In a recent article we pointed out the obvious- western Missouri is under a full attack from Fall webworms.
At this point, the damage is done- the little buggers will continue to eat leaves, which in some cases will cause the tree to die.
Some insecticides such as Sevin or Malathion can be beneficial. Burning the web 'nest' with a torch, albeit dangerous, is effective too. One local even tried using a 12 gauge to blow the clusters of worms to smithereens, but is not recommended...
According to experts, the solution lies in an early Spring application of dormant oil due to its low toxicity and easy availability; most any garden supply store sells it and it works well to kill eggs that are hidden under and around the bark.
More info here: