Browning leaves on Oak Ridge Reservation harbinger of next cicada generation
Jul-21-2021

On the road leading to the Department of Energy’s Oak Ridge National Laboratory, drivers may notice that many of the green trees lining the entrance to the lab are dappled with brown leaves. At first glance, the sight isn’t extraordinary, as deciduous tree leaves turn hues of oranges and browns before falling to the ground each autumn.

Yet, just weeks past the summer solstice, this phenomenon is out of place and is in fact evidence of another natural occurrence: cicada “flagging.”

This spring, Brood X cicadas emerged from the ground after 17 years burrowed and swarmed across the eastern United States, leaving a trail of exoskeletons and echoes of mating calls. Cicadas emerge in such large quantities to withstand predation and successfully maintain their populations, and trees actually play a key role in their life cycle.

A male cicada attracts a female through a mating call, the sound responsible for cicadas’ shrill hum. After the two mate, the female cicada uses a sharp tubular organ called an ovipositor to make an incision in a tree stem and deposit her eggs there. This incision, however, damages a tree’s vascular system and can cause stems beyond the incision to die and wither, leaving behind brown leaves that resemble flags dangling from the trees.

The eggs then grow into nymphs and make their way to belowground, where they remain dormant for another 17 years.

ORNL ecosystem ecologist Verity Salmon said most trees are able to withstand the light damage inflicted by cicadas. However, it’s possible that trees exposed to other risk factors — such as lightning strikes, drought or fire — or particularly old or young trees could be at an increased risk of suffering from cicada flagging.