Magicicada broods and distributions
Although nearly all of the periodical cicadas in a given region emerge in the same year, the cicadas in different regions are not synchronized and may emerge in different years. All periodical cicadas of the same life cycle type that emerge in a given year are known collectively as a single “brood” (or “year-class”). The resulting broods are designated by Roman numerals — there are 12 broods of 17-year cicadas (with the remaining five year-classes apparently containing no cicadas), and 3 broods of 13-year cicadas (with ten empty year-classes). As a result, it is possible to find adult periodical cicadas in almost any year by traveling to the appropriate location. The table below is a guide to the approximate locations of periodical cicada broods. On a local scale, periodical cicadas can be patchily distributed.
Straggling and spurious broods
Sometimes periodical cicadas emerge “off-schedule” by one or more years. This phenomenon is often referred to by the general term “straggling,” although straggling cicadas can emerge either later or earlier than expected. Straggling makes it difficult to construct accurate maps of periodical cicada brood distributions, and historical reports of emergences often contain little or no information about how many cicadas were seen. Straggling emergences in which only a few cicadas are present are common, but larger unexpected emergences of thousands of individuals have also been reported. Stragglers are almost certainly responsible for reports of “spurious broods” that are not generally recognized. For more information about stragglers, see this page.
Periodical cicada broods fit together like puzzle pieces, in both time and space. Broods are neither species nor are they populations; they are best described as regional, multispecies groupings of periodical cicadas that emerge on a common schedule. Broods may have complex histories, exemplified by the fact that some broods have geographically separated disjuncts. More subtly, the different species found in any given brood may also have separate evolutionary histories and may have joined the brood at different times or from different sources. The video below describes one simple hypothesis for brood formation drive by climate shocks and temporary life cycle anomalies.
Periodical Cicada Broods
|I||1978||1995||2012||2029||TN, VA, WV|
|II||1979||1996||2013||2030||CT, GA, MD, NC, NJ, NY, OK, PA, SC, VA|
|III||1980||1997||2014||2031||IA, IL, MO|
|IV||1981||1998||2015||2032||IA, KS, MO, NE, OK, TX|
|V||1982||1999||2016||2033||MD, OH, PA, VA, WV|
|VI||1983||2000||2017||2034||GA, NC, SC|
|VIII||1985||2002||2019||2036||OH, PA, WV|
|IX||1986||2003||2020||2037||NC, VA, WV|
|X||1987||2004||2021||2038||DE, GA, IL, IN, KY, MD, MI, NC, NJ, NY, OH, PA, TN, VA, WV|
|XIII||1973||1990||2007||2024||IA, IL, IN, WI|
|XIV||1974||1991||2008||2025||KY, GA, IN, MA, MD, NC, NJ, NY, OH, PA, TN, VA, WV|
|XIX||1985||1998||2011||2024||AL, AR, GA, IN, IL, KY, LA, MD, MO, MS, NC, OK, SC, TN, VA|
|XXIII||1989||2002||2015||2028||AR, IL, IN, KY, LA, MO, MS, TN|