Common Stalk Borer Activity Update Back »

Written collaboratively by Adam Varenhorst, Philip Rozeboom, Laura Edwards, Amanda Bachmann, and Patrick Wagner.

Throughout South Dakota, corn is beginning to emerge. In its early vegetative growth stages, corn is susceptible to feeding by the common stalk borer. For the most part, common stalk borer is considered to be only a minor or occasional pest. However, it is capable of reducing plant stands near the edges of fields or in any area of the field that borders a grassy or weedy area. Common stalk borers are an early season pest that initially feeds on corn leaves. As they grow, they move to the whorl of the plant and bore into the growing point. This activity can result in dead heart and subsequent plant death.

Predicting Common Stalk Borer Migration Into Corn Fields With Degree Days

The hatching and movement of common stalk borer caterpillars can be estimated by using degree days with a developmental threshold of 41°F. Common stalk borer eggs typically begin to hatch at 575 degree days. The caterpillars finish hatching and begin development on weeds and grasses at 750 degree days. At 1300 degree days, 10% of the caterpillars will begin moving to corn. At this point corn should begin to be scouted. At 1400 degree days, 50% of the caterpillars will or have moved into corn.

As a reminder, the equation for degree days is:

(Maximum Daily Temperature - Minimum Daily Temperature) ÷ 2 - The Developmental Threshold

In South Dakota, most of the state is still between conditions approaching egg hatch and egg hatch occurring. In Hot Springs and Vermillion, enough degree days have been accumulated for the common stalk borers to be developing on weeds and grasses. Scouting should begin immediately in those areas.

Table 1. Color coded key for common stalk borer caterpillar activity based on accumulated degree days.

Accumulated Degree Days Common Stalk Borer Caterpillar Activity Recommendation
0-574 Conditions becoming favorable for egg hatch. Scouting is not necessary
575-749 Eggs will begin to hatch. Scouting is not necessary
750-1299 Young caterpillars will bore into stalks of grass and weeds. No scouting necessary. Avoid spraying grass and weeds along field edges
1300-1399 10% of caterpillars begin moving into adjacent corn. Begin scouting field edges for defoliation
1400-1700 50% of caterpillars begin moving into adjacent corn. Continue scouting for defoliation.


Table 2. Common stalk borer activity based on degree day accumulation for South Dakota.

Location Accumulated Degree Days
Since January 1, 2018
Buffalo 547 No scouting necessary
Newell 532.5 No scouting necessary
Rapid City 714.5 No scouting necessary
Hot Springs 860.5 Begin scouting within 5 days
Lemmon 574.5 No scouting necessary
Faith 486.5 No scouting necessary
Cottonwood 637.5 No scouting necessary
Mission 619.5 No scouting necessary
Selby 492.5 No scouting necessary
Gettysburg 495.5 No scouting necessary
Pierre 616.5 No scouting necessary
Winner 715.5 No scouting necessary
Aberdeen 549.5 No scouting necessary
Huron 604 No scouting necessary
Tyndall 656.5 No scouting necessary
Sisseton 547 No scouting necessary
Brookings 464.5 No scouting necessary
Vermillion 768 Begin scouting within 5 days


Identifying Common Stalk Borer Defoliation

The initial defoliation caused by common stalk borers will leave sections of the leaves with ragged holes. In addition, the holes will be surrounded with a sawdust like excrement (Figure 1). As mentioned previously, the caterpillars will eventually move to the whorl and begin feeding at the growing point. If defoliation is observed on the leaves, search for caterpillars by pulling the whorl and slowly unraveling it (Figure 2).

Green plant with holes in the leaf.
Figure 1. Defoliation caused by common stalk borer caterpillars. Courtesy: Adam Varenhorst

Green plant leaf with hands on each side. Small caterpillar at the center.
Figure 2. Common stalk borer caterpillar in the whorl of a corn plant. Courtesy: Adam Varenhorst

Identification of Common Stalk Borers

Common stalk borer adults are small brown moths. The caterpillars range in size during development with the last instar typically 1 ½ to 2 inches in length. The caterpillars can be identified by their solid orange head that has a black stripe along each side. In addition, younger caterpillars will have a distinctive purple-brown-black band between their last true leg and first abdominal proleg. This is often referred to as the saddle, which will fade as the caterpillars age. The caterpillars also have alternating dark and light bands behind the saddle (Figure 3).

Caterpillar with orange head, and dark patch in middle of the body.
Figure 3. Common stalk borer caterpillar. Courtesy: Adam Varenhorst

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