Stalk borer damages cornfields

From July through August, producers and researchers unexpectedly observed significant damage resembling European-corn-borer injury along the edges of several corn fields in Martin and Mower counties of Minnesota.

Affected plants showed rows of holes on the leaves and boring damage in the stalks. But dissection of the corn stalk revealed the culprit was common stalk borer, Papaipema nebris, rather than the European corn borer.

Bt trait testing of the affected plants confirmed the expression of several Bt proteins, including Cry1F, Cry2A, Cry3Bb and Cry34/35. Among those, Cry1F and Cry2A target lepidopteran insects and are expected to suppress common-stalk-borer infestations. Whereas Cry3Bb and Cry34/35 are only effective against corn rootworms.

Damage was concentrated in the first few rows next to grassy areas, while interior rows remained largely undamaged. In one field about 60 percent to 70 percent of the plants along the first few rows, based on about 200 sampled plants, showed visible injury near the ditch. That edge pattern of injury is characteristic of the common stalk borer’s migration behavior from grassy weeds into corn early in the growing season. Further investigation is needed to determine whether the survival of common-stalk-borer larvae in the Bt corn plants is resulting from the movement of large larvae or potential resistance to Bt proteins.

The common stalk borer is native to North American and completes one generation per year. Females lay eggs in late summer and early fall, typically on grasses and weeds along fencerows, waterways, contour strips or weedy field margins. The insect overwinters in the egg stage on plants such as bromegrass, bluegrass, and ragweed, and eggs hatch in late spring. Young larvae feed inside grass stems and as they grow, or when weeds are mowed or killed by herbicides, they migrate into adjacent corn, typically infesting the first four to six rows along field edges.

Larval feeding causes two main types of injury to corn, leaf feeding and stalk tunneling. Early feeding produces ragged holes in whorl leaves, while tunneling into the stalk can destroy the growing point, causing wilted centers or “deadheart” symptoms. Severely injured plants may become stunted, tiller excessively, or produce small or barren ears. Damage is most severe on young corn -- stages V2 to V5. Although tolerance increases after the V6 stage, infestations along grassy borders can still cause localized yield losses if larvae are not identified and managed promptly.