Time for monitoring corn seedling insect pests

Colorado

There are several insects and diseases that may attack corn during early and late seedling stages. These include pale western cutworm, seedcorn maggots, wireworms, western corn rootworm, damping off and seedling blights. This week we will focus our discussion on western corn rootworm and wireworm problems and their management.

Western corn rootworm

The western corn rootworm overwinters in egg stage. Eggs start to hatch in late spring depending on soil temperatures. Hatch in southern Colorado usually starts in mid May, while eggs in cooler areas do not hatch until early June.

After hatching, the small rootworm larvae move to nearby corn roots and begin feeding on root hairs and small roots. Larger rootworms feed on and tunnel in primary roots. Under heavy pressure, root systems can be completely destroyed.

In general, economic losses occur after one or more primary roots are chewed to within 1.5 inches of the plant. Grain losses from Colorado State University studies have averaged 10 percent over the last decade. Root damage also can result in extensive "gooseneck" lodging and harvest difficulties.

By mid July, most larvae have finished feeding and transformed into adult beetles. They emerge from soil to feed, mate and lay eggs. These eggs produce the larvae that will attack the following year's crop.

Females prefer moist area, so eggs are concentrated in the soil between rows in irrigated corn. Heavier soils with organic matter content of 1.5 percent also are preferred.

Management strategies:

Crop rotation

This is the most consistent and economical means of controlling rootworms.

Early planting

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

Other cultural practices

Practices that promote strong root systems and generally vigorous crop will make corn more tolerant to rootworm and damage.

Insecticides

Some, but not all, continuous cornfields will have rootworm populations large enough to justify the expense of protecting the crop with an insecticide. The number of adults present during the previous growing season is the best guide for selecting the fields to be treated. If records from the previous year show that beetle counts for a field averaged more than one beetle per plant on any scouting day during August or early September, then an insecticide treatment should be considered.

Continuous cornfields that were not scouted during the previous growing season also should be protected with an insecticide.

Chemical applications to first year corn are not recommended. 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.

If a field is to be treated, then one of the several recommended application methods must be selected. One of these methods may be more convenient than others for a particular farming operation.

Granular rootworm insecticides can be applied at planting in-furrow or banded over the row either at planting or cultivation. Granular insecticides applied at cultivation have proved the most consistent control in Colorado State University tests.

Liquid rootworm insecticides can be applied at planting or at cultivation in a spray directed at base of the plant. No liquids should be applied before May 15.

In general, application at cultivation is more reliable than at planting. Apply only before June 15.

Wireworms

Wireworm populations in corn are most severe in cornfields following sod or fields having a prolonged grassy weed problem. Wireworms feed on germinating corn seeds and early corn seedlings, and may bore into stalks at the soil level.

Wireworms are larval stages of a family of beetles commonly called "click beetles." There are several species of wireworm whose life cycle may require one or more years per generation.

The insects usually overwinter in the adult stage. Adults are brownish or even blackish in color, elongate and tapering toward each end but more so toward the rear. The earliest stages of larvae are very small and white, later stages have a characteristic hard shell appearance and a yellow brown color. Mature larvae range from one-half to one inch in length, depending on the species.

Females deposit eggs in the soil among grass roots. When the eggs hatch the larvae feed on the roots of corn and other grasses. The larvae mature in from two to five years. Fully developed larvae form pupation chambers in the soil and pupate. Adults emerge from the pupae and remain in the soil until the following spring.

If severe infestations are anticipated, use a protectant and a soil insecticide labeled for in-furrow use.

Registered western corn rootworm and wireworm insecticides and the correct rates are given the High Plains Integrated Pest Management Guide (http://highplainsipm.org). This guide is also available from Colorado State University Cooperative Extension Offices.

Date: 5/24/06

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