Western Bean Cutworm: Genetic Traits

One of the big field crop stories this past fall was the reported inability of a genetically modified trait, Cry1F, to adequately control western bean cutworm – not in beans, but in field corn. This trait became available about 15 years ago, at which time western bean cutworm was primarily found in the western Corn Belt states of Colorado, Idaho, Nebraska, and Wyoming. Of these, only Nebraska and Idaho are major corn-growing states.

At the time, seed company marketing literature barely mentioned the Western bean cutworm, but this changed after Iowa State University entomologists discovered economic damage from this insect in that state. Formerly a pest of the Western Plains, now the western bean cutworm has rapidly spread eastward into the northeastern U.S. including all of New York, most of Pennsylvania and parts of New England. It’s been confirmed in damaging numbers in northern New York, so cold winters apparently aren’t a deterrent!

In 2016 corn-growing farmers generally assumed that the Cry1F trait adequately controlled the western bean cutworm, but damage was severe in many fields. By the time people realized that the trait wasn’t effective on this pest it was too late to apply insecticide as a rescue treatment. The western bean cutworm is now the primary insect pest affecting corn ears, and in a still-expanding area. The situation is so serious that entomologists from five land-grant universities (including Cornell University and Penn State) have written an open letter urging the seed industry to remove the “control” designation from the Cry1F trait regarding western bean cutworm.

Be a good scout – or hire one

Unlike the western corn rootworm, the failure of the Cry1F trait to adequately control western bean cutworm has little to do with farmers’ crop rotations or a failure to follow recommended pest control measures. That’s why as the insect moves into new areas it can wreak so much havoc in corn fields.

It’s important for someone – farmer, crops consultant or trusted employee – to scout corn fields so that action can be taken if necessary. Cutworm larvae overwinter in the soil, emerging in early summer and becoming moths that mate and lay eggs on corn leaves. The larvae feed on the leaves and pollen, later moving into the ear.

The best way to scout for western bean cutworms is to check for egg masses on the upper flag leaf, checking at least 20 plants each in several parts of a field. Field scouting is key because as is true with many insect pests, cutworms are much more effectively controlled if the problem is detected early. One source recommends an insecticide application if at least 5 percent of plants have egg masses on the upper leaves or in the whorl. Many insecticides are labeled for western bean cutworm, but the registration status of the insecticides may vary by state so check with local authorities and as always – read the label.

This is a rapidly evolving situation, and recommendations may changed by this summer. Just because a farm is outside the current “known area” for the cutworm doesn’t mean you won’t have problems. It’s likely that by this summer the insect will have spread throughout most corn-growing areas in New England. Discuss the various genetically modified trait options with your Extension educator and or seed dealer. Field evaluations by Cooperative Extension in Northern New York last September found a wide range in western bean cutworm damage depending on which genetic trait was used.

Prevention vs. control

Management of the western bean cutworm will likely be similar to that of other cutworms in that crop rotation will have little impact on infestation level. A first-year corn field may be about as likely to have an infestation as a fifth-year one, especially if there are other corn fields in the area. That’s because cutworm moths can fly from field to field in depositing eggs.

The most effective means of control appears to be use of genetic traits proven to be more effective than Cry1F in controlling this insect. Unlike the situation with the western corn rootworm, the control problem with western bean cutworm isn’t that the insect developed resistance to a particular genetically modified trait. It took years for rootworms to develop resistance. The process undoubtedly sped up by some farmers planting continuous corn while using the same GM traits year after year, and in some cases by ignoring the required planted refuge area.

I’ve noted before that to some extent rootworm resistance is a case of (to quote Walt Kelly’s Pogo) “We have met the enemy, and he is us.” Although entomologists think that resistance would have occurred regardless of farmer action (or inaction), this isn’t any excuse for ignoring the requirements. (These aren’t suggestions or recommendations but requirements that farmers must agree to in writing.)

My recommendation: If you see an online article or article in a farm magazine about western corn cutworm, be sure to read it since it may contain new information and or suggestions for control. There may be specific recommendations of what GM traits are most effective in combating this pest, and those that either should be avoided or used only as part of a “stack” with other GM traits.

Unfortunately, organic farmers and others who choose to use neither insecticides nor genetically modified corn have no obvious means of prevention or control given the life cycle of this insect. Furthermore, western bean cutworm affects field and sweet corn. It may become a serious pest of sweet corn growers. Even a modest infestation can be a turnoff for sweet corn lovers, who won’t be thrilled by husking an ear only to discover that a cutworm has set up housekeeping there.