Healthy hardwoods generally can endure one or two years of heavy defoliation (around 60-100% of leaf loss) before tree mortality occurs. Healthy trees will normally put out a replacement set of leaves a few weeks later (see photo), though the new leaves may be smaller and lighter in color.
A heavily defoliated tree must use its energy reserves to grow a replacement set of leaves. This weakens trees and makes them more susceptible to disease, drought stress and other insect attacks, which in combination can kill the trees.
You can help valued trees recover from defoliation by watering them deeply each week if there hasn't been a soaking rain. Keep watering the tree until it has dropped its leaves in the fall. Trees generally need the equivalent of 1 inch of rain per week.
Also, avoid giving trees any nitrogen fertilizer as this will overstimulate the tree to produce more replacement leaves than it can support. That may deplete energy reserves so much that the tree won't be able to produce leaves the following spring.
Trees most likely to be heavily defoliated include oak, apple, birch, aspen, willow, tamarack and linden. Other deciduous trees and conifers also can be stripped of leaves or needles. If you remain alert to increasing numbers of caterpillars and egg masses, use management options to reduce damage to your trees and provide adequate water through the growing season, you can greatly reduce the chance of your tree dying.
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