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How Good Is Your Scouting Eye?

Take a look at the pictures on this page. If the soybean leaf damage wasn’t labeled, could you guess the percentage of total leaf material that has been defoliated by insects and worms?

Those numbers are important, because there’s a point at which pest damage passes the threshold of economic damage. Before that point, treatment may not pay. Beyond the threshold, yield losses ramp up quickly, and it’s time to apply an insecticide treatment.

There’s more at stake than just the economics, says Erin Hodgson, Iowa State University Extension entomologist. Unnecessary treatments can place some beneficial insects, such as pollinators, at risk. Unneeded insecticide applications can also contribute to resistance development. 

Leaf-Mashup
John Obermeyer, Purdue University

The question of when plant and leaf defoliation impacts soybean yields has been studied a number of times, says Hodgson. In the 1950s, researchers put the tipping point at about 30% of leaf damage.

Other researchers revisited it in the 1980s and slightly modified the recommendations. Before bloom, 30% defoliation is still a good threshold. After bloom, they determined 20% defoliation is the line for considering treatment.

“Entomologists from Mississippi State studied it again just a few years ago, in 2012,” Hodgson says. “That research said that 30% to 40% defoliation is the threshold in the soybean vegetative stage. Once it gets to reproductive stage (bloom), then 20% defoliation is the line for treatment.

“I’m still pretty comfortable with that as a general rule,” Hodgson points out. “It works pretty well for all defoliator pests. A bite is a bite when it comes to soybean leaf damage – be it a beetle, a caterpillar, or a grasshopper.”

The trick is to train your scouting eye to know when you’re approaching a threshold, Hodgson says. She offers these additional tips.

Study up. Since most people overestimate the percentage of defoliation, you may need to study pictures like those above to improve your skill. You could even carry pictures with you for on-the-spot comparisons. There is also visual help online at various crop Extension sites.

Scout early and often. Hodgson says it’s important that you’re in fields every week looking for damage. At certain points, it should be more often, as you can easily pass a threshold level in a week. 

Discount the edges. “I disregard the outside 20 rows,” Hodgson says. “Damage is always worse there because that’s where pests enter a field. It may not represent the entire field. Skip those rows, then zigzag your way through the rest of the field as you scout.”

More Scouting Tips 

• Sample regularly.

• Recognize direct and indirect plant injury.

• Estimate defoliation both on whole plants and field-wide.

• Use recommended thresholds.

• Use labeled rates of pesticides and leave no survivors.

• Continue to scout after treatment. 

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