The 2020 Periodical Cicada Emergence

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 IX is emerging in 2020.  13-year periodical cicada Brood XIX will emerge in 2024, but significant numbers appear to be emerging this year, 4 years early.   Brood IX has not been thoroughly mapped, and the phenomenon of early emergences in 13-year cicadas is not well understood.  Citizen Scientists can help us understand these phenomena by reporting periodical cicada emergences.

Brood IX is of interest, because even though our existing maps of it are fragmentary, it appears to exemplify the puzzle-piece nature of periodical cicada broods.

17-year periodical cicada broods in NC, VA, TN, and WV. Purple circles are Brood I, blue circles are Brood II, gold circles are Brood V, brown squares are Brood VI, red circles are Brood IX, blue squares are Brood X, and green circles are Brood XIV. Open symbols are negative (absence) records.

There are several outstanding questions concerning Brood IX:

  • Brood V and Brood IX are offset by four years.  Four-year offsets are of special interest.  The spatial association of brood pairs offset by 4-year differences (XIV/I/V/IX, II/VI/X/XIV, XIX/XXIII) suggests a special role for 4-year jumps in periodical cicada brood formation (Cooley et al. 2018).  Characterizing the contact, if any, between these broods is key to understanding the role of 4-year jumps in brood formation.
  • Brood I has a disjunct in the Bristol TN/Bristol VA area that is widely separated from the main body of the brood in the Shenandoah Valley (Cooley 2015). An outstanding question is whether Brood IX occupies this gap in Brood I.
  • Magicicada cassinii is absent from Brood VI, but present in all other 17-year broods except Brood VII.  Our 2003 records of Brood IX suggest that M. cassinii is rare in Brood IX.  An outstanding question is how rare this species is in this brood.
mtDNA haplotypes in Brood IX
17-year periodical cicada broods in NC, VA, TN, and WV. Purple circles are Brood I, blue circles are Brood II, gold circles are Brood V, brown squares are Brood VI, red circles are Brood IX, blue squares are Brood X, and green circles are Brood XIV. Open symbols are negative (absence) records.
  • Brood IX is not genetically homogeneous; in the map above; blue boxes show Brood IX mitochondrial DNA (mtDNA) haplotypes.  Our sampling to date (which is limited) indicates that the portion of Brood IX in NC is characterized by eastern mtDNA haplotypes, while the interior portion, in VA and WV, exhibits Midwestern mtDNA haplotypes.  This difference suggests either introgression of one or the other mitochondrial haplotypes into the brood, or it suggests that the brood is of composite (multiple) origins. The absence of mixed-haplotype population samples argues for the composite-origin hypothesis; however, sampling is extremely sparse, limiting our ability to make strong conclusions.
Detail of 17-year periodical cicada broods near Elkin, NC. Blue circles are Brood II, brown squares are Brood VI, red circles are Brood IX, blue squares are Brood X, and green circles are Brood XIV. Open symbols are negative (absence) records.
  • Elkin, NC (map above) is an interesting periodical cicada hotspot.  Brood VIBrood IXBrood X, and Brood XIV all have populations in the area.  An outstanding question is the extent to which these broods are in contact or overlap.

What about stragglers?

2020 is four years before the emergence of Brood XIX and of Brood XIII in 2024.  Both 13- and 17-year cicadas can emerge early or late, and both have a tendency to do so in increments of 1 and 4 years (Cooley et al. 2018; Marshall et al. 2017).

Brood XIII is located in northern Illinois.  Our group traveled to Chicago in November 2019 to dig up nymphs, and we found many with red eyes, a sign that they would emerge the following spring.  So we expect many stragglers this year across the range of Brood XIII.  This brood has had a number of notable straggling events;  Dybas (1969) reported choruses of cicadas 4-years early in Bemis Preserve, and we (Cooley et al. 2016) noted light choruses of cicadas in Hinsdale, IL along Ogden Avenue (across the tollway from Bemis Preserve) in 2003, four years before the 2007 emergence of Brood XIII. Because Brood XIII is well-separated from Brood IX, Brood XIII stragglers are unlikely to be confused with Brood IX emergences.

An overriding philosophy of the mapping project is: A Misleading Map is Worse Than No Map At All.  There are a lot of Brood XIX stragglers out, and if they are mistaken for Brood IX emergences, these mistaken records will create the impression that Brood IX has grown suddenly and substantially larger, or that Brood IX broadly overlaps Brood XIX, when no such thing is true.  When we collect mapping records, we keep track of densities– in all likelihood, true Brood IX emergences will be dense and widespread, while Brood XIX  straggler emergences will tend to be more patchy and scattered (More details about how we take records are what we use for mapping criteria are found here.).  Furthermore, Brood IX contains M. septendecim, while Brood XIX contains M. tredecim, and these two species are relatively easy to tell apart by song pitch and by general appearance.

Two outstanding questions:

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Cooley, J. R. 2015. The distribution of periodical cicada (Magicicada) Brood I in 2012, with new, previously unreported populations (Hemiptera: Cicadidae). The American Entomologist 61: 52-57.

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.