Dr. Dominic Reisig
Associate Professor and Extension Specialist
Entomology - NC State University
Looper numbers have really picked up this week in soybeans. Loopers are migratory pests that sometimes show up late season and eat leaves, but not pods or seeds. Remember that the threshold for soybean loopers (and all defoliating pests) is 15% defoliation throughout the canopy (thresholds and defoliation guide here). Loopers generally defoliate from the bottom of the canopy up so peel back those plants when you scout.
So, should you treat your soybeans for loopers now?
If you have beans below 15% defoliation throughout the canopy, relatively low populations of loopers (5 in 15 sweeps and lower), beans with a low-yield potential, and beans that are at or past R6, I would not recommend treatment. Remember that the “safe point” for defoliation is R7 and that they begin feeding in the middle of the canopy, working their way up. Also keep in mind that our 15% defoliation threshold is a yield prevention threshold that has not taken into account the money you will spend on insecticide, fuel, and drive-down loss over the beans.
The hardest call will be beans with a high-yielding potential that have not reached R6 and have lots of loopers and/or defoliation. So you really need to assess what your canopy looks like. Although pyrethroids will knock loopers back initially, they will often resurge because they are tolerant to these chemicals and because natural enemies are removed. My top choices for soybean looper are Belt, Prevathon and Steward. There are premix products that will also work, such as Besiege, but these should only be used if you need the non-worm portion of the premix (such as stink bugs or kudzu bugs at threshold). Remember that you want to preserve natural enemies if possible. Tracer/Blackhawk and Intrepid are also good effective chemicals for loopers.
Source : ncsu.edu