A corn rootworm (CRW) beetle infestation can decimate corn yield. They have a keen ability to adapt to control strategies, which makes vigilant scouting and management a priority. Regions with heavy continuous corn acres can harbor high CRW populations. However, rotation-resistant beetles and extended diapause beetles contribute to increased populations, even with a rotation in place. With careful monitoring, a timely insecticide application can manage CRW pressure and mitigate damage the following year.
Factors to Consider
- Crop rotation
- Corn rootworm pressure
- Economic thresholds
- CROP ROTATION PATTERNS. Planting continuous corn fails to break pest and disease cycles. This allows CRW populations to increase rapidly. Rotation to a nonhost crop is beneficial. Understand whether you are in an area where the western corn rootworm beetle variant or northern corn rootworm extended diapause populations are common. These variants are examples of the ability of CRW to overcome crop rotation, reinforcing the need for multiple strategies for sustainable control.
- ROOT DIGS. Direct observation of larval damage by digging, washing and rating roots is essential for assessing the level of field damage. Start with a test strip or refuge area. Compare samples from three spots in the field for a representative sample. Use the root injury rating scale from Iowa State University to record results. Fields with a history of rootworm pressure or local environments conducive to beetle success are the priority.
- FIELD SCOUTING. Begin scouting in June or July, as soon as beetles start emerging from the soil, and continue until September. Weekly evaluation helps with data-driven control decisions. Adult beetles are attracted to pollen and fresh silk, so concentrations in cornfields in early reproductive stages should be a priority. The goal is to scout and take control while beetles are mating, prior to laying eggs. Gravid females have swollen abdomens, and a gentle squeeze will exude milky or opaque contents that feel granular, indicating the presence of eggs.
- ECONOMIC THRESHOLDS. NUTo calculate the economic threshold, count beetles on at least two plants in 25 different areas of a field. Include western and northern CRW in a single count. If the average meets or exceeds 0.75 beetle per plant in corn-on-corn, or 0.5 beetle per plant in corn-on-soybeans, and 10 percent of the females are gravid, consider initiating adult beetle control. If female populations are greater than 10 percent gravid, application might not be economical because so many eggs already are in the soil. Continue monitoring after insecticide treatment. Population fluctuations might warrant additional treatment.LL
To prevent egg laying, control of adult corn rootworm beetles is essential in high-pressure situations. Crop rotation, proper scouting and timely insecticide applications can improve yield potential.