June 8, 2007

Figure. 1. Corn rootworm larvae

Based on observations by UNL Entomology Professor Lance Meinke at UNL's Agricultural Research and Development Center near Ithaca (Saunders County) and at UNL's South Central Ag Laboratory near Clay Center, corn rootworm egg hatch began on May 25. Hatch will begin somewhat later in northeast Nebraska and in western Nebraska.

This timing is about "normal" for these locations. After hatch occurs, begin scouting continuous corn fields 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.

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. There are three larval instars (stages). The greatest amount of damage is done in the last stage. Degree-day accumulations needed to complete development of different stages are shown in Table 1. 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. Search through the soil and roots over a sheet of black plastic to make it easier to find the small white worms. There is no established treatment guideline 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. If needed, cultivation time treatments should be applied soon after egg hatch begins. Cultivation time applications of insecticides are an effective means of reducing injury to corn plants from rootworm feeding damage. Most granular soil insecticides for corn rootworms (except for Aztec and Fortress) that are labeled for application at planting time also are labeled for use at cultivation. Incorporate granules with 1-2 inches of soil after application; effectiveness may be decreased unless the insecticide is incorporated.

Other options include the use of Furadan 4F and the use of chemigation treatments with Lorsban 4E. Control with Furadan 4F will generally be improved if the treatment is cultivated into the soil, unless sufficient rainfall or irrigation occurs after application to move the insecticide down into the root zone. Lorsban 4E applications should be timed for the first appearance of second instar corn rootworms. Additional information on suggested insecticides, rates and restrictions is available on the UNL Department of Entomology Web site.

Bob Wright
Extension Entomologist