Question
I live in southwest Nebraska and have a mature locust tree (50ft. at least).  Two years ago there were about 1 inch green worms dropping from the tree and this last summer there less foliage and some dead branches.  What should we do?  I would hate to loose this tree.

Answer
There are a least two caterpillars that will feed on locust trees. The most common is Clouded Sulphur caterpillar and Silver Spotted Skipper. Both are green and around 1-2 inches long at maturity. They eat the foliage and can be controlled with insecticides. The safest insecticide would be Bt. This material is sprayed on the foliage as soon as the smallest caterpillar is observed. The caterpillar will eat the Bt killing the insect. Check with your local nursery/garden type store for Bt.
Caterpillar type insect populations vary greatly form year to year so it maybe there will not be any this year. I would fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. Do this in the early spring as the leaves are budding out. This will help the overall health of the tree and cause it to produce more foliage.