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Good reasons to keep scoutingGood reasons to keep scouting

Soybean Source: Watch for leaf injury by insects, and dig some bean roots to check for SCN.

Clarke McGrath

July 20, 2018

6 Min Read
JAPANESE BEETLE: This invasive insect can feed on soybeans and corn. For soybeans, it prefers to feed between the leaf veins.

Late summer is when we start hearing terms like “rounding third,” “in the home stretch” and “fourth-quarter time” used to describe where our corn and beans are in the growing season. The analogies make sense. Even late in a baseball or football game, a lot can still happen.

As Yankee legend Yogi Berra said, “It ain’t over till it’s over.” With that wisdom in mind, it might not hurt to talk about a few late-season soybean issues. There may be a few things we can do to finish stronger this year or to get a head start for future crops.

Finding a bunch of holes in our soybean leaves is certainly cause for alarm, but we need to investigate before taking action. Beans can withstand a lot of defoliation, especially early in the season; 30% is a common economic threshold in vegetative stages. After the beans start flowering, defoliation has the potential to be more problematic, so most thresholds drop to 20%.

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

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

Since the “usual suspects” haven’t been problematic for a few years, I feel like we are at a critical juncture. Besides falling short of convincing folks to keep up the weekly scouting, a few might be debating whether to turn the page or hang with this article a little longer. Well, as Yogi once said, “When you come to a fork in the road, take it.” We’re taking that fork!

That brings us to a pest that’s become a wildcard for both corn and soybeans: the Japanese beetle. They seemingly came out of nowhere and starting clipping corn silks and defoliating soybeans in parts of Iowa just a few years ago. We use degree days to predict when they will start to emerge, and based on their life cycle can guess about how long they might feed in our fields. Beyond that, we have a lot to learn.

One thing is clear: 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.

Insecticide decisions
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.”

DEFOLIATION: A 30% loss of leaf area before plants begin to bloom is the general threshold for when to spray soybeans with insecticide. After beans have bloomed, 20% defoliation is the threshold for treatment.

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.

One more thing to consider
I had a fairly long list of things to scout for that we discussed in last month’s column, and then added the defoliators we also mention this month. It’s not a new topic, but it’s one we need to be on top of, and it can slip off the radar if we don’t think about it now. Once the combines start to roll we might not have time to think about much else.

We might have good intentions to address it after harvest, but experience tells us the window is small that time of year. If we think about it now and work with our local agronomists on a plan between now and harvest, odds are it will get done. You might have already guessed where this is going; it’s another plea to make sure your fields are sampled on a regular basis for soybean cyst nematode.

We talk about SCN a lot, and you’ve been hearing about this pest for decades. Remember the SCN Coalition, launched back in the 1990s, and its slogan “Take the test, beat the pest”? Well, they got back together and relaunched it. Talk about déjà vu!

Stop losing yield to SCN
The SCN Coalition, comprised of university researchers, Extension specialists and ag company representatives, didn’t get back together just to relive old times. They got back together because SCN — soybean producers’ No. 1 yield-robbing pest — isn’t going away. It is adapting to the way we fight it, and we are losing ground — and yield.

When I said the coalition is recycling the slogan “Take the test, beat the pest,” I should have mentioned they also updated it with the term “What’s your number?” To really explain why the coalition wants farmers to sample soil, have it tested and know their nematode numbers is worthy of an entire article, so we may revisit this in more depth later. But the short version is this: The first step in the integrated management plan for SCN is testing your fields to know your numbers. If you have sampled before, but it was a few years ago, it is probably time to do it again.

So that’s it. Before harvest, sit down with your local agronomist to set up nematode sampling cycles based on the SCN Coalition’s new active management recommendations. Out of all the mid- and late-season soybean management topics we’ve talked about in the last few Soybean Source columns, this last one might be the most important. Have a great, and safe, rest of the growing season.

McGrath is the on-farm research and Extension coordinator for the Iowa Soybean Research Center at ISU. Contact him at [email protected].

About the Author(s)

Clarke McGrath

Clarke McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at ISU.

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