Mystery Feeding in Corn: Sap Beetles?

July 2, 1999

· "Nicks" and "windowpanes" have been found in corn leaves, but no insects found.

· Sap beetles have been found in corn whorls throughout central and northern Illinois.

The most common reports of insect injury I received during the past week and a half did not have an identifiable cause. Several people throughout central and northern Illinois have observed "nicks" or small "windowpanes" in corn leaves, but they've been unable to find any insects in some of the fields. I have seen the injury on both fresh and dried samples of corn leaves, and it looks very similar to feeding injury caused by first-instar European corn borers. In fact, one of my first guesses was that people were finding the injury in Bt cornfields but were not able to find the larvae because the borers had died from ingesting the Bt endotoxin. However, many folks also have observed the same type of injury in fields of non-Bt corn.

Picnic beetles.

So what is it? I suppose it is possible that some of the injury can be attributed to young corn borer larvae that since have perished as a result of heavy rains or some other environmental calamity. However, several of the reports, submitted independently, also included observations of small, brown beetlesas many as four to five per plant. I have received two samples of these beetles and have identified them as sap beetles. Most of us are familiar with sap beetles, especially the well-known, four-spotted "picnic beetle." Two other species of sap beetles, the corn and dusky sap beetles, are less recognized but just as common as, if not more common than, the "picnic beetle." We usually expect to find sap beetles later in the season as secondary invaders of corn ears or stalks damaged by corn earworms, fall armyworms, or European corn borers. But the literature indicates that sap beetles have three to four generations each year in the central Corn Belt. So their presence right now is not unusual.

Most people have been finding the sap beetles in corn whorls. Look for very small, rather nondescript beetles. The adult corn sap beetle is about 1/8 inch long, ranging from red-tinged black to brown-yellow. The dusky sap beetle is similar in appearance except slightly larger, about 1/6 inch long, and dull black. Both species have characteristic short wing covers and club-shaped antennae (Figure 3).

A logical explanation for their presence in cornfields and feeding on corn leaves right now is that the corn leaf tissue might already have been injured in some way. However, in one of the samples I received, the leaves that had been fed upon appeared quite healthy. Regardless of the reason for the presence of sap beetles and the cause of the "nicking," the injury is not significant and control of these relatively innocuous pests is not warranted. I have been asked, "Will the feeding wounds enable disease pathogens to infect the corn plants?" Who knows? Many things are possible. After all, once upon a time we stated that western corn rootworms would not lay eggs in clean soybean fields.--Kevin Steffey

Author: Kevin Steffey