Wallaces Farmer

Determine percent of infested plants and consider applying insecticide at peak larval movement.

June 20, 2019

5 Min Read
Common stalk borer larva
CORN PEST: Common stalk borer larva is a caterpillar that attacks corn plants along the edges of fields. Adam Varenhorst

In mid-June Iowa State University Extension field agronomists began hearing reports of higher-than-normal stalk borer infestations along the edges of cornfields compared to previous years. According to degree-day tracking so far in the 2019 growing season, the caterpillars should be moving from overwintering hosts to corn throughout Iowa during the week of June 17.

Tracking degree days is a useful tool to estimate when common stalk borer larvae begin moving into cornfields from their overwintering hosts, says ISU Extension entomologist Erin Hodgson. She provides the following guidelines and management recommendations.

Foliar insecticide applications, if needed, are only effective when larvae are migrating and exposed. Start scouting corn for stalk borer larvae when 1,300 to 1,400 degree days, (base 41 degrees F) have accumulated. Much of Iowa has reached this important benchmark. Thus, scouting for migrating larvae should begin now to make timely treatment decisions.

Female moths prefer to lay eggs in weedy areas in August and September, so managing weeds (especially giant ragweed) in and around corn during that time will make those fields less attractive. Long-term management requires mowing grassy edges around field edges, so the females will not lay eggs in that area during the fall.

Making insecticide treatment decision

Treatment is necessary if you see stalk borers infesting corn plants during the growing season. To prevent stand loss, scout and determine the percent of infested plants. Consider insecticide application at peak larval movement, or 1,400  to 1,700 degree days (base 41 degrees F). The use of an economic threshold (Table 1), first developed by ISU entomologist Larry Pedigo, will help determine justifiable insecticide treatments based on corn’s market value and plant growth stage. Young plants have a lower threshold because they are more easily killed by stalk borer larvae.

corn borer chart.JPG

Stalk borers tend to re-infest the same fields, so prioritize fields with a history of stalk borers for scouting first with extra attention to the field edges. Applying insecticides after larvae have entered the stalk is not effective. Instead, target foliar applications to larvae as they migrate from grasses to corn. Look for larvae inside the whorls of corn plants to determine the number of plants infested.

The larvae are not highly mobile and typically only move into the first four to six rows of corn. Look for new leaves with irregular feeding holes or for small larvae resting inside the corn whorls. Larvae will excrete a considerable amount of frass pellets in the whorl or at the entry hole in the stalk. Young corn is particularly vulnerable to severe injury, but plants are unlikely to be killed once reaching V7.

Common stalk borer adults are moths that lay eggs that overwinter and hatch into larvae that attack corn plants

ADULT STAGE: Common stalk borer adults are moths that lay eggs that overwinter and hatch into larvae that attack corn plants. 

Using burndown herbicides before corn planting can force stalk borers to move and infest emerging corn. If an insecticide is warranted based on stalk borer densities, the application must be well-timed to reach exposed larvae before they burrow into the stalk. Border treatments should be considered since the infestations are usually localized. Make sure to read the label and follow directions, especially if tank-mixing with herbicides, for optimal stalk borer control.

Don’t confuse common stalk borer with other kinds of caterpillars. Stalk borer larvae have three pairs of true legs and four pairs of fleshy prolegs. The body is creamy white and dark purple with brown stripes. Often there is a creamy white stripe running down the back of the thorax and abdomen. A distinctive feature of stalk borer larvae is an orange head with two dark lateral stripes. The adults are dark grey and brown colored moths, with jagged white lines and two to three clusters of white spots.

How stalk borers thrive

Stalk borers have one generation annually in Iowa. Stalk borer eggs are laid on grasses and weeds in the fall and overwinter in this cold-hardy stage. Egg hatch typically occurs around April 19 to June 5, and about 50% of egg hatch happens at 494 growing degree days.

Young larvae will feed on grasses and weeds until they outgrow the stem of the host plant. Migration to larger host plants begins around 1,300 to 1,400 degree days. Fully developed larvae drop to the soil to pupate. About 50% of pupation happens at 2,746 degree days, with 50% adult emergence at 3,537 degree days. Peak adult moth flight occurs during the first two weeks of September.

Corn adjacent to grassy and weedy areas becomes a suitable food source for migrating larvae. The most susceptible corn growth stages for infestation are V1 to V5 or about 2 to 24 inches in plant height. Larvae can defoliate leaves and cause non-economic injury. More often, however, the larvae kill corn plants by entering the stalk and destroying the growing point. A dead heart plant will have outer leaves that appear healthy, but the newest whorl leaves die and can cause barren plants.

Stalk borer larvae can shred corn leaves and destroy the growing point in the plant.

DOING DAMAGE: Stalk borer larvae can shred corn leaves and destroy the growing point in the plant.

For more information on stalk borer biology and management, read a Journal of Integrated Pest Management article by Rice and Davis, called Stalk borer ecology and IPM in corn.

Source: ISU, which is responsible for information provided and is wholly owned by the source. Informa Business Media and its subsidiaries aren’t responsible for any content in this information asset.

 

 

 

 

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