Small white worm-like larvae have been observed infesting corn ears. The unknown larvae belonged to the Nitidulidae family and are either the corn sap beetle, dusky sap beetle, or picnic beetle. These beetles are considered minor pests of corn, and tend to be more of an issue for sweet corn. Although there wasn’t extensive damage to the kernels, there were kernels that had been hollowed out. In addition, some of the ears appeared to have a fungal infection near the ends.
The larvae of these three beetle species look very similar and are difficult to tell apart. Larvae of all three species are approximately ¼ of an inch long and are typically a cream to light pink color. They have a brown head capsule and the segment directly behind the head is mottled brown (Figure 1).
Adults species can easily be distinguished from one another. Adult corn sap beetles are a uniform dull black color and are approximately 1/8 of an inch long. The dusky sap beetle is also a dull black color, but is approximately 1/6 of an inch long. The picnic beetle stands out from the other two species due to both coloration, shiny black with four yellow or orange spots on their wing covers (elytra), and larger size, approximately 1/3 of an inch long.
Figure 1. Nitidulidae beetle larvae feeding on an ear of corn.
Photo by Adam Varenhorst.
Damage to Corn
Adults beetles are typically attracted to corn ears that were previously damaged by other insects or abiotic factors. However, they are also capable of entering undamaged ears any time from silking to maturity. Both the adults and larvae may damage corn ears by feeding on developing kernels. Larvae are capable of hollowing out kernels that are present on the upper half of the ear. Each ear may have several larvae present, however larvae of these beetles rarely cause economic losses.
In addition to their feeding, these beetles are capable of vectoring fungi from the genera Aspergillus, Penicillium, and Fusarium. These fungi are capable of producing mycotoxins. Aspergillus and Penicillium ear rots tend to develop under hot and dry weather conditions while Fusarium ear rot develops in warm, humid weather. Scout and harvest corn early if >10% of the ears have ear rots. Corn from such fields should be dried immediately to <15% moisture content (MC). Corn from fields with high ear rot incidence should be stored separately after it has dried to <15% MC.