1/22/2023

Corn Rootworm Levels in the Central Corn Belt - 2022

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Agronomy Research Update
Written by Mary Gumz, Ph.D., Agronomy Manager

Key Findings

438 corn and soybean fields were monitored for corn rootworm (CRW) beetles across Illinois, Indiana, Iowa, Minnesota, and Wisconsin in 2022.

Populations in South Dakota, Iowa, and Wisconsin differed from those in Illinois and Indiana, with higher maximum weekly counts, higher prevalence of northern corn rootworm (NCR) and peak counts occurring 4 to 5 weeks after initial trap placement.

All corn growers should monitor for CRW populations and use best management practices.

Objective & Study Description

Objective: Assess CRW populations across Illinois, Indiana, Iowa, Minnesota, South Dakota and Wisconsin.
Locations: 438 corn and soybean field locations across Illinois, Indiana, Wisconsin, Minnesota, Iowa, and South Dakota.
Sampling Methods:
- Six sticky traps placed per field starting at blister stage (R2) (Figure 1).
- Northern and western CRW beetles were counted every seven days, with traps replaced every week (Figure 2).
- Trapping continued for six consecutive weeks by Pioneer sales professionals and agronomists.
  
  Figure 1. Placement of a new Pherocon^® AM/NB sticky trap on a corn plant (left). Arrangement of six traps in the field (right).

Results

Weekly average trap counts ranged from zero to 269 beetles per trap.
86% of fields sampled showed some level of CRW pressure.
The highest average weekly trap counts were found in Wisconsin and Northern Illinois.

Map - Peak weekly CRW beetle counts by location.

Figure 3. Peak weekly CRW beetle counts by location.

Corn rootworm populations were characterized at four different levels for each sampling location (Table 1).
- Zero = no beetles collected
- Low = traps averaged <21 beetles/week
- Moderate = traps averaged 21-50 beetles/week
- High = traps averaged >50 beetles/week

Table 1. CRW levels across the entire study area and by state.

CRW Level	Locs	Zero	Low	Moderate	High
		-- % of sample locations --
Overall	438	14	45	23	18
Illinois	57	30	42	21	7
Indiana	29	45	52	3	0
Iowa	14	0	86	14	0
Minnesota	24	4	71	8	0
South Dakota	4	0	100	0	0
Wisconsin	279	6	40	27	24

Western CRW was found in nearly all sampling locations across all states in the sampling area.
Northern CRW was prevalent across most of the sampling area except for Eastern Illinois and Indiana.

Map - CRW species found at trapping locations in 2022.

Figure 4. CRW species found at trapping locations in 2022.

Sticky traps were put in the field at R2 and the first week’s counts were taken seven days later and continued on a weekly basis. Since the traps were placed based on crop development rather than a calendar date, traps in the southern part of the study were placed in the field earlier than traps in the northern part of the study.
Timing of peak beetle trapping (based on weeks following R2) varied depending on geography. Locations in Indiana, Illinois, and Iowa tended to have peak beetle counts during week 1 or 2 of trapping.
Locations in Wisconsin and Minnesota tended to have peak beetle counts occur later in trapping.

Map - Week of highest CRW beetle count at 2022 trapping locations.

Figure 5. Week of highest CRW beetle count at 2022 trapping locations.

What Does This Mean in the Field?

Farmers throughout the Central Corn Belt need to be aware of CRW pressure on their farms, especially in continuous corn rotations.
Northern CRW, and the potential for its extended diapause variant, is a greater concern in Wisconsin, Minnesota, Iowa, and South Dakota than in Illinois or Indiana.
Farmers who are monitoring CRW beetles using sticky traps may be able to make a good population estimate after 1 to 2 weeks of trapping in central Illinois and Indiana. Farmers in the northern and western portions of this study area should trap at least 4 to 5 weeks for more accurate counts.

CRW Best Management Practices

When using CRW beetle trapping to monitor populations:

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 Lumisure^® 1250 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 beetles prior to egg-laying and use a corn rootworm Bt corn product or soil-applied insecticide the following year.
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.