The most common caterpillars that affect the leaves of your fruit trees are forest tent caterpillars, eastern tent caterpillars, webworm or gypsy moths. The tent and webworm caterpillars form tent-like homes which the larvae (caterpillars) crawl into in the evening and lay their eggs. Gypsy moths lay eggs in white egg masses, which are visible to the naked eye. Typically, trees are not greatly affected by the caterpillars since they are a natural part of the ecosystem, but over time or in great quantities, it is possible for them to cause irreversible damage or affect the quality and quantity of the harvest.
Wrap a sticky tree wrap around the trunk of your fruit tree to catch and kill the caterpillars as they crawl up and down your tree. These are available at most home and garden centers.
Remove the egg masses or tents where larvae develop manually from your tree and dump them into a bucket of soapy water. You can also prune smaller twigs on which tents are located. This is best done in the evening or in the early morning when the larvae are most likely in the tents.
Spray the tents and egg masses with an insecticide labeled to kill caterpillars. Bacillus thuringiensis (Bt) is the most common product used and is harmless to people, animals and trees. Bt is most effective on young caterpillars (smaller than an inch) and is best applied in the early morning or in the evening. Apply Bt according to the manufacturer directions. Bt does not kill the caterpillars on contact. It is a bacteria that they eat and become paralyzed until death.