Much Ado About Bean Leaf Beetles

In my most recent ILSoyAdvisor post, while discussing late-season scouting for insects, I mentioned bean leaf beetles. I wrote that they rarely are present in high enough numbers to warrant control action. But once again 2020 has thrown a curveball at us, and there are numerous reports in the central and northern portions of Illinois of heavy infestations of this normally minor pest. The damage can be significant in some cases, so how can a farmer decide whether a spray will be cost-effective in the current, challenging commodity environment?

First, the pest

Adult bean leaf beetles are around ¼-inch long and can vary widely in color; they can be green, yellow, tan or red. They usually have spots on the wing covers, but these are not always present. However, they always have a black triangle behind the head. There are two generations per year in the state of Illinois. They are most likely to be a serious problem when the first generation appears and attacks the earliest emerging soybean fields. However, neonicotinoid seed treatments provide early season protection, and by the time treatments have lost their effectiveness there generally are enough leaves to make up for any foliage lost due to feeding. It is extremely rare for the second generation to cause economic impact because by the time it emerges, the plants have plenty of leaves and can withstand quite a bit of feeding.

So, what is different about this year?

Obviously, something is different about this year which has caused this normally minor pest to appear in much higher than normal numbers and cause significant damage to soybean fields. To understand the reasons behind this phenomenon, I reached out to Dr. Nick Seiter, professor of entomology at the University of Illinois. Dr. Seiter explained that, like all living organisms, bean leaf beetles are highly affected by the environment. Since the larvae of this pest develop in the soil, they are susceptible to changes in soil moisture, among other environmental factors. Perhaps the soil moisture patterns this year have been favorable for robust development of adult beetles, leading to higher levels than we have seen in recent years.

Should I apply a late-season insecticide to control bean leaf beetles?

Most integrated pest management strategies for bean leaf beetles recommend spraying if after-bloom defoliation is greater than 20 percent or if beetles are feeding on 5 percent or more of the pods. However, once the crop reaches the R6 growth stage, even very high levels of defoliation are unlikely to have a yield impact. In fact, growers in the southern US commonly apply a defoliant at the R6-R6.5 growth stage, without reducing yield.

Pod feeding is of greater concern. For growers concerned about seed quality, such as in seed production fields, an insecticide may be needed to protect the germination potential of the seeds. Also, in some cases, “clipping” can occur when beetles feed near the base of the pods. Remembering that an average of one pod/plant across an acre is equivalent to about two bushels can help when determining whether to spray. It may seem obvious, but don’t forget that an insecticide can only be effective if the beetles are still present in the field. Ensure that they have not moved on before making the decision to apply. Finally, if you are planning to spray, be sure to use an insecticide that has a residual component. You don’t want to kill the beetles in the field only to have them move back in from a neighboring area a few days later.

Bean leaf beetles are just the latest in the long line of challenges growers have encountered over the past couple of years. In most cases, they probably aren’t going to cause economically significant levels of yield loss, but some fields may deserve a closer look. Hopefully not many growers will have to put their harvest planning on hold and get the sprayer back out of the shed.