Caterpillars have been extremely active this year, especially in soybean. We suspect that late-season infestations of caterpillars in corn are no exception. These infestations are serious because these caterpillars feed on the ear and/or tunnel into stalks. Their feeding can also lead to secondary infestations of other, less common insects, and severely infested fields can be at risk for ear molds and mycotoxins. August is an excellent time to scout for ear-feeders.
Scouting is essential to determine what species are feeding on the ear. Determining the species present will tell you whether Bt traits or insecticide applications are performing as expected and inform future management decisions. Once these insects are in the ear, it is too late to treat for them the present year. Below we will discuss typical symptoms of caterpillar injury and how to identify four key species: corn earworm, European corn borer, western bean cutworm, and fall armyworm.
Typically, caterpillars enter the ear by chewing through the husk or via the silks. All caterpillar injury is similar because they have chewing mouthparts: kernels will be destroyed or missing. Another characteristic of caterpillar feeding is the presence of frass (insect waste). Ear molds, mycotoxins, and picnic beetles may also be present as a result of these infestations. Typically, folks begin looking at the ears closer to harvest after the caterpillars have vacated the ear: at this point, it would be difficult to determine exactly what species was present, so scouting should begin now to ensure proper identification.
Corn earworm (CEW)
Corn earworm is considered an infrequent field corn pest, although we have seen a lot of CEW activity in Iowa the past few years (in both Bt and non-Bt hybrids). CEW has an orange head capsule, but the body is highly variable in color (brown, yellow, green, pink, red, etc.). However, there are black tubercles across the body, each with a single erect hair. CEW typically produces a lot of frass, which resembles oatmeal (mushy, tan, and wet).
CEW are initially found in the silks but eventually make their way inside the husk and feed at the ear tip or on the side of the ear. CEW are cannibalistic, so usually only one is present on each ear. They occasionally create an “exit hole” in the husk, which may be confused with the “entry holes” of other species.
European corn borer (ECB)
European corn borer was the target of the first Bt corn hybrids in 1996. Once considered a major corn pest, ECB is now of little economic importance but could be an issue for farmers who plant non-Bt corn hybrids. ECB has a dark head capsule and a smooth body that is uniformly gray or brown in color. Its frass is usually powdery and white or tan in color.
ECB can be found feeding on every part of the corn plant except the roots: silks, ear, ear shank, stalk, and leaves. Compared to other ear-feeding insects, ECB is more likely to burrow into individual kernels or the cob. Injury is most likely a result of ECB feeding in the ear shanks and from tunneling into the stalks, which is where older larvae overwinter.
Western bean cutworm (WBC)
Western bean cutworm injury is often associated with sandy soils. Young WBC is dark with black heads, but this color eventually lightens to a pinkish-brown hue. Later instars have reddish-brown head capsules with two thick, dark stripes behind the head. The body is smooth with subtle stripes.
Similar to CEW, kernel feeding typically occurs at the ear tip or on the side of the ear. Injury varies in severity from surface feeding (scraping the kernels) to completely consuming large areas of kernels. WBC are not cannibalistic, so it is not uncommon to see more than one per ear. Usually, WBC leave an “entry hole” through the husk on the side of the ear, but they may also enter via the silks.
Fall armyworm (FAW)
Fall armyworm is not a common pest in Iowa field corn. However, they migrate north similar to other species and may occur. The diagnostic feature of FAW larvae is the inverted white ‘Y’ on their head capsule. They have a smooth body with light striping on the back. Their color is highly variable (green, light brown to almost black).
FAW injury is similar to the other species described: they feed on kernels at the ear tip or on the sides of the ear and may chew “entry holes” in the husk similar to WBC.
Picnic beetles are sometimes blamed for ear feeding, but they are typically not the prime suspect. They do not normally feed on healthy plants, but rather work on previously injured spots. Adults may be found in the ears and stalks of corn that have been previously fed on by caterpillars and are typically present after the pest species has vacated the ear.
Adult picnic beetles are black or brown and usually have four prominent orange/yellow spots on their wings. They have distinct, clubbed antennae.