By Adam J. Varenhorst
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 | Recommendation |
| 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).
Figure 1. Defoliation caused by common stalk borer caterpillars.
Figure 2. Common stalk borer caterpillar in the whorl of a corn plant.