The 2021 Periodical Cicada Emergence (Brood X)

Periodical cicadas are unique in that all (or nearly all) members of the population emerge in one year and then are absent in the intervening years.

17-year periodical cicada Brood X is emerging in 2021.   Brood X was mapped during the 2004 emergence, but since this is one of the larger broods, some parts of the brood have yet to be mapped in detail.  The brood is divided into three major parts.

You can Help!

You can report periodical cicadas using the Cicada Safari App, available on the Google Play Store or the Apple Store.  For reasons noted below, we’re especially interested in separating records of stragglers from records of full Brood X emergences.

Why stragglers are an issue for Brood X.

A perennial issue with Brood X is that it occurs 4 years after Brood VI and 4 years before Brood XIV, and these three broods are adjacent to each other in parts of their ranges.  From a biological perspective, 4-year stragglers from either of these broods are of interest because they can cause gene flow among these broods.  From a practical perspective, 4-year stragglers from any of these broods complicate mapping efforts, because populations may be difficult to assign to a brood.  Some straggling emergences are easily identified as such, since they involve only small numbers of cicadas.  Others are less easily interpreted, and the difficulty may be compounded by the problem of “shadow brooding” in which repeated instances of straggling resupply populations of stragglers that are on the verge of becoming self-sustaining.  We expect to see emergences in 2021 that are difficult to assign to a single brood in Kentucky (where Broods X and XIV are in contact) and in Georgia (where Broods VI and X are in contact).

2021 will also see an emergence of Brood X in the Washington DC Metro area.  This area saw substantial emergences of 4-year early stragglers in 2017, and because 2017 was a Brood VI emergence year, these stragglers were sometimes confused with Brood VI.  Expect to see heavy Brood X emergences in the DC metro areas with stragglers in 2017.

An overriding philosophy of the mapping project is: A Misleading Map is Worse Than No Map At All.  There will be substantial numbers of stragglers in 2021, and if they are mistaken for Brood X emergences, these mistaken records will create the impression that Brood X has grown suddenly and substantially larger, when no such thing is true.  When we collect mapping records, we keep track of densities– in all likelihood, true Brood X emergences will be dense and widespread, while straggler emergences tend to be more patchy and scattered (More details about how we take records are what we use for mapping criteria are found here.).

Key points about stragglers:

  • Stragglers typically occur at low densities
  • Stragglers are typically patchy
  • Stragglers populations generally don’t last for long!

Brood X Outstanding Questions:

  • What is the northern limit of Brood X in Michigan, and what populations exist in northern Ohio?
  • Are the three species groups (-decim, -cassini, and -decula) equally represented among stragglers?
  • Brood X comes into contact with Brood XIX in south eastern Illinois.  To what extent, if any, do these broods overlap, and are their boundaries stable?
  • Brood X comes into contact with Brood XXIII in southern Indiana.  To what extent, if any, do these broods overlap, and are their boundaries stable?


Many reprints are available through the “Reprints @ ResearchGate” link on my homepage.

Cooley, J. R., G. Kritsky, M. J. Edwards, J. D. Zyla, D. C. Marshall, K. B. R. Hill, R. Krauss, and C. Simon. 2009. The Distribution of Periodical Cicada Brood X in 2004. The American Entomologist 55(2): 106-112.

Cooley, J. R., G. Kritsky, D. C. Marshall, K. B. R. Hill, G. J. Bunker, M. L. Neckermann, J. Yoshimura, J. E. Cooley, and C. Simon. 2016. A GIS-based map of periodical cicada Brood XIII in 2007, with notes on adjacent populations of Broods III and X (Hemiptera: Magicicada spp.). The American Entomologist 62:241-246.

Cooley, J. R., N. Arguedas, E. Bonaros, G. J. Bunker, S. M. Chiswell, A. DeGiovine, M. D. Edwards, D. Hassanieh, D. Haji, J. Knox, G. Kritsky, C. Mills, D. Mozgai, R. Troutman, J. D. Zyla, H. Hasegawa, T. Sota, J. Yoshimura, and C. Simon. 2018. The periodical cicada four-year acceleration hypothesis revisited: Evidence for life cycle decelerations and an updated map for Brood V (Hemiptera: Magicicada spp.). PeerJ 6:e5282.

Dybas, H. S. 1969. The 17-year cicada: A four year mistake? Bulletin of the Field Museum of Natural History 40:10-12.

Marshall, D. C., J. R. Cooley, and K. B. R. Hill. 2011. Developmental Plasticity of Life-Cycle Length in Thirteen-Year Periodical Cicadas (Hemiptera: Cicadidae). Annals of the Entomological Society of America. 104(3): 443-450.

Marshall, D. C., K. B. R. Hill, and J. R. Cooley. 2017. Multimodal life cycle variation in 13- and 17-year periodical cicadas (Magicicada spp.). Journal Of The Kansas Entomological Society 90: 211-226.