Why Do Periodical Cicadas Emerge Every 17 Years (or 13 Years)?
Magicicadas cicadas have 13-year or 17-year life cycles - the longest known for an insect. They spend virtually their entire lives beneath the ground, then emerge, become adults, and breed only to die within a few weeks.
Why do they emerge in 13- and 17-year intervals? And why, within an area, do they all emerge at the same time?
Researchers in Brazil pose a few evolutionary theories on these questions:
- 13 and 17 are prime numbers. Perhaps this prime time emergence was an evolutionary development enabling them to outwit parasites. That is, parasites life cycles are much shorter, generally a few years. Because a prime number is not divisible by any number except itself and 1, it would be very difficult for a parasite to try to match its own life-cycle emergence with that of the periodic cicada. Thus, if a parasite species were dependent on the cicada, it is likely that the parasite species would become extinct.
- By emerging in such high numbers (there can be as many as hundreds of thousands of cicadas emerging simultaneously in a single acre), it is almost impossible for predators of the periodic cicada to be able to eat them all, thus enabling its species to survive.
How Do Periodical Cicadas Emerge?
- These cicadas live 13 or 17 years about 6 to 18 inches underground as nymphs feeding on tree roots.
- In the spring of their emergence, they begin tunneling exit holes, of about 1/2 inch in diameter, to the surface.
- When the soil temperature in the exit hole reaches 64 degrees F, the nymphs emerge from the holes at sunset, and climb up trees or any nearby structure.
- At night, the skin is shed and the adult emerges, spending just under a week harboring to enable their skin to harden and become fully adult.
- Although southern cicadas tend to emerge sooner than northern cicadas, due to the warmer temperatures, all cicadas in a region will emerge at the same time.
- Additionally, the periodical cicadas of a brood will tend to emerge at roughly the same time of year every cycle, with some variation for temperature.
When Will Cicadas Emerge in Your Area?
According to Dr. Gene Kritsky, Professor of Biology, College of Mount St. Joseph:
- There are three species of 17-year cicadas: Magicicada septendecim, Magicicada cassini and Magicicada septendecula. These will emerge in 2013.
- There are four species of 13-year cicadas: Magicicada tredecim, Magicicada neotredecim, Magicicada tredecassini,and Magicicada tredecula.
- Cicadas emerge in "broods," designated by Roman numerals, beginning with those that emerged in 1893 and at 17-year intervals thereafter as Brood I [1]. The cicadas that emerged the next year, 1894, and every 17 years thereafter are Brood II (2) and so on. The cicadas that emerge in 2013 are Brood II.
- Numbering for the 13-year cicadas began at 18 [Brood XVIII] with those that emerged in 1894, and the numbering continues on to Brood XXX [30].
These cycles can then be used to determine when and where the broods will emerge. Although they are not always exactly on time, the University of Connecticut created an online database query providing information on past emergence.
You can search a vast array of factors in the Magicicada Database Query Page, but to find out when broods have been in your state or county, select these and click search. You can also limit your search to only population samples, by selecting that line in Record Type.
Or, for a more general reference, refer to the Magicicada Brood Chart to find out if periodical cicadas are coming to your town!
How do you know if it is a 17-year Periodic Cicada ... or just a regular everyday brand of cicada?
Magicicada periodical cicadas:
- have black bodies and red eyes. Their wing veins are reddish-orange with a black W shape near the tip.
- are about 3/4 to 1 3/8 inch in length as adults.
- are found in eastern North America. The 17-year broods are generally in the northern states and the 13-year broods in the southern and Midwest sates.
- emerge in spring to early summer, when temperatures are about 64 degrees F.
- make distinct species-specific sounds, or songs. Audio recordings of these can be heard at The University of Michigan Museum of Zoology page.
- feed on woody plants during the day, however only the females feed. insert 10 to 25 eggs at a time into the twig of a tree; a female may lay more than 500 eggs in her short lifetime.
- hatch from their eggs as nymphs within a couple months, then drop to the soil to tunnel beneath the soil to begin feeding on roots.
Part 2: Cicada Damage and Control
References and Resources
Source...