Periodical cicadas, sometimes called 17-year locusts, emerge from the ground by the thousands every 13 or 17 years. The cicada nymphs cover trees, shrubs, and other plants, and then molt into adulthood. Adult males congregate in loud choruses, and fly together in search of females. Homeowners may be concerned about damage to their landscapes or gardens.
Periodical cicada nymphs feed underground on tree roots, but will not cause significant damage to your landscape trees. In fact, 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 will not damage your trees.
Adult cicadas exist for one reason – to mate. Egg laying by mated females does 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, even this cicada activity should not concern you. 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 1 1/2" diameter, even the trunk may be excavated by a mated female.
So how do you keep your new landscape trees safe from cicada damage? If periodical cicadas are due to emerge in your area, you should place netting over any young trees. Use netting with openings less than one half inch wide, or cicadas will be able to crawl through it. Drape the netting over the entire tree canopy, and secure it to the trunk so no cicadas can crawl under the opening. Your netting will need to be in place before the cicadas emerge; remove it once all the cicadas are gone.
If you are planning to plant a new tree in a year when cicadas are due to emerge in your area, wait until the fall. The tree will have 17 years to grow and establish itself before the next generation arrives.