Corn throughout South Dakota has either emerged or is beginning to emerge. During this time, it is important to scout border rows for the common stalk borer. The common stalk borer is considered a minor and occasional pest in South Dakota; however, it is capable or reducing plant stands near the edges of fields or wherever grass or weeds are adjacent to the field. The common stalk borer is an early season pest that initially feeds on corn leaves, but eventually tunnels into the stalks during the early vegetative growth stages. This tunneling can cause “dead heart” and subsequent plant death.
Profile & Appearance
The adults of the common stalk borer are small, brown moths. The front wings span approximately 1 to 1.4 inches and are typically gray-brown in color with a few clusters of small white spots. Common stalk borer caterpillars are distinctive and easier to identify than the moths. The common stalk borer 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 caterpillars age. The latter part of the abdomen is cream in color. Fully developed caterpillars are 1.5 to 2 inches in length.
Figure 1. Common stalk borer caterpillar. Credit: A. Varenhorst.
Lifecycle & Behavior
Common stalk borers have one generation per year in South Dakota. Adult moths emerge between August and October and are short lived. Females lay eggs predominately on dead vegetation, and prefer narrow-leaved perennial grasses (e.g., giant foxtail, chardgrass, 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 available. The caterpillars eventually outgrow their smaller host plants, and begin searching for larger hosts such as corn and occasionally soybean. Usually only one caterpillar is found within smaller host plants as common stalk borer caterpillars are cannibalistic. However, larger host plants can contain multiple caterpillars as long as they are not in close proximity to one another.
Predicting common stalk borer migration to corn fields.
The hatching and movement of common stalk borers to corn can be calculated by using degree days (41°F base temperature). Common stalk borer eggs typically begin to hatch at ~575 degree days, and the majority of eggs will finish hatching by ~750 degree days. The caterpillars begin moving from smaller grass host plants to corn at ~1,400 degree days, with 50% of common stalk borer caterpillars moving into corn around ~1,700 degree days (Table 1).
In South Dakota, we have exceeded the 575 degree days mark and much of the state has exceeded 750 degree days. This indicates that common stalk borer hatch is well underway, but movement to corn fields has yet to begin (Table 2). Scouting should begin within a week.
Table 1. Color coded key based on accumulated degree days and caterpillar activity.
|Accumulated Degree Days Since January 1, 2017||Caterpillar Activity|
|0-574||Conditions approaching egg hatch|
|750-1300||Development on weeds and grasses|
|1300-1400||10% of caterpillars moving to corn
(begin scouting corn)
|1400-1700||50% of caterpillars moving to corn|
Table 2. Accumulated degree days for locations within South Dakota in 2017.
|Location||Accumulated Degree Days
Since January 1, 2017