Brood’s Clues: New Mapping Approach Puts Cicadas in Focus
By Leslie Mertz, Ph.D.
Vast numbers of creatures are preparing to arise from beneath the ground this year, but it isn’t a TV episode of Stranger Things or a scene from a vampire movie. Rather, it is the real-life, millions-strong arrival of periodical cicadas, known collectively as Brood X, that emerge en masse every 17 years.
The short-lived show, complete with its deafening racket of buzzy male songs, will only last a few weeks while mating frenzy and egg-laying are accomplished. The adults will then die off, and the larvae will hatch from the eggs and burrow back underground to remain until the next big group emergence in 2038.
Entomologist John Cooley, Ph.D., is especially excited about Brood X this year, because its timing coincides with the release of a new approach that promises a better understanding of cicada broods and whether their ranges are shifting due to climate change or other causes. Cooley, a professor in the Department of Ecology and Evolutionary Biology at the University of Connecticut-Hartford, is lead author of a report describing the new cicada-mapping approach, published last week in the Annals of the Entomological Society of America.
Cooley has been interested in cicadas since his Midwest childhood summers spent listening to the annual summer cicada Neotibicen pruinosus, sometimes known as a scissors grinder. “It had dusk choruses of EEEEEEE-oo, EEEEEEE-oo, EEEEEEE-oo that were just mind-blowing,” he says. “Then you’d catch them: As nymphs, they’re like little tanks, and when they’re adults they’re these big fight-y insects, so it was endless fascination!”
His enchantment with cicadas continued into college when he began exploring the so-called periodical cicadas, or those that emerge in large groups either every 13 years or every 17 years. Brood X is the most widespread and largest of the emergences, and includes all three of the 17-year-cycle species—Magicicada septendecim, M. cassini, and M. septendecula—that arise together every 17th spring to congregate on trees throughout much of the eastern-central United States. Besides Brood X, 11 additional 17-year broods plus three more 13-year broods of periodical cicadas occur in the United States.
“When you’re talking about periodical cicadas, you quickly get into questions about the evolution of the species and the broods, and many of the hypotheses make an appeal to their geographical distribution, or the biogeography,” Cooley says. “To do that, you need fine-scale maps, but the maps that were in existence at the time were county-level at best, which are too crude to determine whether broods overlap or ranges are changing or to understand differences in the life cycles of broods (13- versus 17-year cycles) and the potential for gene flow (interbreeding) between different broods or life cycles.”
To fix that problem, Cooley and University of Connecticut colleagues Christine Simon, Ph.D., and David Marshall, Ph.D., set out to construct much more precise maps for these intriguing bugs.
Building better maps meant hitting the road. For the work described in the new paper, the researchers set their sights on Brood VI, which extends over parts of North Carolina, South Carolina, and Georgia, and was due to emerge in 2017. In both 2000 and again in 2017, the researchers spent a month “driving down every little road we could get the car through” to pinpoint the location—aided by GPS technology in 2017—of singing males and listening to song choruses to estimate population size.
The data from these back-to-back Brood VI emergences provided the first published examples of such highly detailed mapping of consecutive periodical cicada brood generations, Cooley says. With such comprehensive information, the researchers not only had a precise picture of the brood’s range boundaries and population density, but they can also now use that as a baseline from which to clearly identify future density changes or range shifts.
One of the most challenging aspects of their work was to delineate range boundaries so they could clearly assess the occurrence of overlaps between neighboring broods or shifts in a brood’s range. After what Cooley described as “a lot of arguing and discussion about it,” the researchers finally settled on a conservative description that would indicate range shifts but wouldn’t show false overlaps. A preliminary analysis of data from Brood VI, as well as soon-to-be-published data they have gathered on other broods, suggests that brood overlaps are rare, which shores up their impressions from the field, Cooley says. “It looks like broods don’t really overlap that much, no matter what the old papers and records say.”
The researchers did, however, notice something odd while they were in the middle of the 2017 Brood VI field work. That year, UConn invited the general public to submit online reports of periodical cicadas. “We became aware that something funny was happening when the website quickly got overwhelmed with reports from Washington, DC, which is way out of range for Brood VI but is within the range of Brood X, so these cicadas in DC were emerging four years early,” Cooley says. “If we begin to see lots of mistakes in counting—and periodical cicadas get to the right number of years not with an internal clock but with a counter (counting seasonal pulses of fluid flow in plant roots)—then something is going on, and a leading hypothesis is climate change.”
Asking for Public Help
This year, Cooley is again encouraging the public to get involved by reporting periodical cicadas via the new Cicada Safari app. “Not only is this the year when Brood X will be coming out, but it’s also four years before the Brood XIV emergence, so if there are any strange, off-cycle emergences of Brood XIV, they’re going to show up this year,” he says.
He adds, “We’d like to get people out there and watching. It’s going to be an exciting year, and hopefully this new Brood VI paper will help us think about how to deal with all the new information that will be coming in.”
Annals of the Entomological Society of America
Leslie Mertz, Ph.D., writes about science and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.