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Cicadas stir up a ruckus

Evidence shows little negative impact on trees and forests

Posted by Korey Morgan, Office of Communication, USDA Forest Service

Pedestrians scurry up and down the central boulevard, peering nervously upwards at the interlocking limbs of enormous oak trees lining the street. A traffic light flashes from red to green, the chime of the crossing signal barely audible over the pulsating of tens of thousands of insects clambering among the branches of the urban canopy.

Seemingly straight out a sci-fi movie, the Mid-Atlantic region is experiencing a unique natural phenomenon: Billion of periodical cicadas are starting to emerge across 15 states, including the Baltimore-Washington metro area. So-called Brood X, the largest of 12 periodical cicada broods, is creating quite a buzz in a region home to more than 9 million people.

As the periodical cicadas take flight in awesome numbers; rumors and misunderstandings fly around along with them. Residents fear for their vegetable gardens, their yard trees, and their favorite parks and forests. For some, the insects are even evoking imagery of a biblical plague.

“People really shouldn’t worry. Cicadas are not defoliating insects and have nothing to do with locusts,” said Sandy Liebhold, research entomologist with the Forest Service’s Northern Research Station in Morgantown, West Virginia. “They won’t eat your plants, vegetables, or even the leaves of trees. They are emerging only to mate and lay eggs.”

Even people familiar with annual cicadas are likely unaccustomed to their periodical relatives. Thousands of cicada species are commonplace across the temperate world. For residents of far flung cities such as Seoul, Rome, and New York City, their annual cacophony is a reminder of the dog days of summer.

Periodical cicadas, however, are unique to an area that spans from Texas to Massachusetts.

“There is nothing quite like the periodical cicadas in existence anywhere else in the world,” said Liebhold. “Most cicadas, in fact most insect species, complete a lifecycle in one year or less. Not so with periodical cicadas. A single generation lives for more than a decade underground and comes up every 13 or 17 years in extraordinary abundance.”

Scientists have studied periodical cicadas for more than two centuries. Benjamin Banneker, a free African American surveyor and naturalist from Maryland with little or no formal education, accurately predicted the emergence of various broods in the late 1700s. By 1900, USDA entomologist Charles Marlatt mapped the boundaries of different periodical cicada broods, a system that entomologists use today to predict where the insects will emerge.

As for their effect on trees and forests, USDA scientists today are investigating cicada behavior both above and below ground.

“Before they emerge, juvenile cicadas feed by sucking water and nutrients from tree roots,” said Liebhold. “Once they emerge, they tend to aggregate on trees grown in open spaces. The females lay their eggs by cutting slits in the green shoots of tree limbs. Neither of these behaviors is known to significantly harm trees. With one notable exception: very young trees can be overwhelmed by too many females cutting slits to lay eggs.”

One way to protect your recently planted saplings is to secure a fine mesh netting around the canopy for a few weeks. Coping with the sheer abundance of the periodical cicadas is another question and depends on who you ask.

“In North America, we are blessed with these very interesting and unique species. We are very lucky to have them, and I hope people enjoy this amazing natural phenomenon,” said Liebhold.

More information about periodical cicadas is available from the Forest Service’s Northern Research Station, including a recent special episode of the podcast “Forestcast.” Forestcast is produced by the Northern Research Station, and uses cutting edge science to show what is happening in the forests of the Northeast and Midwest, and where those ecosystems may be headed. Get more episodes of Forestcast wherever you get your podcasts.

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