MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

The year 2010 was a banner year for all kinds of pests in Michigan agriculture. So much so, in fact, that MSU’s field crop entomologist, Dr. Chris DiFonzo, has dubbed the growing season as the “year of the worm.”

Western bean cutworms were no small part of that declaration. MSU’s Western Bean Cutworm Trapping Network showed a significant increase in both the number of moths detected in traps and corresponding damage. It is interesting to note, however, that not all unprotected corn fields adjacent to high moth catch traps exhibited high levels of damage. Many researchers across the region have reported that the growth stage of the corn field at the time of moth flight is important. Fields that have begun to shed pollen by the time of moth flight and those still in the mid- vegetative growth stages generally are less attractive to female moths than fields that are just approaching tassel emergence when the females are in flight.

Chris reported that in one of her research fields in Montcalm County, 90 percent of the ears exhibited at least some damage. While damage was not nearly that severe in fields that I walked in southwest Michigan, there certainly will be fields that have suffered some significant economic yield losses from the pest. I have seen fields that have between 25 to 30 percent of the plants showing signs of ear feeding. Some ear feeding is not that severe, while other plants may have lost a portion of 10 to 15 percent of the kernels to feeding and subsequent ear rot damage. Damage from multiple larvae is quite common in heavily infested fields.

So why the rapid upsurge in western bean cutworm activity this year?

The answer may lie in the increase in numbers we saw in trapping in 2009. More larvae laid their eggs in fields last season. Western bean cutworm larvae tend to overwinter well in light textured soils. Possible reasons why include the ease of larval movement down below the frost line in the sandy soils and the fact that these soils tend to be less saturated during the fall, winter and early spring. This means that south and west Michigan are likely to maintain high numbers of this pest in the future.

So what are the treatment options?

We have two different approaches, using insecticide applications or planting a Bt corn variety, which provides resistance to the pest. The good news is that there are a lot of insecticides that are capable of providing excellent control of western bean cutworm. The bad news is sort of twofold. Since western bean cutworm moths lay their eggs as the plants are approaching pollination, either a high boy sprayer or aerial applications must be used to apply insecticides. Also, the timing of pesticide application tends to be somewhat tricky. When we look at the duration of western bean cutworm moth flight, it was spread out over three weeks in 2010. If you are trying to use the treatment threshold of 5 percent of the plants having egg masses, you will have to record cumulative numbers to make a spray decision. Also, it may take two applications to be able to get effective control of the pest. For this reason, I suspect that many growers will be interested in using a Bt-based control strategy for controlling western bean cutworm in their fields in 2011.

Bt corns containing Syngenta’s Viptera trait (Vip 3a protein) and the Cry 1F Protein (Herculex and Smart Stax) provide good control of WBC larval feeding. Chris considers the level of western bean cutworm control of the Herculex Bt to generally be comparable to a well-timed insecticide spray. You will probably find some feeding injury on the ears, but kernel damage is usually much less severe than on non-Bt and non-treated hybrids. If you plan to use Bt hybrids, keep in mind that understanding how to maintain refuge areas will be critical in helping this technology to remain effective. 

If you are considering controlling western bean cutworm with insecticides, become familiar with the life cycle of the pest, the timing of when to scout and the optimal timing and best insecticide to use to control western bean cutworm in your fields. If the moth flight continues to remain spread out, we may be looking for the materials with the longest active window to maintain the best control.

What will be the longer term population dynamics of western bean cutworm in our area? Often pests get a boost when they first move into a new area because they move ahead of some of the biological control organisms (pathogenic fungi, predators, etc.) that will eventually help to keep their numbers in check. It remains to be seen what organisms will play an important role in their biology in the future in Michigan.