The adults of tent caterpillars are moths which can fly long distances, so even though you control the pests on your property, the moths can fly in from nearby areas to lay their eggs on your fruit trees. If you have just a few trees you may be able to control them by physically removing the egg cases over winter. Look for cluster of several hundred small brownish eggs on small branches and limbs. The egg case cluster is usually about an inch long and 1/4 inch wide. It will look a bit like a raised bump of mud on the branch. Simply scrape off and destroy the egg clusters that you see while the tree is dormant. (They're a lot easier to see when the leaves are off the tree.) The tent caterpillars always attack the tree on which their egg case is laid, and only would move on to another tree after they strip the first one bare. So removing the egg cases in winter, prevents the tents from forming.