CROPS

Late-season soybean insect pests on the rise
By Dr. Paul Tracy

Over the past several field seasons, I have received an increasing number of inquiries into late-season soybean insect pest problems. Soybean aphids have received much of the news attention and have been a problem in the northern Midwest. We need to continue to monitor for soybean aphids and treat for them should they become an economic problem.

My personal observations are that bean leaf beetle and stinkbugs (especially green stinkbugs) have been the most serious late-season soybean insect pests in our region. These insects overwinter locally. Therefore, I am reasonably sure that our milder winters have contributed to their population explosion in recent years. This June, levels of first generation bean leaf beetles and stinkbugs were extremely high. I am concerned that later generations will be present in very high numbers during August and September.

The bean leaf beetle has been a sporadic soybean pest for decades. However, its numbers have increased dramatically since the mid-1990s. It resembles a ladybug, but is shaped slightly more oval and always has a dark triangle-shaped patch directly behind its head. It ranges in color from yellow-green to red. It may or may not have large dark spots present on its back.

The bean leaf beetle has two generations per season with adult beetles feeding on foliage and larvae feeding on roots, nodules and underground stems. Larval feeding rarely affects soybean yield, but early-season larvae activity may help predict late-season pressure. Early-season spraying is recommended when 30 percent of the soybean foliage has been removed; one or more plants per foot of soybean row have been killed; or if five or more beetles per foot of soybean row are present.

Generally, we lose more soybean yield to bean leaf beetles when feeding occurs in August and September. From bloom to pod fill, the economic threshold is to treat when defoliation reaches 20 percent, or when 16 beetles per foot of soybean row are present. As soybean seed matures, the threshold occurs when 5 percent of pods show feeding damage and 10 or more beetles are present per foot of row.

Most of us are familiar with stinkbugs, but we generally do not recognize them as soybean pests in the Midwest. Over the past few years, I have observed them to be a very serious soybean pest in our region. In fact, I predict that stinkbugs will become our No. 1 soybean insect pest in the near future.

There are many different stinkbugs species. Some are actually beneficial predators. The green stinkbug is our No. 1 economical stinkbug pest on soybeans. It is very easy to identify. As its name implies, it is bright green in color and has narrow orange-yellow lines along its edges. Stinkbug nymphs resemble adults in shape but start out blackish, and become green with orange and black markings before reaching the green adult age.

Like the bean leaf beetle, early season stinkbug damage can affect soybean seedlings, but most of the economical damage occurs mid-late season. Unlike bean leaf beetles, stinkbugs do not have chewing mouthparts. They have piercing-sucking mouthparts and do not cause physical foliar feeding damage. They puncture soybean stems, leaves, blooms and pods. Once punctured, stinkbug damage shows as deformed, undersized, shriveled or aborted pods and seeds. Stinkbug injury also delays grain maturity through a condition known as “green bean syndrome.” Stinkbug damage reduces germination and seedling vigor in seed production situations.

I recommend scouting for stinkbugs starting at first bloom. Economic threshold is reached when one stinkbug per foot of soybean row occurs. Stinkbugs are mobile and difficult to observe visually. Drop cloths or sweep nets are needed to accurately estimate their populations in fully canopied soybean fields.

Based upon June observations, I expect stinkbugs and bean leaf beetles to be major economic pests on soybeans this August. I have listed common soybean insect control products used for these two late-season pests. Please note that stinkbugs are slightly tougher to control than bean leaf beetles and often require higher application rates. Also note the labeled harvest intervals which may have some influence on your late-season treatment selection. Familiarize yourself with the bean leaf beetle and stinkbug. Scout fields, pay attention to pest alerts in your area and be prepared to treat soybean fields if necessary.