Bt for fall webworm
Most fall webworm needs no treatment, because the damage is late and usually cosmetic. Where a small tree or a pecan does need protecting, Btk works only if the caterpillars eat it, and they feed inside the web. Spray while the webs are small, and tear larger ones open first: the 2017 national review notes that big webs make it harder for the spray to get in.
Before you order
A bacterium sprayed on leaves. Caterpillars that eat it stop feeding and die within a few days. It works only on the larvae of moths and butterflies, and has little or no effect on bees, lady beetles, lacewings or parasitic wasps. How it works, how to tell it has established and the supplier list are on the full Bacillus thuringiensis kurstaki profile.
Other controls for fall webworm
No other reviewed control is listed for this pest yet.