Does weather or climate play a role in the emergence of the 17-year periodical cicada?

"Seventeen Year Blues" to the tune of the Rolling Stones "Miss You"

They've been living b'lo ground
Now they're nearing their rebound
Yeah, we miss you
By late May they'll all emerge,
Sooner if they feel the urge
Yeah, we miss you
By the zillions gonna swarm
Four weeks is bout the norm

Oh, cada why you wait so long
Won't you come on! Come on!

Is this real or hocus-pocus?
Are they cicada or locusts?
Yeah, we miss you
Ten plus seven they've had to wait
Just to court a bit, then mate
Yeah, we miss you

Look for this at record stores, if not next week, perhaps in 17 more years.

Our brains seem to work better when we can celebrate events or birthdays having zeroes or fives affixed to the end. Maybe that's why we designate those dates for special celebrations. For example, fifty years ago today, (May 6) Roger Banister first broke the 4 minute mile. Not many people are throwing grand parties for whatever it is that happened in May of 1987. It just so happens, though, that a brood of periodical cicada (Brood X) will emerge across much of the eastern U.S. this spring after spending 17-years below ground. Let's be honest, after enduring almost 2 decades buried beneath the surface, if anything deserves a party, they certainly do.

Periodical cicada are definitely the Rip Van Winkle of the subterranean world. At this very moment, while we may feel little or no apprehension, literally billions of these red-eyed critters are lurking at our feet. However, within a couple of weeks, they'll invade the above-ground world with a multitude of exoskeletons and a cacophony of sound. There can be as many as one million per acre in heavily infested locales, typically in hardwood groves. In these areas, the sound will be simply overpowering Fortunately, they neither sting nor bite, thought, their egg laying can injure young trees.

Prior to this spring, they've resided a foot or more below the surface, surviving on the juices of roots and staying nearly stationary, except to occasionally shed their armour. They've missed all of the above ground activity since the summer of 1987. President Reagan was in his second term when the last batch was upon us, no one had yet heard of the world wide web, Friends was still years from its first episode, and the Cubs won the World Series (ok, they didn't miss this yet).

Cicada aren't blessed with the biggest ganglia of the world's creatures nor the most aerodynamic set of wings -- sometimes you can observe them repeatedly flying into a wall, falling to the ground, righting themselves, and doing it again. Their single-minded intent is to mate and reproduce, and for this they're exquisitely designed.

While they're euphemistically called "locusts," cicada are in a different order of the animal kingdom. Locust are in the order Orthoptera and cicada are in the order Hemiptera. Locusts are basically migratory grasshoppers, while cicada don't migrate at all (except upward from the soil). It's thought that they were first referred to as "locusts" by Pilgrims on Cape Cod when they were besieged by them in 1634 (these happened to be Brood XIV). The Pilgrims were terrified by their sudden appearance and astounding numbers and assumed that this was some plague reminiscent of locusts in ancient Egypt. They're eerie call even sounds like they're saying Pha-a-a-a-a-raoh.

The Latin name for the 17-year cicada is Magicicada septendecim. These cicada take 17 years to mature -- other species take different amount of years to reach maturity, such as the 13-year cicada. Magicicada septendecim is one of the longest living insects and also one of the loudest. It's believed that periodical cicada appear every year somewhere in the U.S. since there are 17 different broods, each numbered in order of their appearance. Most of these broods aren't so numerous or make sufficient noise to deserve our scrutiny, but others, including Brood X, the biggest and most widespread brood, most certainly get our attention whenever it's their turn to greet the Sun. The realm of Brood X is the Middle Atlantic and Mid West states -- an especially dense knot of cicada emerge in Maryland, Delaware, New Jersey, eastern Pennsylvania, and northern Virginia.

For the past month or so, cicada have been working their way toward the surface. They'll eventually emerge en mass on consecutive nights when the soil temperature is approximately 63 degrees F (17 C), but the air temperature needs to be around 68 F (20 C) for them to lay eggs. Because they're such tasty little morsels, apparently, they required a means to arise from the earth without being totally consumed. Their strategy is to overwhelm any would be predators by popping out of the ground by the millions (referred to as predator satiation). This assures that a new cycle will occur and that those predators lucky enough to be around at the same time, will enjoy a hardy cicada feast.

Cicada are driven by instinct. Since soil temperatures reach 63 degrees each spring, if periodical cicada were solely guided by climate considerations, they would emerge every year. In addition, there are no known 17-year climate cycles. It's as if some mysterious signal beacons them to the surface every 17 years, transforming them from pulpy pupae (actually nymphs) into winged adults.

New research has shed some light on the curious cicada behavior. It seems that both math and climate have played a role in cicada evolution. Biologists believe now that by emerging every 13 or 17 years, over thousands of years, cicada are better positioned to survive calamitous episodes of climate, which could potentially wipe out those creatures that prefer warmer weather, can't migrate, and aren't quite able to put on another layer of clothing when there's a chill in the air. According to Robert Cox of the University of Memphis and C.C. Carlton of Arkansas University, it turns out that the odds of cicada surviving particularly cold spring and summer weather increase dramatically if they emerge periodically. For instance, if one out of every 50 summers happens to be too cold for cicadas to live, those cicadas broods having a cycle of every 2 years will completely perish within 5 decades. If the cycle is lengthened to 7 years, the probability that at least one brood having that 7-year cycle will survive for a 1,500 year period is 8%. However, if the cycle is lengthened still further, to 17 years, the probability of survival over a 1,500 year period increases to 96%! Evidently, over the eons, those genes leading to longer live cycles in the cicada world have been favored.

Interestingly, both 13 and 17 are prime numbers -- divisible only by themselves and 1. It makes sense that in order to improve their chances of surviving, it's best to emerge when cicada having a different cycle aren't out at the same time. Otherwise, if they mate with a critter having a shorter cycle than theirs, over thousands of cycles, their longer cycle could possibly morph into a less advantageous period, or the synchrony of their emergence could be altered. Emerging in 15 years instead of 17, greatly diminishes the likelihood of those cicadas passing on their genes. Even if a 17-year cycle is maintained, there would be fewer cicada around to sate their predators appetite. Moreover, cicadas not emerging at just the right time, when nearly all others emerge, quickly lose the advantage large numbers offer. Come out even a few days ahead of the rest of the brood, and they're easy pickings for a mockingbird with nestlings to feed.

Prime numbers help ensure that there will be a minimum of interaction between the different cycles -- only once every 221 years for the 13-year and 17-year cicadas. It should be said that choice played no part in these prime numbers. Rather cicada having cycles of 13 and 17 years have been thus far able to withstand the temperature anomalies and other disasters that probably resulted in extinction for broods having shorter cycles.

Because the 17-year cicada live to the north of the region preferred by the 13-year cicada, and because both groups are aligned with known climate/vegetation zones, there's a strong suggestion that temperatures plays a role in the length of their life cycles. On average Brood X and other broods of the Mid-Atlantic states first appear and become noticeable from May 15 to May 20, but this depends somewhat on the weather and your particular location. If you live in northern Virginia, the cicada may emerge a few days earlier than in central Pennsylvania, but by May 20 they'll be noticeable to almost everyone living in the geographic area corresponding to Brood X. Sure, they'll create havoc for a fortnight or two, and most of us will be glad when they're gone, but they're indeed a reminder as well as testimony to the power of evolution and the importance of acquiring the fittest genes. Perhaps by the spring of 2021, we'll miss them.

To participate in a cicada survey see http://www.cicadas.info
For more about the 17-year cicada, see the National Geographic Magazine July 1953 issue and the Washington Post for Monday, May 3, 2004.

This week's question is provided by Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for SEVEN YEARS! This year Dr. Foster has decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.