Bean Leaf Beetle, Japanese Beetles, Grasshoppers
...It's All the Same When it comes to Thresholds

Many are asking what the thresholds are for each of these insects in soybeans. But it really comes down to the same thing no matter what the insect is..how much defoliation are they causing? The soybean plant responds the same whether it is bean leaf beetle, Japanese beetle or even a thistle caterpiller that is making the holes. A hole is a hole. What is important is how holes there are.

Defoliation is often overestimated. Most of the defoliating insects feed on the tops of the plants so that when you are scouting you think you are seeing a lot of feeding damage. But once you pull the plant out of the ground and take a good look at the rest of the leaves, the holes really are only concentrated at the top of the plant. And with soybeans, they keep putting out more new leaves so they can compensate for a lot of feeding. And leaves that are positioned below the leaves with all of the holes actually grow bigger, increasing their surface area since they are getting more sunlight through the canopy and can absorb all of the energy from the sun.

So before pulling the trigger too soon, take a good estimate of what the defoliation is in your field. During the Bloom (R1) to Full Pod (R4) growth stages, there needs to be 15% defoliation before a spray is warranted. All of the leaves on the plants should have about the same amount of feeding as the leaf in Figure 1.

Figure 1. What 15% defoliation looks like. Try to visually push all of the holes to onespot on the leaf and determine just how much leaf is gone when all of those holes are combined. Photo credit: Marlin Rice, Iowa State University.

It is actually more important once pods are forming to pay attention to pod feeding, especially by bean leaf beetles. IP, food grade and seed soybeans can only tolerate 10% of the pods having feeding on them before a spray is warranted. The feeding is not as much of a concern as is the pod diseases that will enter these damaged pods, impacting seed quality.

Final consideration before you spray..look for spidermites and aphids in your field. If you are in a region that has started to miss these frequent rain events, spidermites could be starting up. Spraying with Matador for defoliators when spidermites are present could actually cause the spidermite population to flare up. Only dimethoate (Cygon or Lagon) kills spidermites so keep that in mind before pulling the trigger. And if aphids are present, spraying for defoliators will kill the natural enemies that have been keeping the aphids in check. The aphid population can then increase in the absence of the natural enemies. Just another reason why you should only be applying insecticides when the pest population reaches threshold.

Figure 2. Red Phase of Green Leaf Beetle

Figure 3. Japanese Beetles