Swarms of cicadas are unexpectedly crawling out from under trees from North Carolina to New Jersey. The red-eyed insects are almost impossible to miss; they fly around lazily, plunking into backyard barbeques and crashing into cars. They litter the ground with their crunchy husks as they molt. Most noticeably, they chirp en masse for their mates, producing a relentless, shrill buzz that is recognized as a song of summer. And within a month they are gone.
Different populations, or broods, of “periodical” cicadas emerge in distinct geographical regions during specific years, after spending a 13- or 17-year span growing underground. (Some “annual” species just emerge yearly.) Scientists were expecting to see Brood VI bugs in South Carolina and Georgia, which happened, but they got a surprise when Brood X cicadas also started appearing in North Carolina, Virginia, Maryland, the District of Columbia, Ohio and Indiana last week—four years earlier than anticipated.
Experts suspect a warming climate, with more warm weeks a year during which the underground nymphs can grow, could be triggering some cicadas to emerge ahead of their brood. “Temperature is everything,” says Marlene Zuk, an entomologist at the University of Minnesota. “When temperature changes, insects don’t just feel hot or cold. Their whole body doesn’t function normally.” And cicada nymphs may be growing to a threshold size so quickly that their internal biological clock is miscalculating when it is time to emerge, says Keith Clay, a biologist and cicada expert at Indiana University Bloomington. To calibrate these clocks, periodical cicadas likely rely on a variety of environmental cues such as changing seasons and ground temperature, he says. Nymphs feed on the xylem fluid (sap) from tree roots, and changes in the fluid composition as trees leaf out each spring may also help them gauge the passage of time. Entomologists reached this conclusion back in 2000 when they artificially sped up the blooming cycle of peach trees supporting cicada nymphs that were in their 15th year and tricked the insects into emerging a year early.
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Nymph growth is key. “They have to be a certain size to come out as adults,” says Chris Simon, a cicada biologist at the University of Connecticut. Nymphs go through five stages of development, called instars, which last an average of four years each, except for the first one. The nymph’s internal clock may synchronize to these four-year instars, she says, which may explain why this year’s premature arrivals are ahead of schedule by four years. Even if they grow at different rates, most cicada nymphs wait for the rest of their group to catch up before squirming out of the dirt, she adds. When the soil reaches 64 degrees Fahrenheit in a given year, they all emerge together. Experts think evolution has favored this strategy as a way of overwhelming predators like birds and squirrels so cicadas can mate frantically and lay eggs before they die in a few weeks. But thousands of Brood X sightings implies the bugs are not waiting for their fellows. They are risking being eaten before they can reproduce, Simon notes.
Citizen science projects like magicicada.org are helping researchers keep track of more cicadas than ever before. People can report the location and type of cicada they see as well as whether the insects are singing or not. Simon and her team plan to head to the Washington, D.C., area this weekend to gather more data on how long the cicadas keep chirping and inspect wilted branches and leaves for scars left by laying eggs.
If Brood X cicadas continue to emerge and mate in large numbers over the next week or two, it could mark a rare acceleration event in which a significant number of cicadas survive and establish a new brood, Simon says. But she’ll only find out if the early risers are truly successful much later. “We need to keep tracking them for 17 more years, and 17 years after that to get a clear picture,” she adds.