Corn Rootworm Population Levels Increased in West-Central Illinois in 2020

• This project builds on CRW trapping in Northern Illinois in 2019.
• In 2020, Pioneer undertook research to:
  • Quantify corn rootworm (CRW) beetle populations across Illinois and Indiana with Pherocon^® AM/NB sticky traps.
  • Understand how management practices influence CRW population levels.
  • Identify best management practices for growers to make informed decisions for the following growing seasons.
• The study revealed areas with potentially heavy CRW infestations and identified best management practices for farmers to make informed decisions for the following growing seasons.

• Locations: 522 field locations in Illinois and Indiana
• Sampling Methods:
  • Six sticky traps placed per field starting at blister stage (R2).
  • Northern and western corn rootworm beetles were counted every seven days.
  • Trapping continued for 5 consecutive weeks by Pioneer Sales Professionals and Pioneer Agronomists.

• High beetle counts were concentrated in northwest Illinois but were scattered throughout both states (Figure 1).

Figure 1. Peak populations at CRW beetle trapping locations in 2020. Inset area of West-Central Illinois is magnified in Figure 2.

Figure 2. Peak populations at CRW beetle trapping locations in West-Central Illinois.

• CRW populations were characterized at four different levels for each sampling location. The percentage of locations at each level is summarized in Table 1.
  • Zero = no beetles collected
  • Low = <21 beetles/week
  • Moderate = 21-50 beetles/week
  • High =  >50 beetles/week
• Northern Illinois had the greatest percentage of locations with high beetle populations in this study followed by West-Central Illinois. East-Central Illinois and Indiana had the fewest.
• A gradient can be seen (Figure 1) with high CRW populations found in northern and far northwest Illinois and decreasing populations in sites farther south and east. 

Table 1. Corn rootworm population levels by region.

• Corn rootworm species compositions varied among locations depending on population levels (Figure 3)
  • High CRW pressure locations largely consisted of western corn rootworms (82%).
  • Moderate pressure locations had a more even mix of species, with a 70:30 ratio of western to northern CRW.
• In Indiana and Eastern Illinois, only western CRW was found.

Figure 3. Species compositions for high and moderate population locations across northern Illinois in 2020.

• Planting date had less influence on the timing of peak trap counts in 2020 than it did in 2019, when planting dates were spread out over a much longer window (Table 2).
  • Most locations had peak CRW counts during the third week of counting (usually around August 3).
• Plots with extremely high counts (100+ beetles) early returned to more moderate counts later in the study. There were no sites with very early low counts that spiked to higher counts later.

Table 2. Date of peak CRW counts by planting month.

• Crop rotation affected CRW pressure levels (Table 3).
  • Fields of continuous corn or with two years back-to-back corn in past three years were more likely to have high or moderate trap counts.
  • Fields with at least one year of soybeans in between corn crops in the past three years had lower trap counts than the back-to-back corn sites but had more sites with low pressure compared to 2019, when most rotated sites had no CRW pressure.

Table 3. Distribution of pressure levels based on crop rotation.

• Overall results show increased total CRW populations compared to 2019 counts.
• Ratios of western and northern CRW beetles were similar to 2019 in West-Central Illinois. In Eastern Illinois and Indiana, populations were composed almost entirely of western CRW.
• Fields with a soybean rotation in the past three years were more likely to show some CRW infestation in 2020 compared to 2019.

• If traps average <21 beetles per week:
  • Low rootworm populations are anticipated next year
    • Rotate acres to another crop.
    • Plant a corn rootworm Bt corn product.
    • Plant a non-Bt rootworm product with Poncho^® 1250/VOTiVO^® insecticide treatment OR a soil insecticide for larvae.
• If traps average 21-50 beetles per week:
  • Moderate rootworm populations are anticipated next year
    • Rotate acres to another crop.
    • Plant a corn rootworm Bt corn product.
    • Apply a soil insecticide at planting for larvae.
• If traps average >50 beetles per week:
  • High rootworm populations are anticipated next year
    • Rotate acres to another crop.
    • Apply foliar insecticide in the current year to control adult beetles prior to egg-laying and use a corn rootworm Bt corn product or soil-applied insecticide the following year.

• These results show an increased risk of CRW infestation across the entire study area. Fields rotated to corn for two or more consecutive years need to have a plan to manage CRW.
• Pioneer and university research suggests that continuous, uninterrupted use of the same corn rootworm Bt technology can lead to reduced product efficacy against these insects.
• To maintain efficacy of Bt corn rootworm products, it is essential to develop a rootworm management plan that:
  • Breaks the cycle
  • Manages populations
  • Protects the Bt trait
• Please contact your Pioneer Sales Professional for more information.