How To Prepare
Cicadas do no serious harm to any healthy, mature tree. When they first emerge, they feed briefly on the sap of woody plants, but not enough to affect a mature plant. The greatest damage they do is when female cicadas cut slits in slender twigs at the ends of tree branches and stems of some woody shrubs to create a place to lay their eggs. The twigs affected are between ⅛ and ⁷⁄₁₆ inches in diameter. After the eggs hatch, those twigs may die and break off. A mature tree can spare those twigs and will soon grow new ones.
The only trees at risk of significant damage are very young trees that were planted within the last couple of years, or have a trunk diameter of 2 inches or less, and older trees that are unhealthy or stressed. The special organ that female cicadas use to make slits and deposit eggs is called an ovipositor and cannot harm anything other than a twig.
How To Protect Your Trees
To protect your trees, first, make sure all your trees, old or young, are well watered and healthy, with no unaddressed disease problems. Healthy trees are always better able to withstand any pests and diseases. For young trees, wrapping their branches in netting will keep cicadas out so they can’t lay eggs in twigs, while still allowing air and sunlight to reach the tree.
Tulle netting on a young tree
Wrap the tree by early May, after leaves emerge and ideally harden off, and before the insects emerge. Keep the netting on for about six weeks, until the insects have died. Tulle–the nylon or polyester mesh used to make ballerina tutus–works well. It is sold in fabric and some hobby stores. Landscape supply companies also sell insect netting. The netting needs to have a fine enough mesh to keep out cicadas, with holes ¼ inch wide or less. The fabric must also be strong enough to last for several weeks on the tree. Don’t use netting with holes ½ inch wide or more; that won’t stop a cicada.
Wrap the entire branching area of the tree. It will likely take at least several yards of fabric. Where sheets of fabric meet, overlap the edges, tuck them under once or twice, and secure the roll with binder clips, clothespins, or staples. Below the branches, gather the netting around the trunk or the base of the shrub and clip it, staple it or tie it with twine. Make sure there is no gap more than ¼ inch. Do not tie anything around the trunk so tightly that it damages the bark.
Netting must stay on the tree for about six weeks, beginning in late April or early May. Be sure to remove the netting and any string or rope tied around the trunk once the cicadas have died. Wait until the cicadas have subsided to prune out any visible damage. It is often not necessary; dead twigs will likely fall off a tree, and larger branches are not affected. Studies have shown that most wounds left in twigs by cicadas callus over with new wood in one to two years.
Source: The Morton Arboretum website: mortonarb.org