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Heat-Unit Accumulations Indicate Stalk Borers on the Move

May 18, 2001
It's time to start thinking about the insects that will "step up to the plate" when flea beetles, southern corn leaf beetles, white grubs, grape colaspis, and wireworms are nothing more than memories. Stalk borers may threaten corn in some areas of Illinois quite soon, so keep your eyes peeled.

Stalk borers are one of the most challenging insect pests to manage effectively. Stalk borers overwinter as eggs on weed hosts and pass through one generation per year. Fields infested with ragweed, wirestem muhly, quackgrass, and giant foxtail are attractive egg-laying sites for stalk borer moths in late summer. Fence rows infested with these weeds offer excellent staging sites for stalk borer larvae to move from and into adjacent border rows of corn.

Small stalk borer larva.

Overwintering stalk borer eggs hatch in late April and early May. Stalk borers complete 7 to 10 instars and actively feed for 8 to 10 weeks. Larvae are 1/2 to 1-3/4 inches long, depending upon instar. Newly hatched larvae appear to be brown, but they have prominent white and purple stripes running lengthwise along their bodies and tunnel into aboveground stems, usually of grasses. Larvae usually are about 1/2 to 3/4 inch long when we first find them in corn. Older larvae outgrow the grass stems and crawl to nearby larger-stemmed plants, such as corn. The last larval instar is often dirty gray or whitish and may be more difficult to identify.

Late-instar stalk borer larva.

Plant injury occurs in two ways. If larvae enter the plant through the lower stalk and tunnel upward, the feeding injures the growing point and the damage appears as "dead heart."

Larvae that enter the top of the plant tunnel downward into the stalk. Injury first appears on newly emerging leaves as ragged holes that increase in size as the leaves expand. Later, the growth of the tassel may be affected, and the upper part of the plant may be deformed.

Corn plants injured by stalk borer.

Damage occurs most often in corn rows adjacent to fencerows, road ditches, grass conservation waterways, and terraces, or in spots within the field that had weed infestations the previous year. There are no effective rescue treatments for controlling stalk borers after they have entered the cornstalk. Destroying weed hosts will reduce egg-laying sites in late summer. Postemergence insecticide treatments are effective when larvae are moving from weed hosts to corn plants. It can be very difficult to pinpoint the exact time that movement from weeds to corn plants occurs.

You need to begin watching for stalk borers when the larvae outgrow their initial host plants (typically weeds along field margins or in waterways) and crawl to nearby corn. This movement takes place over several weeks, and we can estimate it by accumulating heat units above a base temperature of 41°F. Stalk borers first begin to move into corn when about 1,100 heat units have accumulated from January 1; 50% movement occurs when about 1,400 to 1,700 heat units have accumulated. We recommend scouting when 1,300 to 1,400 heat units have accumulated, and a decision to treat with an insecticide should be made between 1,400 and 1,700 heat units.

Figure 2 shows actual heat-unit accumulations (base 41°F) from January 1 through May 13, 2001. Bob Scott, Illinois State Water Survey, supplied the map. Initial movement of stalk borers is slightly ahead of last year in the southern one-third of the state. However, based on actual heat-unit accumulations, 50% movement has not begun in Illinois. Last year a 50% movement was under way in a small portion of southeastern Illinois, as of May 8. We will continue to supply these maps in the next few issues of the Bulletin.

Insecticides suggested for control of stalk borers are *Ambush at 6.4 to 12.8 oz per acre; *Asana XL at 5.8 to 9.6 oz per acre; *Capture 2EC at 2.1 to 6.4 oz per acre; Lorsban 4E at 2 to 3 pt per acre; *Pounce 3.2EC at 4 to 8 oz per acre; and *Warrior T at 2.56 to 3.84 oz per acre. (Use of products preceded by an asterisk is restricted to certified applicators.) Some Bt-corn hybrids also provide some control of stalk borers.

Economic injury levels for stalk borers have been developed and published by Iowa State University. These thresholds (Table 1) are based on six corn-leaf stages, three corn prices, control costs of $13 per acre, and a control level of 80%.

We have received no reports of stalk borer activity yet, but we will share that information with you as soon as we do.--Susan Ratcliffe, Mike Gray, and Kevin Steffey

Author: Kevin Steffey Mike Gray Susan Ratcliffe

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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