Sap Beetles Infesting South Dakota Corn Ears, SDSU Extension Warns Producers

Figure 3. Sap beetle larvae feeding on an ear of corn. (Credit: Adam Varenhorst, SDSU Extension)

Northeast Radio SD News – South Dakota - South Dakota producers are reporting a noticeable increase in small black beetles infesting corn ears this season, according to SDSU Extension entomologists. The insects — commonly referred to as sap beetles — are appearing in unusually large numbers and are being found feeding on ear tips and inside damaged kernels.

SDSU Extension Associate Professor and Field Crop Entomologist Adam Varenhorst, along with co‑author Philip Rozeboom, says three species are responsible: the corn sap beetle, the dusky sap beetle, and the picnic beetle. Although typically considered minor pests, both adults and larvae are actively feeding on developing kernels this year.

Figure 1. Sap beetle adults on an ear of corn. (Courtesy: Eugene E. Nelson, Bugwood.org)

Identification

The three beetle species vary in size and appearance:

·         Corn Sap Beetle: Dull black, about 1/8 inch long.

·         Dusky Sap Beetle: Also dull black, slightly larger at 1/6 inch.

·         Picnic Beetle: The largest at 1/3 inch, shiny black with four yellow or orange spots on its wing covers.

Larvae of all three species look similar — cream to light pink, about ¼ inch long, with a brown head capsule and mottled segment behind the head.

Damage to Corn

Sap beetles are typically attracted to ears already damaged by insects or environmental factors, but they can also enter healthy ears from silking through maturity. Both adults and larvae feed on developing kernels, and larvae may hollow out kernels on the upper half of the ear.

Each ear can contain several larvae, and their feeding can create holes and expose kernels to fungal contamination. The beetles can carry fungi from the Aspergillus, Penicillium, and Fusarium genera, which are capable of producing mycotoxins.

Despite the increased activity, SDSU Extension notes that sap beetle larvae rarely cause economic losses in commercial corn.

Figure 2. Damaged kernels with holes and beetles feeding inside. (Credit: Adam Varenhorst, SDSU Extension)

More Information

Producers can find related guidance on stored grain protection, aphid monitoring, and corn rootworm scouting through SDSU Extension’s crop management resources.

For questions, SDSU Extension can be reached at 605‑688‑4792 or sdsu.extension@sdstate.edu.

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