NDSU Extension - Stark & Billings County

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HOW MUCH DEFOLIATION CAN MY TREE HANDLE?

Each year we watch the deciduous trees in our communities, forests, parks, and yards set new leaves to help them produce energy during the hospitable months of our region

These leaves are important energy producing organs that help trees grow and produce new leaves, shoots, seeds and fruits. Often times we find insects eating the leaves of our trees and their feeding damage can be evident and extensive. This begs the question; will my tree recover?

                Nearly all plant species have insects that feed on them and one strategy of defense that plants have is compensatory growth. Plants can usually grow new leaves to replace those that are lost in order to recover from a defoliation event. Many insect defoliator pests of our region have one or two generations per year, however we have different insect pests that are problematic throughout the season.

How much defoliation can my tree handle?

                If your trees are healthy and have lost less than half of their leaves, they have a good chance of recovering. Healthy trees that lose half of their leaves for 2-3 years in a row will still likely recover, but repeated defoliation events greater than half for more than three consecutive years may make your tree susceptible to other diseases or weather events.  

                Trees store starch in their vascular system, so they have resources available for later in their life (either during the same year or a following year). They use these reserves to produce new leaves after every winter and after defoliation events. With early season defoliators like tent caterpillars, trees have enough time to replace these leaves and continue to grow. Midseason defoliators cause the most stress on trees, because the tree has less time to replace the leaves and less time to recover. Late season defoliators like fall webworm do not place as much stress on trees, because the defoliation occurs just a few weeks before leaves drop in autumn.

                Adult defoliators are best controlled with insecticides when they first become active and start feeding or laying eggs. Insect larval defoliators like caterpillars and sawflies that have only one generation per year can be controlled with a single application of product before the insects are one inch long. Insecticides like Spinosad, a natural-occurring soil bacterium, work better at controlling these larvae, because they are safer and conserve natural enemies that eat other tree insect pests, such as scale insects and other piercing-sucking foliage pests (aphids). 

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