Start Watching for Western Bean Cutworm

To say that the planting season was tough this spring is an understatement. However, the growing season will continue to provide us with fresh challenges. This week, I want to talk western bean cutworm (WBC), a pest I am keeping an eye out for.

Western bean cutworm has become a major problem across large swaths of the corn belt in recent years. Its tolerance for common transgenic crop traits makes it harder to control, so proper management is important. If you miss the critical window between egg hatch and when the caterpillars move into the ears, you could be out of luck. Therefore, early monitoring with devices like Smart Trap™ and scouting will be key to keeping your crops safe.

What makes western bean cutworm unique?

Western bean cutworm counts other cutworms and armyworms as family, but has more in common with corn earworm in the way it eats. Rather than feeding on leaves or stalks like other cutworms,  young WBC caterpillars first consume pollen from the tassel. When that is not enough, they crawl into the ear to feed directly on the kernels. This preference makes these moths most damaging to maturing plants that are just tasseling or silking.

Many fields won’t be close to tasseling when the moths arrive, because of the late planting across most of the affected region. But the moths don’t know that; they’re still in the ground and preparing for their debut. Each season, they’ll get one shot to emerge, reproduce, and have their offspring find enough food (our food) to survive.

What can we expect with current growing conditions?

More likely than not, the first moths that emerge will be starved for choices on good places to lay their eggs. In my experience, this typically happens around the third week of June here in Indiana, but the timing may vary in your area. They’ll locate the most mature plants they can; if these plants are not close to tasseling, the caterpillars will have nothing to eat and will starve. Mortality can be high during this period, as much as 90%. Once pollen shed occurs, however, things get easier for the caterpillars and we can expect them to survive long enough to get into the ears.

In 2017, several fields I monitored were planted late and the WBC moths began flying well before tasseling. Very few caterpillars survived to infest ears and damage was low. This year, planting across the region is far later than even 2017 and we may have a similar outcome. Ultimately, it will come down to when the flight of WBC moths peaks and how long the tail end of the flight will be. The earlier the peak and the shorter the flight, the less damage we can expect from WBC. But given the continued chill of this season, we could see that post-peak tail to last longer as moths emerge slowly over time; probably beyond tasseling for even some of the late-planted corn.

Make sure you scout your fields once WBC has been reported in your area. Look for the egg masses on the leaves. These will be tight clusters of a few dozen spheres and can range in color from white to purple. If you find five or more infested plants out of a hundred, consider how close your field is to tasseling. If there’s still time, you may want to continue to monitor weekly and see if new eggs continue to be laid. If you still have high numbers of infested plants as you approach tasseling, start control practices.

Stay Informed

I will be tracking WBC and other pests throughout the summer. Follow me at or on Twitter @ScottW_DTN.

More on Smart Trap

The self-counting Smart Trap delivers accurate, daily counts of pest populations in the field. Several times the precision of traditional trapping, in a fraction of the time. These trap count western bean cutworm, as well as many other insects.