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Keep an eye on those soybeans

July 23, 2021

Soybean Source: It’s easy to get riled up over holes in soybean leaves, but investigate before taking action. Beans can withstand some defoliation.

By: Clarke McGrath

It’s been a pretty challenging summer, with most of the state fighting serious drought, storm damage in many areas, and some regions even dealing with excess moisture.

Even with all these challenges, as I write this in mid-July, the latest USDA National Agricultural Statistics Service report shows two-thirds of Iowa’s soybeans in good to excellent condition. Sure, any government report is open to debate, but there are some decent-looking beans out there. We can’t do much to manage the weather over the next few months, but we can keep a good eye on our fields with some scouting, which may help head off a few issues.

Quit eating my &@%$ beans!

Finding a bunch of holes in our soybean leaves tends to get us riled up, but we need to investigate before taking action. Beans can withstand a lot of defoliation; but after they start flowering, defoliation has the potential to be more problematic, so most thresholds drop to 20%.

When pest damage passes the threshold, it creates economic damage. Beyond that threshold, yield losses increase quickly, and it’s likely time to apply an insecticide treatment.

The usual suspects are grasshoppers, various caterpillars and bean leaf beetles. Calls about these have been few and far between so far, but keep them in mind when scouting, since some pests can pop up on short notice.

DEFOLIATION THRESHOLDS of soybean leavesDEFOLIATION THRESHOLDS: Is what we are seeing out there really 20% defoliation? The phrase “a picture is worth a thousand words” sums up soybean defoliation pretty well, since describing the various percentages seems impossible to me. Knowing what 20% looks like on a soybean leaf is only part of the “picture.” (Courtesy of University of Nebraska-Lincoln CropWatch)

That brings us to a pest that has become a wild card — for both corn and soybeans — the Japanese beetle. These beetles seemingly came out of nowhere and starting clipping corn silks and defoliating soybeans in parts of Iowa just a few years ago, and have subsequently spread across most of Iowa. I have gotten a fair number of calls about them the last few weeks, and they are likely to hang around most of the summer.

In areas where this pest has been a problem the last few years, it’s a lot easier to get folks talked into scouting later in the season. They know how much damage Japanese beetles can do in a short time. Scouting is the key step, but before we pull the trigger on any insecticide applications, there are a few more things to consider that might fine-tune our decisions.

Evaluating defoliation

Is what we are seeing out there really 20% defoliation? The phrase “a picture is worth a thousand words” sums up soybean defoliation pretty well, since describing the various percentages seems impossible to me. Smartphones with good cameras have helped a lot, and scouting apps are handy as well. But knowing what 20% looks like on a soybean leaf is only part of the “picture.”

It seems like every piece of entomology literature ever written contains the phrase “soybean defoliation is often overestimated.” As clichéd as that line seems after all these years, it has stuck around because it is spot-on for several reasons.

Different pests often feed at different levels of the canopy, so evaluate the entire picture. Fields with 30% or 40% defoliation in the upper canopy look awful. It can be tempting to pull the trigger after seeing that; but if the rest of the canopy is in good shape, the overall percentage may not approach 20%. Field borders and edges usually look tougher than the rest of the field, so wading further in and scouting the field as a whole is necessary. A couple of things to remember when it comes to Japanese beetles: Foliar insecticides are generally effective on Japanese beetles. So, shortly after a field is sprayed, you’ll feel pretty good because they will be toast. But adults are highly mobile and can reinfest a field within a few days — so don’t stop scouting.

Other issues

The two-spotted spider mite can be a problem in soybean fields — and sometimes in corn — in hot and dry conditions. Mid- to late July through August is typically when they hit economically damaging levels. The literature says, “Two-spotted spider mites can increase whenever temperatures are greater than 85 degrees F, humidity is less than 90% and moisture levels are low. These are ideal conditions for the two-spotted spider mite, and populations are capable of increasing very rapidly.”

Mites can reduce soybean yield by 40% to 60% when left untreated, and shatter loss can also increase significantly in fields that had spider mite pressure. A general guideline for soybeans is to treat the crop between R1 to R5 stage of growth (bloom through beginning seed set), when most plants have mites, and if there is heavy stippling and leaf discoloration apparent on lower leaves.

With that said, spider mite treatment economics depend on a lot of dynamic factors, which are worthy of an entire article on their own. Read an Iowa State University Integrated Crop Management news article on spider mite scouting and management from last summer.

Good luck out there scouting, and stay cool and hydrated.