Pest of the Week

Corn Earworm Chewing Through The Corn Belt

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Entomologists from Iowa, Maryland, Michigan, Ohio and Ontario are reporting above-average populations of corn earworm, which can put field corn at risk for ear molds and mycotoxins later in the season. (DTN photo by Pamela Smith)

ROCKVILLE, Md. (DTN) -- It's been a tough year for most corn growers, but at least one creature out there appears to be a fan of the 2019 corn crop.

The corn earworm is making a surprising number of appearances in the Corn Belt and beyond this year, with entomologists from Iowa, Maryland, Michigan, Ohio and Ontario reporting above-average populations of the ear-chewing pest.

"The corn earworm doesn't overwinter in Ohio, Michigan or points north of there, so most of what we are getting is coming up from more southern states," explained Ohio State University Field Crop Entomologist Kelley Tilmon. "A lot of our infestations of this insect are weather dependent, when we have fronts that blow up moths. This year we've received more of them than is typical."

While some corn earworm damage is an annual occurrence, it is usually only a serious economic problem in sweet corn, where ear damage is more costly and Bt traits less common. But populations are high enough to cause concern in field corn this year, entomologists said.

The biggest threat for these acres is insect feeding can open corn ears up to diseases that will compromise the quality and safety of the grain, Tilmon noted. "Damaged ears may be colonized by fungi such as Fusarium, Gibberella and Aspergillus that produce harmful mycotoxins," she said.

CHECK YOUR CORN

Don't assume the Bt traits in your corn hybrid will protect your fields from this insect.

The corn earworm feeds on a wide variety of crops, most notably cotton, where it is called the cotton bollworm. In southern states, the insect can be exposed to the same Bt traits in both corn and cotton fields within the same season, Tilmon explained. As a result, entomologists have confirmed resistance in the bollworm/earworm to multiple Bt Cry proteins. Only the Vip3A protein, found in Viptera, Leptra and Trecepta corn hybrids, still provides reliable protection against this insect, Tilmon said.

Michigan State University Extension entomologist Chris DiFonzo is particularly concerned about reports of earworm infestations in Bt hybrids containing Cry1A.105 + Cry2Ab2, found in VT Double Pro, VT Triple Pro and SmartStax. While earworm resistance to this trait has been confirmed in sweet corn in Maryland and field corn in North Carolina, it is not common in the Midwest, and growers with unexpected feeding in these hybrids should report it, DiFonzo said.

"The bottom line is if you are scouting fields and finding caterpillars, be aware that they could be earworm," she told growers in a university pest alert. "If it's a Bt hybrid, check the trait package to determine which species should be controlled. If the damage levels are unexpected, then it's important to investigate and try to explain why."

Use the Handy Bt Trait Table, created by DiFonzo, to determine which Bt traits your hybrids contain: https://agrilife.org/….

KNOW WHAT TO LOOK FOR

Corn earworm moths appeared north earlier than normal this summer, in June rather than mid- to late-July, DiFonzo noted. Due to the challenging spring, moths encountered cornfields in a wide variety of developmental stages, including their favorite -- corn in the early silking stages, Tilmon added.

"Moths lay their eggs in the corn silks, so as soon as they hatch, the larvae are right there in the ear tip," she explained. The caterpillars feed inside the husk, primarily on the tip of the corn ear, protected from insecticides.

Earworm caterpillars can be confused with other ear-feeding insects like western bean cutworm and fall armyworm. In general, corn earworm larvae will remain near the tip of the ear, whereas western bean cutworm will cut a trail down toward the center and bottom of the ear, Tilmon noted.

Don't rely on the old adage that you will only find one corn earworm per ear of corn, however. While it is true that corn earworm is cannibalistic, past studies have shown that sub-lethal effects of some Bt traits on the caterpillars can interrupt this trait. Perhaps because of this, it is increasingly common to find multiple corn earworm in a single ear of corn in the field, Tilmon said.

Color isn't too helpful, either, as the earworm can come in a range of shades, from yellow, green, brown, red and even black. More useful diagnostic clues include an orange head, small black spines or hairs, colorful alternating stripes running the length of the body, and black spots.

See detailed graphics for identifying the corn earworm from Purdue University here: https://extension.entm.purdue.edu/… and Michigan State's DiFonzo here: https://www.canr.msu.edu/….

While it's too late to treat infested fields, growers should be on alert for certain ear molds later in the season and may want to target those fields for early harvest and grain segregation. You can read more about the types of molds and mycotoxins infested fields are at risk for here, from Ohio State University here: https://agcrops.osu.edu/….

Read more from Tilmon on this topic here: https://agcrops.osu.edu/…. Read DiFonzo's pest alert here: https://www.canr.msu.edu/….

See a University of Maryland article on earworm here: http://blog.umd.edu/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter @Emily_unglesbee.

(PS/CZ)

Emily Unglesbee