Check corn now for western bean cutworm damage before harvest

Alert! There have been numerous reports of high western bean cutworm infestations and damage in south and central Michigan, including in Bt corn hybrids.

Kernel damage and ear rot on non-Bt corn. Damage along the side of the ear is characteristic of western bean cutworm feeding. Photo: Chris DiFonzo, MSU.
Kernel damage and ear rot on non-Bt corn. Damage along the side of the ear is characteristic of western bean cutworm feeding. Photo: Chris DiFonzo, MSU.

A lot of western bean cutworm damage is being found in central and southern Michigan, in both non-Bt and Bt corn (including Cry1F). I would appreciate any observations on percent infested ears by hybrid, especially feedback on Bt hybrids with the Cry 1F Bt trait (email me at Cry1F is in Herculex 1 and Xtra; most Acremax (AM), Intrasect and TRIsect packages; in Smartstax; and Agrisure E-Z Refuge products (3122, 3220, 5122, 5222). Caterpillars will be tucked in the ear tip or between the ear and stalk; they may not be obvious at first glance unless you peel back the husk. To quote a caller from yesterday, Aug. 25, 2016, “When you walk into the field, it looks OK.”

Recall that western bean cutworms were a concern in 2008-2011 in corn and dry beans when it was colonizing the Great Lakes region from the western states. In the 2012 drought, the population tanked and biocontrol picked up for the next few years. However in 2015, numbers started to increase again. This year, flights were fairly heavy in central Michigan (a known hot spot), but also in southern counties. Damage is being reported most often in fields in areas with sandy soil, which likely increases overwintering success. Entomologists in neighboring states are reporting damage too. Up-and-down population cycles are typical of many agriculture pests.

Some people in southern Michigan may be dealing with western bean cutworms for the first time, so let’s recap a few important points.

Western bean cutworms overwinter here in Michigan (i.e., we grow our own), and seem to do especially well in areas with sandy soil. My lab found larvae overwintering as deep as 12 to 16 inches in sandy soil. Hot spots in previous years were counties along Lake Michigan, in southwest Michigan along the Indiana border, in central Michigan in Montcalm, Gratiot and Isabella counties, and in the Upper Peninsula, all areas with sandy soil.

Western bean cutworm females are attracted to pre-tassel corn for egglaying. Fields that are infested now were at that perfect stage back in late-July or early August. Larval management (a pyrethroid spray) needs to happen during and just after egg-hatch. Scouting for egg masses is relatively easy if you know what to look for. In Michigan, I recommend a lower threshold (5 percent plants infested) compared to western states. Once small larvae move down the plant and inside the husk, spraying is neither recommended nor effective. On all the pyrethroid labels under corn is this statement about western bean cutworms: “For control before the larva bores into the ear.” And late August is far too late since western bean cutworms are starting to drop to the ground to overwinter. The damage is mostly done.

Larvae usually feed in the ear tip. If the tip isn’t completely filled, yield loss can be minimal. Larvae sometimes bore into the side where the ear presses against the stalk. Again, the number of kernels eaten may only be a few. But yield loss doesn’t tell the whole story. Often, the more important impact from the feeding is the husk damage and opening of the ear. As western bean cutworms feed, they chew tissue and spew out messy frass. The open husk lets in secondary insects like sap beetles, which do the same. The result is a moist, wounded area for pathogen growth. Depending on environmental conditions, mycotoxins can then contaminate damaged grain at harvest.

Note it is too late – in time and plant stage – to spray fungicides in an attempt to prevent or “clean up” ear infections. Reduction of ear molds and mycotoxins must begin much earlier, by scouting and spraying for western bean cutworms at the optimal timing to avoid ear damage altogether.

The Bts that kill European corn borer do not give 100 percent control of western bean cutworms. Cry1Ab (Yieldgard) and Cry1A.105 plus Cry2Ab2 (Double Pro) are not effective. Cry1F is listed as giving western bean cutworm control, but it is having a tough time this year – many Cry1F hybrids are being chewed on, especially in southern Michigan. It is my opinion that it only provides suppression; bigger larvae are able to eat up ear tips regardless of the Cry1F. Vip3A (in some Agrisure hybrids and AcreMax Leptra), a Bt optimized for secondary caterpillars, should be effective against western bean cutworms.

This information is from a trial my lab did in 2010 during the western bean cutworm outbreak in Montcalm County. Cry1F hybrids had less damage and ear mold, but control of western bean cutworm was not 100 percent. (Note that these hybrids were not genetically equivalent, so yield loss is difficult to attribute all to western bean cutworm.)

Western bean cutworm control

Type of hybrid

Type of Bt(s)

Expected western bean cutworm control

% ears with feeding

% ears with ear rot

# kernels eaten per ear


Non Bt






-18 bu







 -8 bu







Avg. 239 bu/ac

Smart Stax







If you have a field that is heavily infested (beyond what you’d expect from the 5 percent refuge):

  • Consider reporting highly infested Cry1F fields to your seed dealer so that the companies can get an idea of the scope of the problem. For sure report a Vip3A field that appears to have unexpected high damage from western bean cutworms, as this would be of great concern.
  • If you have an option of harvesting an infested field as silage, that will avoid drying costs and further quality loss.
  • Run the grain from infested fields over a screen before it goes into a bin to get out as many damaged kernels as possible.
  • Dry the grain down as soon as possible to prevent pathogen growth and mycotoxin development.
  • Never store bad grain over good, or add it to a bin with good grain. Better yet, don’t store western bean cutworm damaged grain at all, if possible.

I would appreciate your emailed observations on percent infested ears by hybrid, especially feedback on Bt hybrids with the Cry1F Bt trait. You can email me at

Dr. DiFonzo’s work is funded in part by MSU’s AgBioResearch.

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