Common Stalk Borers and Why They Matter in Early Season Corn

Identification

Figure 1. Common stalk borer caterpillar. (Credit: Adam Varenhorst, SDSU Extension)

Adult common stalk borer are small, brown moths. The front wings are typically gray-brown in color with a few clusters of small white spots, and they span approximately 1 to 1.5 inches.

Common stalk borer caterpillars are distinctive and easier to identify than the adults. The caterpillars have solid orange head capsules that have a single black stripe along each side (Figure 1). Younger caterpillars have a distinctive purple-brown band (sometimes referred to as a saddle) behind their true legs that extends to the second pair of abdominal prolegs. The purple-brown band is distinctive during the early stages of caterpillar development, but this band fades as the caterpillars age. The latter part of the abdomen is cream in color with black stripes. Fully developed caterpillars are 1.5 to 2 inches in length.

Life Cycle

Figure 2. Stalk tunneling of a V4 corn plant by a common stalk borer caterpillar. (Courtesy: Mike Dunbar)

Common stalk borers have one generation per year. Adult moths emerge between August and October and are short lived. Females lay eggs predominately on dead vegetation, preferentially on narrow-leaved perennial grasses (for example, giant foxtail, orchard grass, or winter wheat) over broad-leaved plants or annual grasses. Eggs are laid either within curled leaves or between the plant stem and the leaf sheath. The eggs overwinter and hatch in late spring. Common stalk borer caterpillars feed on a wide range of plant species, and newly hatched caterpillars tunnel into the first suitable host plant that they find. The caterpillars eventually outgrow smaller host plants and begin searching for larger hosts, including corn. Usually only one caterpillar is found within smaller host plants, as common stalk borer caterpillars are cannibalistic; however, larger host plants can be infested by multiple caterpillars.

Scouting and Injury to Corn

Figure 3. Leaf feeding injury associated with common stalk borer feeding. (Credit: Adam Varenhorst, SDSU Extension)

Common stalk borer caterpillars injure corn when feeding on early developmental stages (VE through V5). Infestations are typically observed in the first 4 to 8 rows that are adjacent to grassy areas. Although caterpillars feed on corn leaves, stalk tunneling is the principal cause of yield loss (Figure 2). Feeding on the corn leaves results in ragged holes that are often accompanied by a sawdust-like substance around them (Figure 3). Stalk tunneling can delay corn development, resulting in deformed plants and reduced yields via poor pollination and smaller ears. If caterpillars feed on the growing point of the corn, they can cause a condition known as “dead heart.” This causes the plant to stop growing, and it will eventually wilt and die. As corn plants develop, they are more capable of tolerating common stalk borer infestations, and the caterpillars are less likely to feed on the growing point. This is why delaying early season movement into corn is very important.

Management

Figure 4. Unfurled corn whorl containing a common stalk borer caterpillar. (Credit: Adam Varenhorst, SDSU Extension)

The common stalk borer caterpillars are difficult to manage because they tunnel into corn stalks and are very well protected. Cultural control for common stalk borer includes a weed management plan that limits or eliminates grasses near field margins. Management of grasses reduces favorable egg laying sites for female common stalk borer. To be effective, this management should occur in the fall or prior to planting. Another option is to plant corn early, as older plants are more capable of tolerating common stalk borer caterpillar feeding. Populations of common stalk borer are also reduced by burning grassy areas surrounding cornfields during the winter.

Timely insecticide applications that target either newly hatched caterpillars or their migration from grasses to corn can effectively reduce common stalk borer populations and subsequent feeding. However, these applications are only economical if: 1) common stalk borer caterpillars are present in alternative hosts in field borders, or 2) common stalk borers are observed moving to corn prior to settling in the whorl.

There are Bt corn hybrids available that produce toxins that manage common stalk borer caterpillars. However, only the Bt toxin Vip3A is labeled for common stalk borer. The other Bt toxins, Cry1Ab and Cry1F, are labeled for suppressing common stalk borer, indicating that infestations could still occur.