Several insect species including wirewoms, white grubs, chinch bugs, cutworms, and corn root aphids can cause damage to seedlings. Pale western cutworm appears to have the greatest economic impact on dryland corn in Colorado. Injury to corn roots can be caused by white grubs, corn rootworm larvae, and several other pests. Western corn rootworm is among the major insect problems.

Pale Western Cutworm
Pale western cutworms cut off small seedlings below ground. As corn plants get larger, they enter the plant and kill the growing point. It is a subterranean, soft-bodied caterpillar; grayish-white in color, unmarked by spots or stripes, with two distinct vertical brown bars on the front of the head capsule. A fully developed larva is about 25 millimeters (one inch) in length. Eggs are deposited in loose soil and usually hatch within two weeks. Hatch may be delayed for up to several months if moisture and temperature conditions are unfavorable. Larvae prefer loose, sandy or dusty soil and are found most easily in the driest parts of the field such as hilltops.

Outbreaks are associated with dry conditions in the previous spring. If the preceding May and June had fewer than 10 days with ¼ inch or more of rainfall, then pale western cutworm populations can be expected to be high. If the preceding May and June had more than 15 such days, the cutworm will almost totally disappear.

Because of the sporadic nature of pale western cutworm outbreaks, management options are limited to the use of insecticides. Pale western cutworms seem to feed more under dry conditions, so yield relationships are difficult to define. Consider insecticide treatment if one plant in 20 is injured, and cutworms are present.

Western Corn Rootworm (WCR)
The WCR larvae feed on the underground root systems of corn plants. Western corn rootworm (WCR) larvae are white and slender with brown heads and a dark plate on the top side of the terminal body segment. Mature rootworm larvae are about 12 millimeters (1/2 inch) long. Peak feeding usually occurs from late June to mid-July. Lodging (goose necking) of corn plants due to larval root feeding is a typical symptom of damage. Adults often feed on corn silks, and severe silk pruning may result in yield reduction due to poor pollination. However, most damage is due to larval root feeding.

Crop rotation is the most consistent and economical means of controlling WCR populations. There are no commercial rootworm resistant corn varieties, but rootworm resistant Bt hybrids are being tested for commercial use.

Fields that have completed pollen shed are not very attractive to rootworm beetles. Early planted fields can be through withpollination before the majority of the adults have emerged, and, therefore, have less egg laying activity. Early fields also will have relatively larger root systems when rootworm feeding starts. This makes them somewhat more tolerant to rootworm damage.

Chemical applications to first year corn is not recommended. Incorporation of soil insecticides into the soil protects wildlife. If corn is planted prior to May 15, post emergent treatments are preferable. Under Colorado conditions, post-emergent treatments are generally more reliable than planting-time treatments.

The granular formulations (except Fortress and Aztec) can be applied at the same rates, as a band on either side of the row, by cultivator shoes and disc hillers at cultivation. Liquids for cultivation applications include Dyfonate 4ECR and Furadan 4FR (may also be broadcast with ground or aerial equipment). Application at cultivation is usually more reliable than at planting. Apply only before June 15.

Control of rootworm adults is intended either to protect silks during pollination or to prevent egg laying and damage to roots in next year's crop. Adults rarely become numerous enough to interfere with pollination. Control may be justifiable if there are more than 10 beetles per ear zone during the wet silk stage (R1).

If treatments are intended to prevent egg laying, then treatment is recommended when beetle counts exceed 18,000 beetles per acre. Adult treatments applied too early, that is, before 10 percent of the females are carrying fertile eggs, may not have much effect on egg laying. Determining the percentage of females with fertile eggs can be difficult, but generally the proper time for the application of adult treatments occurs two to three weeks after the first adult emerges.

A second adult treatment should be considered if beetle densities rebound to above 12,000 per acre (one beetle in two plants at 24,000 plants per acre). The products registered for control of western bean cutworm, as well as Furadan 4FR, malathion, Penncap-MR, and Slam, may be used at label rates to control adult corn rootworms.

Assefa Gebre-Amlak
Golden Plains Area Extension Entomologist