by Erin Hodgson, Department of Entomology and Rich Pope, Integrated Pest Management Program
Stalk borer is native to Iowa and has only one generation per year. Adult stalk borers are grayish-brown moths with white spots along the forewing with a 1-inch wingspan. Adults emerge in early August and lay eggs in grasses and broadleaf weeds until October. Larvae hatch in late April and early May, or when 500 degree days (base 41°F) have accumulated.
Small larvae are cream-colored with a dark brown or purple band around the body; lengthwise stripes may also be present along the abdomen. Young larvae burrow and feed in grass and weed stems until they outgrow the stem. These older larvae move to larger plants around 1,400 degree days, or earlier if the grass is cut or killed with herbicides. Fully-grown larvae drop to the soil to pupate, emerging in August to complete the life cycle.
Stalk borers have a wide host range, with larvae feeding on over 175 different plant species. In the spring, young larvae are commonly found on brome grass and giant ragweed. Eventually, older larvae move to corn and occasionally soybean.
Stalk borer feeding in corn can cause wilting of upper leaves or may make irregular holes in newly unrolled leaves. Stalk borer larvae, as the common name suggests, typically burrow into the stalk at soil level and tunnel upward. Infested corn plants will be stunted and not produce viable ears. In some cases, young corn plants die.
Iowa State University research has found Bt corn (YieldGard® corn borer hybrid) will suppress but not completely control stalk borers. Therefore, non-Bt corn is more likely to be damaged, especially fields adjacent to grassy areas, waterways, ditches and fencerows.
Start looking for migrating larvae when 10 percent movement is predicted. This will vary by location and year since the prediction is based on temperature. The 2009 forecast for 10 percent stalk borer movement in Iowa is shown here.
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. Exposed larvae can be killed with a foliar insecticide treatment, but tunneling larvae are not susceptible. Young corn is particularly vulnerable to severe damage, but plants are unlikely to be killed once reaching V7 (seven true leaves).
Regular weed management within and around corn fields is crucial for reducing stalk borer populations. Stalk borers can cause damage throughout a field if grasses and broadleaf weeds are not controlled in a no-till system. Just killing weeds in a highly infested area will force larvae to feed on corn - this practice could significantly reduce a stand.
To prevent stand loss, scout and determine the percent of infested plants. The use of an economic threshold (Table 1), first developed by ISU entomologist Dr. Larry Pedigo, will help determine justifiable insecticide treatments based on market value and plant stage. Young plants have a lower threshold because they are more easily killed by stalk borer larvae.
If an insecticide is warranted, an insecticide can be tank-mixed with a fast burndown herbicide, or applied seven days after a slow burndown herbicide. A number of insecticides are available for stalk borer control (Table 2). Border treatment should be considered if infestations are localized. Insecticides must be well-timed so that products are reaching exposed larvae before they burrow into the stalk. Make sure to read the label and follow directions, especially if tank-mixing with a herbicide, for optimal stalk borer control.
Table 1. Economic thresholds for stalk borer in corn.
Table 2. Insecticides labeled for stalk borer control in corn.
Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at email@example.com or phone (515) 294-2847. Rich Pope is an extension program specialist with responsibilities in integrated pest management