Question
All the leaves were eaten from our pecan trees by worms.  No webs were visible.  What do we use to prevent this from happening this year?  Southwest Texas , border. Trees are not in very good shape.

Answer
Sounds like Walnut caterpillar. The caterpillars develop in large clusters and rapidly defoliate ornamental and orchard trees if not controlled. Spraying with insecticides is the best control measure. To avoid as much damage as possible, spray the caterpillars when they are still young.

Larvae characteristically arch their heads and tails in a defensive posture when disturbed. Since they feed together, damage may be localized to just a branch to two. Isolated trees are more subject to attack than forest or orchard trees. The caterpillars often move in a group to the tree trunk to molt from one stage to the next, leaving a patch of fur-like hair and cast skins. When they finish feeding, they drop to the ground and pupate in the soil. They do not spin a cocoon but form a naked pupal case.

Groups of caterpillars found on tree trunks or branches during molting can also be destroyed. For larger trees and plantings, the application of insecticide sprays is the most practical method of control. Use products containing carbaryl (Sevin?, malathion, diazinon, acephate (Orthene?, methoxychlor, trichlorfon (Dylox? Proxol? or Bacillus thuringiensis. Thoroughly spray leaves, twigs, limbs and tree trunks. Large infested trees may require treatment by commercial applicators with high-pressure sprayers.

Since they do move to the trunk before they drop to the ground I would at least spray the trunk as high up as you can. This will kill they caterpillars as they move down the trunk.
Here is a web link to more information on Walnut caterpillars.
http://www.gaipm.org/pecan/scout/html/walnut.html