Cicada nymphs feed underground on tree roots and do not cause significant damage to trees. The cicada nymphs help aerate the soil and bring nutrients and nitrogen to the surface, benefiting plants. Once the nymphs emerge, they spend a few days on trees and shrubs, allowing their new adult exoskeletons to harden and darken. During this time, they do not feed and do not damage trees.
Adult cicadas exist for one reason, to mate. It is the egg laying by the mated females that damage trees. The female cicada excavates a channel in small twigs or branches (those around the diameter of a pen). She oviposits her eggs in the slit, effectively splitting the branch open. The ends of affected branches will brown and wilt, a symptom called flagging.
On mature, healthy trees, cicada activity is not a concern. Large, established trees can withstand the loss of branch ends and will recover from the onslaught of cicadas.
Young trees, particularly ornamental fruit trees, do require some protection. Because most of its branches are still small enough to attract female cicadas intent on laying eggs, a young tree may lose most or all of its branches. In very young trees with trunks under 40 mm in diameter, even the trunk may be excavated by a mated female.
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