Homeowners may be seeing some lingering effects of last year’s 17-year cicada emergence in smaller branches of trees, but it’s nothing to worry about.
“The trees will be fine in the long run, even if you see some small dead branches now,” said Sharon Yiesla, plant knowledge specialist at the Plant Clinic of The Morton Arboretum in Lisle. “Just prune any dead wood from small trees or shrubs if you find it unsightly.”
Toward the end of last year’s emergence, female cicadas laid their eggs in small slits in slender young twigs and branches. When the eggs hatched, the nymphs fell to the ground and burrowed into the soil, where they will spend the next 17 years. They left behind slits in the wood. To learn more about periodical cicadas and their life cycle, see mortonarb.org/cicadas.
Although trees can seal over most of the slits with new bark and wood, too many slits can kill a small branch. “The tree will grow new twigs to replace the dead ones,” Yiesla said. Branches that were stressed by cicada egg-laying also may lose their leaves early this fall.
“Cicadas are native insects, and our native trees are adapted to dealing with their periodical emergences,” she said.
There is a small possibility that if an egg-laying slit was not completely sealed over, it may admit disease organisms. For example, it can admit the fungal spores or bacteria that cause cankers — infections in the wood of a tree or shrub. These organisms can enter any opening in the tree’s protective bark, such as a broken branch, a scrape or nick, or a pruning wound.
A canker wound may appear as a slit in the wood, a change in the color of the stem, a sunken area, cracked or peeling bark, or oozing sap. Canker diseases spread through the wood both above and below the visible wound.
“If you catch a canker infection early enough, before it spreads too far inside the tree, you may be able to prune it out,” Yiesla said. “But it’s important to catch it early.” The Plant Clinic (mortonarb.org/plant-clinic) can help diagnose canker infections or other plant disease issues.
The big danger is not that cicada damage will lead to cankers, but that cankers will be overlooked by homeowners. “If you assume that damage is caused by cicadas, you may miss a bigger problem,” she said. “Not all slits are cicada slits.”
The insects only laid their eggs in very small branches less than ½ inch in diameter. “Get a wound checked out if it’s in any branch thicker than your little finger,” Yiesla said. “That’s not from cicadas.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.