June 27, 2013

Figure 1.  Rootworm larvae. (Photo by UNL Department of Entomology)

Rootworm egg hatch was first detected at Clay Center and Ithaca on June 10. Continuous corn fields should be scouted for corn rootworm larvae and damage. This will help determine whether an insecticide is needed in non-Bt rootworm corn hybrids, if one was not used at planting, and provide a check of the effectiveness of your management practices. In case of poor control, this will allow you to apply a rescue treatment before too much damage occurs.

Scouting

To check for the presence of larvae in a field, dig a 7-inch cube of soil centered on the corn plant. Sample a minimum of two plants at each of five sites in a field. Carefully search through the soil and plant roots for larvae. Placing the soil and roots on a sheet of black plastic may make it easier to find the small white worms. An alternative method is to place the soil and roots in a bucket of water. Stir the water and soil to break up the soil. Rootworm larvae will float to the surface. Salt or sugar may be added to the water to increase larval recovery. There are three larval instars (stages). The greatest amount of damage is done in the last stage. The first instars are about 1/16 inch long and difficult to find without magnification.

Often the first detected rootworms are second instars. Corn rootworm larvae are slender, cream-colored and have brown heads and a dark plate on the top side of the tail, giving them a double-headed appearance. Mature larvae are 1/2 inch long. There is no established economic threshold for corn rootworm larvae, but some consultants advise treating if there are two or three rootworms per plant. The usefulness of this guideline depends on your ability to find rootworm larvae in the soil.

Treatment

If needed, cultivation time treatments should be applied soon after egg hatch begins. Cultivation-time, insecticide applications can effectively reduce corn rootworm feeding damage. Some granular soil insecticides for corn rootworms (for example, Counter 15G and Force 3G) are labeled for application at cultivation. Incorporate granules with 1-2 inches of soil after application; effectiveness may be decreased unless the insecticide is incorporated. Counter 15G, 20G and Force 3G can be applied only once per season; if used at planting time, they cannot be used post-emergence.

Other options include the use of chemigated insecticide treatments. A variety of products can be applied through chemigation, including Brigade 2EC (and other bifenthrin products) and Lorsban 4E (and other chlorpyrifos products).
Additional information on suggested insecticides, rates, and restrictions is available on the UNL Department of Entomology website.

Bob Wright
Extension Entomologist