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
