this project was to measure adult CRW population levels in corn and soybean fields in 2018 to assist in risk evaluation for 2019.
Research Site Details
- One to four Pherocon® AM non-baited trapping sites were established at 1499 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 (Table 1).
- 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 analyses.
Understanding the Results
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 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.
2018 CRW Beetle Survey Data.
— CRW populations were variable across the corn growing area. This suggests that environment and management are factors in determining CRW pressure levels.
— 19% of corn fields had counts exceeding the action threshold of 2 beetles/trap/day (Figure 1).
— 11% of the corn fields were approaching action threshold levels (Figure 1).
— Corn followed by corn had higher average maximum daily counts than 1st-year corn (4.7 vs. 0.74 beetles/trap/day (Table 1).
— 39% of continuous corn fields exceeded the action threshold (Figure 1).
— Counts from soybean fields in IL and eastern IA were low (0.42 beetles/trap/day) (Table 1).
— The threshold was exceeded in 5% of all soybean fields sampled (Figure 1).
— Counts of 0 were recorded in 14% and 38% of corn and soybean fields, respectively (Figure 1).
2018 Data Interpolation (Figure 2).
— 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 maximu values calculated within a systematic grid applied to the estimation area.
— On a broad scale, CRW populations, and consequently risk potential, is elevated in corn fields across eastern and southwest NE, northeast CO, west KS, southeast SD, as well as northwest, central, and east central IA.
— Corn rootworm populations continue to be relatively low in many parts of ND, MO, IL, and southern WI; however, localized hot spots can be found every year.
— Notable CRW beetle presence in soybean fields was isolated to small areas in north central IL and northeast IA.
Comparison of 2017 vs. 2018 CRW Beetle Data (Figure 3)
— Absolute comparisons between 2017 and 2018 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 are dissipating in others (e.g. portions of IL and southern WI).
What Does This Mean for Your Farm?
- CRW pose a 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 2019 season by selecting B.t.-protected corn products to protect your risk of CRW larvae damaging roots the following year.
1Western corn rootworm. Diabrotica virgifera virgifera LeConte. Extension & Outreach. Department of Crop Sciences. University of Illinois. http://extension.cropsciences.illinois.edu/fieldcrops/insects/western_corn_rootworm
2Hodgson, E. and Gassmann, A. 2016. Guidelines for using sticky traps to assess corn rootworm activity. Integrated Crop Management. Iowa State University. https://crops.extension.iastate.edu/cropnews/2016/06/guidelines-usingsticky- traps-assess-corn-rootworm-activity
3Evaluating corn rootworm risk and economic impact. 2017. Agronomic Spotlight. Monsanto Company.
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