Root Digs Can Help Assess Corn Rootworm Damage

Delayed planting or replanting due to cold and extremely wet seedbeds may have an impact on the amount of insect pressure this year’s corn crop suffers, especially from corn rootworms. Entomologists advise corn growers to conduct root digs during July to determine the level of rootworm feeding in their fields.

“Generally speaking, in the western and central Corn Belt, the optimal digging time is around the first to second week of July,” explains Corby Jensen, Monsanto Corn Technology Development Manager for Monsanto. “For the northern Corn Belt, you would dig more the third to fourth week of July. In New York and New England, it’s usually around mid-July to early August.”

Jensen suggests the following steps for conducting a root dig:

1. Begin by walking into the field 50 feet and digging up three plants in order to determine overall corn rootworm pressure.

2. Dig about six inches on each side of the stalk in a circle. Remove the stalk and remove what soil you can from the root.

3. Place roots in a tub of water and let them soak for 30 minutes or more. Soil adhering to the roots removes most easily after soaking. If rootworm larvae are present in the rootball, they may be found floating on top of the water after soaking. Roots may be washed immediately after digging, but be careful not to remove smaller roots with the soil.

4. Wash the roots carefully using a pressure washer on low pressure or a strong hose.

5. After washing, carefully pull back the roots at each node and carefully inspect for rootworm scarring and root pruning.

6. Count the number of roots eaten and scarred and make a judgment on the damage to the root mass. A 0.25 root damage rating for corn rootworm using the 0-3 Iowa State University Node Injury Scale is considered the economic threshold for considering management options.

Jensen says growers can find out if the level of rootworm infestation merits switching to in-plant protection, such as YieldGard VT Triple®, by either digging up corn roots and assessing the damage this year or using the sticky trap method in first-year corn. Jensen says that, regardless of the region of the country, it is important that growers know both the type of insect pressure that can be expected and the management options available to treat these insects.

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