Purdue Landscape Report: Dead leaves covering trees (image 1) or on the ground beneath them (image 2) in July would normally be a worrying sign for tree health, but this year much of the damage can be blamed on 17-year cicadas. This damage is unlikely to cause serious trouble for healthy, large trees and management is relatively simple. The choice to prune or not to prune comes down to cost, aesthetics, and concern for the next generation of cicadas.
How Cicadas Damage Plants
Cicada damage is similar to a light pruning and should not be an issue for healthy, mature trees. Cicadas damage trees when they lay their eggs in small twigs (3/16 to 1/2 inch in diameter) on deciduous trees and shrubs. They have a long, thin body part called an ovipositor that resembles a sewing needle that they stab into plants to lay their eggs. This action creates small holes and cracks in the bark (image 3). If enough cicadas lay their eggs in a twig, it can damage the bark enough to kill the twig (image 1).
Recognizing Cicada Damage
The degree of cicada damage depends on insect density and the number of trees in the area. To determine if a tree or bush has been damaged by cicadas, ask the following questions:
1. Were there 17-year cicadas within 50 meters (~164 ft) of the tree this year? Cicadas do not travel very far. If there weren’t noticeable numbers of 17-year cicadas nearby the damage was likely caused by something else.
2. Is the damage on a deciduous tree or bush? Cicadas rarely lay their eggs on evergreen trees and herbaceous plants. Damage on these types of plants is likely caused by something other than the cicadas.
3. What size of branches and twigs are damaged? Cicadas show a strong preference for small twigs (3/16 to 1/2 inch in diameter). As a result, damaged trees may appear as though their outer layer of leaves is dead while the inner leaves remain healthy (image 2). If larger branches are dead, the damage was probably not caused by cicadas.
4. Does the bark have typical egg laying damage? If you can reach the damaged twigs, look for a row of puncture wounds often connected by cracks length-wise along the branch. Their appearance may vary between tree species (image 3), but they will almost always be length-wise.
Billions of Cicadas Are Coming This Spring; What Does That Mean for Wildlife?, Got Nature? Blog, Purdue Extension-Forestry and Natural Resources (FNR)
17 Ways to Make the Most of the 17-year Cicada Emergence, Purdue College of Agriculture
Ask an Expert: Cicada Emergence Video, Got Nature? Blog, Purdue Extension-FNR
Periodical Cicada in Indiana, The Education Store, Purdue Extension resource center
Cicada Killers, The Education Store
Cicada, Youth and Entomology, Purdue Extension
Purdue Cicada Tracker, Purdue Extension-Master Gardener Program
Purdue Landscape Report
Elizabeth Barnes, Exotic Forest Pest Educator
Purdue University Department of Entomology