Answer: There's no point in treating your tree now - the leaves are gone and it isn't likely the trees will have enough time to replace the lost leaves. However, if you find caterpillars munching on the leaves next year, you can use the botanical Bt (bacillus thuringensis), sold as Dipel. It acts as a stomach poison and is effective against caterpillars but does not harm beneficial insects. Be sure to check your tree on a daily basis so you can stop the pests in their tracks the minute they begin to attack your trees. It might also help to use a dormant oil spray on your tree this winter. Horticultural oils can suffocate any insect eggs hiding on the trunk and branches.
Best wishes with your apple tree.
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