Corn Earworm, European Corn Borer, Fall Armyworm, or Western Bean Cutworm: Which One Is Causing the Injury I’m Finding on My Corn Ears?

Several questions about injury on corn ears has made it way to my desk the past week.


Insect injury to corn ear (photo courtesy of Duane Frederking).

Damaged ear tips, missing kernels, and fungal pathogens are all being reported. Several insect pests in Illinois could be the culprit. Corn earworm, fall armyworm, European corn borer, and western bean cutworm are pests of Illinois cornfields. Their larvae all feed on the ears of corn plants.

So how does one determine the cause of ear damage this late in the season? The answer is simple: You really can’t. At this time in the season, it is rare to find any larvae still feeding on corn ears. Without larvae, you can’t be positive if injury was caused by earworms, corn borers, fall armyworms, or bean cutworms, as they cause very similar injury. Let’s look at each insect individually.

Corn earworm. Two generations of corn earworm infest Illinois cornfields each year. Because earworms generally do not overwinter in Illinois, summer populations arise primarily from immigration of moths from southern states in late spring and early summer. Infestations of earworm larvae can cause injury to corn plants, including slight defoliation of leaves, damage to the tassel, and consumption of silks and kernels. The second corn earworm generation usually occurs during pollination. Larvae enter the ear primarily through the silk channel, unlike European corn borer and fall armyworm, which enter through the husks or cob. As silks dry, corn earworm begin feeding on kernels. Larvae feed at the tip and along the sides of the ear near the tip, continuing to feed until they mature. At that time the larvae drop to the ground to pupate. When leaving the ear, corn earworm may drop from the ear tip or create exit holes by chewing through the husk. These exit holes can be mistaken for entrance holes caused by other larvae.


Corn earworm larvae.


Corn earworm injury to corn ear.

European corn borer. Two to three generations of European corn borer occur in Illinois each year. Injury to corn ears is caused by the second and third generations. Loss of grain to larvae’s direct feeding on kernels has not recently been an issue in field corn, but in sweet corn and seed corn, losses can be significant. We’ve also received reports of corn borer feeding in non-GMO corn. Larvae feed on pollen and silks before entering the ear. Entry to the ear is also gained by tunneling through the shank and cob. Ear feeding by corn borer larvae is not focused on any one area. Injury can be found at both ends and along all sides of the ear. Larvae feed until mature; they overwinter as fifth-larval instars in stalks and plant debris.


European corn borer larva (photo courtesy of Marlin Rice, Iowa State University).


European corn borer injury to corn ear (photo courtesy of Marlin Rice, Iowa State University).

Fall armyworm. Like the corn earworm, fall armyworm moths migrate north into Illinois each year. Fall armyworms are a concern for cornfields from mid- to late summer. They cause serious leaf-feeding damage and feed directly on corn ears. Late-planted or later-maturing hybrids are more susceptible to fall armyworm injury. Most common is pretasseled corn. Larvae consume large amounts of leaf tissue, but as corn plants develop, larvae move to the ear. Unlike the corn earworm, the fall armyworm feeds by burrowing through the husk on the side of the ear. Larvae also enter at the base of the ear, feeding along the sides and even tunneling into the cob. They usually emerge at the base of the ear, leaving round holes in the husks.


Fall armyworm larva.


Fall armyworm injury to ear.

Western bean cutworm. A mid- to late-summer pest of corn, western bean cutworm moths begin to emerge in early July. Though some leaf feeding occurs, larvae feed primarily on silks, tassels, and developing kernels. Larvae of the western bean cutworm are not cannibalistic, and several larvae may infest one ear. Entry to ears is gained through silk channels or by chewing through husks, injuring the tip, base, and sides of the ear. Larvae feed on kernels until about mid-September, when they exit through husks. Reports of western bean cutworm injury have been very sporadic the past couple of years.


Western bean cutworm (photo courtesy of Marlin Rice, Iowa State University).


Western bean cutworm injury to corn ear (photo courtesy of Marlin Rice, Iowa State University).

Any one or combination of the aforementioned insects could be the cause of the injury being seen in cornfields. As much as we would like to be able to pinpoint the direct cause of injury, that is often impossible this late in the season. Summer scouting is the key to determining the potential insect culprits.

 

(Updated from 2005 article)


Insect Briefs – Soybean Aphids, Syrphid Flies, and More

–  Russ Higgins, University of Illinois Extension reports finding a couple of soybean aphids in LaSalle County this week. This follows some earlier reports in July of soybean aphids being found in low numbers in Iowa. Despite some of the recommendations floating around, direct yield loss from soybean aphid feeding does not occur when the first (or five or ten) aphids begin feeding. Today’s soybean varieties are equipped to handle minor challenges, including a few aphids. Yield loss from soybean aphid is related to how many soybean aphids are present and for how long the aphids are present and feeding. The amount of aphid population pressure over time is calculated as aphid-days. Simply put, this is the average number of aphids on a plant multiplied by the number of days they are present. A single soybean aphid on a plant for 10 days is equal to 10 aphid-days, 200 aphids on a plant for 20 days is equal to 4,000 aphid-days, and so on. This aphid-day concept proved to be a good indicator of how soybean yield responded to aphid populations. The lowest level of aphid infestation that has been shown to cause yield loss in soybean is several thousand aphid-days.

*This excerpt was taken from a larger document co-written by many entomologists in the North Central region (see list below). To read the full article with citations, please click on the link:​ Just the facts: A review of the biology and economics behind soybean aphid insecticide recommendations.

As always, we continue to encourage growers to scout fields and utilize economic established economic thresholds.

University of Minnesota: Bruce Potter, Robert Koch & Phil Glogoza
Iowa State University: Erin Hodgson
Purdue University: Christian Krupke
Penn State University: John Tooker
Michigan State University: Chris DiFonzo
Ohio State University: Andrew Michel & Kelley Tilmon
North Dakota State University: Travis Prochaska & Janet Knodel
University of Nebraska: Robert Wright & Thomas E. Hunt
University of Wisconsin: Bryan Jensen
University of Illinois: Kelly Estes & Joseph Spencer

 

–  I’ve received a few reports of maggots in corn. I believe these insects are syrphid fly larvae. One report indicated large numbers of these larvae throughout the entire corn field. Syrphid fly larvae are actually predators of soft-bodied insects like aphids. They are not causing injury to the corn. Perhaps some of you may remember a similar story out of Indiana last year: https://www.no-tillfarmer.com/articles/4935-looping-worms-and-sweat-bees-seen-in-corn

–  Despite inconsistent moth flights, there are reports of fall armyworm and corn earworm feeding in corn.

–  We’ve had a few western bean cutworm in traps in northern Illinois. Flights generally peak in mid-July.