Evaluating Corn Rootworm Control Strategies
We are hearing about corn rootworm populations that are resistant to corn rootworm-resistant (Bt) corn traits. Now is the time to evaluate control in your fields. Look for root pruning, or dig up and wash off a few root balls in a bucket of water and look for live rootworms. In early summer, the small, white larvae are about 1/8-inch long and hard to see, but they will float to the water surface.
Presence of rootworm larvae does not necessarily mean that your trait protection is not working. For rootworm traits to work, larvae must take a few bites for control to take effect. With high larvae populations, those bites can add up and start to impact yield.
Larval injury may make roots more susceptible to root and stalk rot fungi, even on hybrids with rootworm-resistant traits. Corn-on-corn growers in particular may need to rotate crops or consider insecticide control of adult beetles. One western corn rootworm beetle female typically lays 800 eggs in the soil to wait for next season’s corn crop. An average of one beetle per plant is considered an economic treatment threshold.
If you find significant rootworm feeding on trait-protected corn, showing scarred roots and few remaining root hairs, your field may have heavy rootworm pressure and require a new control strategy next season.
Evaluate Control Strategies
Finding more than three rootworm larvae per plant would be considered heavy pressure and could require using a soil insecticide in addition to rootworm traits for corn-on-corn rotations. You may want to consider more frequent rotation to alternative crops such as soybeans. Rotation can reduce pressure, but is not a perfect solution because some rootworms have adapted to lay eggs on soybeans or remain dormant for a year to survive in the soil.
Control Volunteer Corn
Volunteer corn is a factor in rootworm control. In addition to hurting crop yield as a competitive weed, volunteer corn allows corn rootworms to leapfrog corn-soybean rotations by harboring in soybean fields. These areas can become rootworm hotspots in the following year’s corn rotation. Heavy feeding pressure in those hotspots may overwhelm trait-protected corn, resulting in yield loss and root lodging.
A good solution for controlling conventional or glyphosate-tolerant volunteer corn in soybeans is DuPont™ Assure® II herbicide (8 ounces per acre) for control of 18- to 30-inch volunteer corn.
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