Answers are from the Indiana certified crop adviser panel: Betsy Bower, Purdue Extension senior research associate, West Lafayette; Abby Horlacher, Nickel Plate Consulting, Frankfort; and Brian Mitchem, Farmer 1st Agronomy Consulting Services, Decatur.

 

While scouting one of my soybean fields, I noticed leaf defoliation and spotted several Japanese beetles. It looks bad, but when does it become concerning enough that I should spray insecticide?

 

Bower: Defoliation starts to negatively impact yield once we get to the reproductive stages. At R1, yield loss starts at 15% leaf defoliation but is estimated to only be a 1% yield loss from 15% to 45% defoliation. An R1 plant at 50% defoliation is estimated to only have a 2% yield loss.

 

As the soybean plant gets further into the reproductive stages, more yield loss is expected. At R3, 15% defoliation is estimated to be 3% yield loss, 25% defoliation is estimated to be 4% yield loss, and 50% defoliation is expected to be 8% yield loss.

 

As we get closer to maturity, however, yield loss due to defoliation lessens. It is most important to protect a soybean from yield loss due to defoliation from R3 to R5.

 

Horlacher: Keep in mind that foliar damage often looks worse than it is. Before flowering, defoliation must reach around 40% to justify insecticide use. The most critical stage is bloom. Aim to keep defoliation under 15%. If it exceeds that, consider spraying. During pod fill, the threshold rises to about 25%.

 

Also remember, insecticides don’t just target pests. They affect beneficial insects too. They should only be used when pest levels clearly justify treatment.

 

Mitchem: A tendency most have is to overestimate foliar damage from leaf feeding and disease. To help more accurately diagnose feeding levels, you can use an app called LeafByte. It allows you to use a photo of the leaf to estimate feeding. Use this free estimator, as it is easy to overestimate and treat when it’s not justified. Pyrethrin insecticides control beneficial insects as well as target pests, so there is always a concern about the use of insecticides when not justified.

 

Japanese beetles are one of several insects that can defoliate soybean leaves in the summer. These insects are easily found midday at the top of the canopy. Before soybeans enter reproductive stages, defoliation needs to be more than 25% total leaf area before any impact can occur. In studies where even 50% of leaf area was removed at beginning flower, yield levels remained at 80% to 95% of potential. Having 25% of the total leaf surface damaged is rare. Spraying an insecticide by itself is rarely justified despite the visual damage.