What is a brood?

The concept of a periodical cicada “brood” is more a bookkeeping construct than a biological reality. Broods are best thought of as all the cicadas emerging in a given year in a given area on a predictable cycle.

Sometimes, cicadas emerge in places where they are not expected to emerge in a given year, but the location has a well-established record of periodical cicada emergences on a different schedule- in other words, cicadas emerge in a brood year, but within the known terrritory of a different brood. These cicadas are best thought of as off-cycle cicadas, or “stragglers.”

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”), though this should not be taken to imply that all subdivisions (region, species, etc.) in given brood have the same evolutionary histories. “Brood” is a bookkeeping concept more than a biological one. Because there are 12 broods of 17-year cicadas (with the remaining five year-classes apparently containing no cicadas), and three broods of 13-year cicadas (with ten empty year-classes), it is possible to find adult periodical cicadas in almost any year by traveling to the appropriate location.

Periodical cicada broods are designated by Roman numerals. Literature prior to 1902 used a different scheme of enumerating broods based on the order of emergence following 1868. However, this scheme quickly became impractical, as the shorter life-cycles of 13-year broods quickly scrambled the order of the broods. In 1902, Charles Marlatt (Marlatt 1902) presented the scheme currently used for designating periodical cicada broods. Roman numerals I-XVII were reserved for 17-year broods, and Roman numerals XVIII-XXX were reserved for 13-year broods (Marlatt 1902 Table 1). Marlatt arbitrarily designated the 17-year brood emerging in 1893 as Brood I and the 13-year slot for that year was designated Brood XVIII (though there is no 13-year brood on this schedule). This scheme, unlike the previous scheme, makes it possible to determine the life cycle from the Roman numeral designation; in fact, we still use Roman numerals to desigate broods and Arabic numerals to designate life cycles in order to avoid confusion- For example, Brood XIII is a 17-year brood.

About the maps on this site

All of the maps created for this site have similar characteristics and limitations. Unlike many maps of periodical cicadas, we display our data as point data and do not attempt to draw brood boundaries or generalize our data to counties. Brood boundaries are complex on an extremely fine scale and attempts to encircle records with a drawn boundary usually end up overestimating the extent of the periodical cicada emergence. Likewise, maps that generalize to a county level greatly overestimate brood ranges. Such overestimations, in turn, create the false impression of overlaps (sometimes apparently extensive) between broods or life cycles (see Marshall 2001 for a discussion of these points). To reduce any such tendency to overestimate, our maps are intentionally designed to be conservative. Where our maps show the presence of cicadas (“positive” record), then one of the principal members of this project verified that cicadas were present at that place and time. Where our maps show the absence of cicadas (“negative” record), then one of the principal members of this project visited that location and at that time verified that no cicadas were present. Where our maps show no records at all, that is an indication that no member of this project has visited that location.

Note also that the records on our maps include date information. The dates shown do not represent dates of adult appearance; rather, they represent dates on which choruses were active and the area was visited by a principal member of this project. Thus, in any given area, adult emergence may have occurred a week or more earlier than the dates shown on this map.

In the map below, different broods are represented by different colors. Click on any point to see which brood it belongs to. This map may not be reproduced without written permission.

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.

Brood Origins

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.

 

Guide to Periodical Cicada Broods

Click the links to see details of each brood.

Life Cycle		Year				General region
17-year
I	Shenandoah Brood	1978	1995	2012	2029	TN, VA, WV
II	East Coast Brood	1979	1996	2013	2030	CT, GA, MD, NC, NJ, NY, OK, PA, SC, VA
III	Iowan Brood	1980	1997	2014	2031	IA, IL, MO
IV	Kansan Brood	1981	1998	2015	2032	IA, KS, MO, NE, OK, TX
V		1982	1999	2016	2033	MD, OH, PA, VA, WV
VI	Brushy Mountains Brood	1983	2000	2017	2034	GA, NC, SC
VII	Onondaga Brood	1984	2001	2018	2035	NY
VIII		1985	2002	2019	2036	OH, PA, WV
IX		1986	2003	2020	2037	NC, VA, WV
X	Great Eastern Brood	1987	2004	2021	2038	DE, GA, IL, IN, KY, MD, MI, NC, NJ, NY, OH, PA, TN, VA, WV
XI		1937	1954	Extinct		CT
XII		Spurious
XIII	Northern Illinois Brood	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
XV		Spurious
XVI		Spurious
XVII		Spurious
13-year
XVIII		Spurious
XIX	Great Southern Brood	1985	1998	2011	2024	AL, AR, GA, IN, IL, KY, LA, MD, MO, MS, NC, OK, SC, TN, VA
XX		Spurious
XXI		1870	Extinct			FL
XXII	Baton Rouge Brood	1988	2001	2014	2027	LA, MS, OH, KY
XXIII	Lower Mississippi Brood	1989	2002	2015	2028	AR, IL, IN, KY, LA, MO, MS, TN
XXIV		Spurious
XXVII		Spurious
XXIX		Spurious
XXX		Spurious