Farmers in parts of the Corn Belt awoke to incredible bean leaf beetle damage last week. While hatch at this time of year isn’t unusual, the sheer populations this pest has shown have taken scouts and farmers by surprise.
The 2019/2020 mild winter could be to blame, as greater populations survived and were able to reproduce and lay eggs throughout the year. With damage this late in the season your No. 1 concern should be what it means for seed development and pod fill.
“The 2 main things we worry about right now are defoliation and pod damage. For our later maturing beans defoliation affects the bean’s ability to capture sunlight,” says Isaac Ferrie, Crop Tech Consulting in Illinois. “That photosynthesis impacts pod fill—if you can’t get light, you’ll have flat pods.”
Agronomists at Crop Tech are seeing widespread damage from bean leaf beetles across all of Illinois—but the severity of damage varies greatly. In some of the worst cases they’ve seen 40% defoliation.
As crops move from R6 into R7 and later stages, the leaf material become less favorable to the beetles and they’ll start feeding on pods, which can impact seed development and seed quality. Be especially mindful of this if you grow soybeans for seed or receive premiums based on seed quality.
Fortunately, at this point in the season insect damage leading to increased prevalence of disease isn’t as concerning as you might think. Focus on photosynthesis and pod fill.
It’s tricky right now to weigh the treat versus don’t treat decision because you’re so close to the finish line. Scout and make distinct considerations for each individual field as damage can vary.
You’ll want to determine how much damage you have, what stage you’re in and use a hail damage calculator for defoliation and check for pod damage to determine how much you’ve already lost, Ferrie says. The decision boils down to what you can still save, versus the cost of treatment.
The later it is in the season, the more likely you are to not see as much value from an insecticide treatment. Use field guidebooks to check to see if you’re at threshold as you make this decision.