Any day now we’re expecting the arrival of the 17-year cicadas in Northern Illinois. Whether you’re excited or uneasy, it will without a doubt be a sight to see (and a loud, buzzing sound to hear). But beyond this spectacular event lies an important consideration for local homeowners and gardeners: the impact these cicadas can have on trees.
A Brief Overview of the 17-Year Cicada Life Cycle
In Northern Illinois, our brood of periodical cicadas emerge en masse every 17 years to mate, lay eggs, and complete their life cycle. For 17 years before they finally emerge, cicada nymphs spend their lives underground, feeding on sap from tree roots. When it’s finally time, the cicadas burrow up from the ground, climb up a tree, shed their exoskeleton, and emerge as an adult cicada. Males then commence their loud singing to attract a mate.
After mating, females cut slits into tree branches using a saw-like ovipositor, a tube-like organ for laying eggs. She deposits up to 20 eggs in each slit and can lay about 600 eggs in her brief time as an adult. In 6-10 weeks, the cicada nymphs hatch. They drop to the ground, burrow in, and feed on roots for the next 17 years.
How Cicadas Impact Trees
Trees play a central role in the life cycle of cicadas, serving as both a food source and the site for mating and laying eggs. While cicadas can use a variety of woody plants as a host, they have their favorites. They prefer apples, hickories, maples, and oaks, and they also may target birch, willow, linden, and elm. Conifers are rarely chosen since their needles can get in the way of laying eggs. Conifer sap can also trap and kill eggs.
Female cicadas choose tree and shrub branches of a certain size, between 1/8” to about 1/2” in diameter. They make slits and deposit eggs near the ends of branches which causes this section of the branch to dry out and die—a phenomenon known as flagging. Healthy, mature trees aren’t hurt by flagging.
Young trees, on the other hand, can be seriously damaged or even killed by these cicada slits. For one, small trees may only have branches that are under an inch in diameter, so they are a prime target for laying eggs. Young trees and shrubs also have a hard time recovering, compared to mature trees. The slits can also open trees up to pathogens and other pests, to which small trees are more vulnerable. As for the annual cicadas that emerge each year, they do little damage to trees and shrubs.
Additionally, trees serve as hosts for cicadas during their underground nymphal stage. While most trees are unaffected by nymphal feeding on roots, some may experience slowed growth as a result.
How to Protect Your Young Trees
If you have young trees on your property, taking proactive measures can help minimize the impact of cicadas.
- Put off planting. Consider waiting to plant young trees in your yard until after the cicadas have come and gone. Fall is also a great time to plant trees and shrubs.
- Cover young trees. Cover young trees with fine netting or cheesecloth to keep cicadas away. If possible, you can cover the whole tree with netting, being careful to gather and secure it at the trunk near the ground. Another option is to cover the leader branch and any other main branches if you have a limited amount of netting.
- Skip insecticides. Insecticides are typically ineffective against cicadas and can be harmful to beneficial insects.
The arrival of the 17-year cicada brood is fast-approaching, so it is important to understand their potential impact on trees. While mature trees can weather the cicada storm with minimal concern, young trees require extra attention and protection. By covering young trees and waiting to plant more until the fall, we can ride out cicada season with little concern. At the same time, remember to enjoy and marvel at this unique nature event. It’ll be another 17 years until we see this cicada brood again!
