Stalk Borer

Stalk borer (Papaipema nebris) is an occasional pest of corn in Iowa, but some fields experience persistent stalk borer issues. Stalk borers typically only infest the first few rows of corn near ditches, fence rows, or waterways, but understanding how to scout for and identify stalk borers can help with management decisions.

Identification

Adult: Adult moths are small with gray-brown forewings. They have a thin, slightly curved line running parallel to the wing tip. The forewings also have a few clusters of small white spots (Photo 1). The wingspan is approximately 1 to 1.5 inches.

Egg: Stalk borer eggs are 0.02 inches in diameter and somewhat flattened. There are longitudinal ridges and cross-ridges that form shallow pits.

Larva: Young larvae are a distinct brownish-purple color with an orange head capsule that has a black stripe on each side. Young larvae also have three prominent stripes at both ends of the body and a purple band in the middle of the body that disrupts the stripes on the sides (Photo 2). Fully grown larvae are uniformly gray and up to 2 inches long (Photo 3).

Pupa: Pupae are reddish-brown and may occur in the soil or plant stalks.

Biology

Stalk borer is native to North America and present in most places east of the Rocky Mountains. In addition to infesting cornfields, stalk borers also feed on over 100 other plant species, including vegetables, broadleaf weeds, and grasses. They may also feed on soybean, though they are not considered economically important.

Female stalk borer moths lay eggs on grasses (e.g., smooth bromegrass or rye and wheat cover crops) and ragweed during late summer and early fall. Eggs can be found on dead vegetation inside curled leaves or tucked between the sheath and stem. Eggs are laid singly or in groups, and females lay an average of 880 eggs in a lifetime. Eggs overwinter and hatch during April and May, then larvae bore into the stems of grasses or other hosts to feed. Larvae migrate to nearby corn plants to complete development either when they outgrow the stems or those plants are terminated (mechanically or chemically). If corn has emerged and other hosts are not available at the time eggs hatch, young larvae may begin feeding on corn. Larvae develop through 7-16 instars in 60-130 days, depending largely on host quality, then pupate in the soil or in plant stalks. The pupal stage lasts approximately 3 weeks (range 16-40 days). Adult moths emerge in late summer to fall to lay eggs. There is only one generation per year in Iowa.

Plant Injury

The larvae are not highly mobile and typically only move into the first four to six rows of corn near their overwintering site. Stalk borer larvae either feed on leaves or tunnel into the stalk. Leaf feeding first appears as irregular rows of holes in the newly unfolding leaves, similar to whorl-feeding by European corn borer. However, stalk borer feeding results in larger and more ragged holes than European corn borer (Photo 4). Heavy feeding can lead to abnormal plant growth, such as twisted leaves. Leaf feeding does not usually cause economic injury but tunneling can severely damage plants.

Stalk borer larvae feed on leaves in the whorl of corn plants before tunneling into the stalk (Photo 5). Sometimes, larvae burrow into the stalk at the base of the plant and tunnel upward. Larval feeding can destroy the growing point, but the growing point is not always injured. Severely injured plants may develop “dead heart,” where the whorl appears dead while the other leaves are green. If plants survive, they will likely grow tillers and be stunted. Tassels may be deformed, and silking may be delayed, which can result in yield losses. Many plants do not produce normal-sized ears, and when plants are attacked at early stages, they may not produce an ear at all. Severely injured plants may die, leading to stand reductions in corn. Young corn is particularly vulnerable to severe injury; plants are unlikely to be killed once they reach V7.

In soybean, plants infested with stalk borer usually have wilted terminals (Photo 6). It is easy to overlook infested soybean plants because neighboring plants grow quickly and fill in gaps. Yield losses have not been reported in soybean, because neighboring plants compensate well for any stand losses or stunted plants.

Risk Factors

Fields with a history of stalk borer injury are at higher risk since stalk borers tend to re-infest the same fields. Grass cover crops (e.g., rye or wheat) may be attractive egg-laying sites for female moths. Fields with late-season grassy weeds or unmanaged grass borders or waterways are attractive egg-laying sites as well.

Scouting

Stalk borers tend to re-infest the same fields, so prioritize scouting fields with a history of stalk borers, paying extra attention to the field edges. Finding “dead heads” in nearby grasses or weeds is an indicator of stalk borers in the area. Larvae excrete a considerable amount of frass pellets in the whorl or at the entry hole in the stalk, which is a good indication that larvae are present. Also, look for new leaves with irregular feeding holes that may indicate the presence of larvae.

Examine at least five sets of 10 plants, prioritizing field edges with grassy borders. If grassy weeds were present throughout the field the previous year, the entire field may need to be scouted. Look for feeding damage and larvae that are still in the whorl. The plant may need to be dissected to detect larvae that are moving down through the whorl but have not yet bored into the stalk. Calculate the percentage of plants infested with stalk borers (# plants with stalk borers ÷ # plants sampled × 100). Remember that larvae need to be present and not already bored into the stalk for insecticides to be effective. The use of an economic threshold, first developed by Iowa State University entomologist Larry Pedigo, will help determine justifiable insecticide treatments based on market value and plant stage (Table 1). Young plants have a lower threshold because they are more easily killed by stalk borer larvae.

Table 1. Economic thresholds (expressed as a percent of infested plants) for stalk borer in corn, based on market value, expected yield, and leaf stage.

Management

No rescue treatments are available for stalk borer because insecticides will not reach them once they have bored into the stalk. Tracking degree days and scouting the field can determine whether larvae are present and still migrating.

Tracking development: Tracking growing degree days (GDD) is a useful tool to estimate when stalk borer larvae begin moving into cornfields from their overwintering hosts (Table 2). Foliar insecticide applications are only effective when larvae are migrating and exposed to the insecticide. Start scouting corn for larvae when 1,300-1,400 GDD (base 41°F) have accumulated since January 1. Larvae begin moving to cornfields when 1,400 GDD have accumulated, and peak larval movement occurs at 1,700 GDD. Use the Pest Forecasting Maps to track GDD for stalk borer.

Table 2. Stalk borer development (growing degree days, base 41°F) and associated activity.

Cultural: Female moths prefer to lay eggs in weedy areas or grass cover crops in August and September, so any weed management that limits or eliminates grasses in or around the field can be effective. For example: managing weeds within the field, such as smooth bromegrass or giant ragweed, especially if they are still growing late in the season when moths lay eggs; or mowing, burning, or using herbicides on grasses and weeds in field margins, terraces, and waterways.

Management targeted while stalk borers are active (planting to July) will force them to move to nearby crops earlier. Weed and grass management should be targeted prior to adult emergence in late summer and fall or during late winter before eggs hatch. Because stalk borers tend to reinfest the same fields, long-term management of grassy areas or grassy weeds is usually required.

Early planting may allow corn to outgrow severe damage since smaller plants are at increased risk.

Biological: Ants, ground beetles, and spiders are predators of larvae that are moving between hosts. Parasitoids also cause a small percentage of mortality. None of these natural enemies are likely to cause significant mortality of stalk borer.

Plant-incorporated: Some Bt hybrids provide stalk borer control or suppression. Suppression indicates that a lower level of mortality is expected. Use the Handy Bt Trait Table to find appropriate hybrids or see if a hybrid has effective traits.

Chemical: 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. Target applications at peak larval movement, or 1,400-1,700 GDD (see Tracking Development section). Applying insecticides after larvae have entered the stalk is not effective. Since larvae are not highly mobile and tend to infest the first four to six rows, consider border treatments.

Since eliminating weeds can often make stalk borers move into corn, using a burndown herbicide and insecticide combination is effective, either tank-mixed or in a split application. The idea is that the herbicide will kill the grass and force larvae to move, and they will be killed by the residual insecticide. Tank mixing works best with a quick burndown herbicide; if using a slow burndown herbicide, the field should be sprayed with insecticide approximately seven days after the herbicide. Make sure to read the label and follow directions, especially if tank-mixing with herbicides, for optimal stalk borer control.

Additional Resources

For more information on stalk borer biology and management, read a Journal of Integrated Pest Management article by Rice and Davis (2010), called “Stalk borer ecology and IPM in corn.” This publication contains many photos of stalk borer injury in corn and soybean and is a good reference guide.