In 2024, 13-year Brood XIX, which is the largest of all periodical cicada broods, co-emerged with 17-year Brood XIII; these two broods are adjacent (but not significantly overlapping) in north-central Illinois.
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The most recent scientific review of periodical cicada biology (Simon et al. 2022) may be found here.
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Generally, a 13-year brood emerges in the same year as a 17-year brood roughly every 5-6 years, though most of the 17-year broods are not in contact with a 13-year brood, so the different cicadas are clearly separated in space. A co-emergence involving adjacent broods of different life cycles is something that happens only roughly every 25 years. Any two specific broods of different life cycles co-emerge only every 221 years.
2024 was a special year for periodical cicadas:
- For the first time since 2015 a 13-year brood emerged in the same year as a 17-year brood.
- For the first time since 1998 adjacent 13-and 17-year broods emerged in the same year.
- For the first time since 1803 Brood XIX and XIII emerged in the same year.
- It was possible to see all seven named periodical cicada species as adults in the same year, which will not happen again until 2037. All seven named species will not emerge in the state of Illinois again until 2041.
For more information, see the table of upcoming co-emergences at the bottom of this page.
What happened in 2024…
Broods XIII and XIX emerged in their respective ranges. Because 2024 is one year in advance of the Brood XIV emergence in 2025 and four years in advance of the Brood XXIII emergence in 2027, off-cycle cicadas also emerged in the ranges of these broods. See our straggler cicada page for more information about off-cycle cicadas.
What did not happen in 2024…
13-year Brood XIX and 17-year Brood XIII do not overlap to any significant extent. They may co-occur in patches of woods, but these patches will be small in size.
Stannard (1975) published a map of all Illinois periodical cicada broods (reproduced at left). We discuss Stannard’s delimitation of individual broods on the relevant pages of this site; note, however, that no significant overlap is shown between Broods XIII and XIX.
Because the broods do not overlap to any significant extent, the 2024 co-emergence of Brood XIII and XIX did not involve areas of extreme or “double” densities outside the norm of what happens in any other periodical cicada emergence year.
The greatest likelihood of contact between these broods was around Springfield IL; however, note that our map (below) shows Brood XIII in the northwest quadrant of the city and Brood XIX in the southeast quadrant; while there may be scattered woods in which both broods are present, there will be no broad overlap– and it was impossible, in 2024, to identify any areas of overlap, large or small, because the cicada species present in each brood in any area of potential overlap are distinguishable primarily on the basis of life cycle.
What would happen if 13- and 17- year cicadas did interbreed? See discussion below.
In the map below, positive (presence) records of Brood XIII are represented by upwards facing cicadas, and negative (absence) records of the brood are represented by blue crosses. Positive (presence) records of Brood XIX are represented by downwards facing cicadas, and negative (absence) records of the brood are represented by red crosses. Zoom into Illinois to explore the boundary between Brood XIII and XIX and hover over records to see details. Only these two broods are shown; all others have been removed for clarity. Note that the dates on this map do not represent dates of adult emergence; rather, they represent dates on which choruses were active. Thus, in any given area, adult emergence may have occurred a week or more earlier than the dates shown on this map. This map may not be reproduced without written permission. See this note about the characteristics and limitations of the maps on this site.
What was this rare co-emergence like?
As a thought experiment, imagine a drive down I-57, starting in Chicago in June 2024. We start our journey southward on the Dan Ryan Expressway. Right in the city, we might encounter patches of cicadas when older tree canopies are present, but it’s so urban that we’re unlikely to hit large populations of periodical cicadas until we get to the Calumet River; even then, they’ll still be sparse and patchy until we get to the intersection of I-57 and I-80. Continuing on, periodical cicadas won’t be reliably present until we get south of Monee (roughly where Dybas’ Raccoon Grove census spot is-the source of the “several million per acre” figure often quoted in the press). But the cicadas will still be patchy because the woods are patchy. Once we get close to Kankakee, the cicadas will be dense in the relatively thick riparian woodlands. Driving south of Kankakee, the woods will thin out, the land will rise a little bit, and you won’t be aware that you just drove out of Brood XIII. You’ll pass through some very sparsely wooded areas, and then right after you cross into Iroquois County, you’ll hit cicadas again, but you won’t realize you’re now in Brood XIX, because the cicadas will look and sound the same. About where you cross US 50, you’ll pick up the species M. tredecim, and the M. neotredecim will exhibit displaced song pitches. As you continue south, you’ll be in XIX right down to Lake of Egypt, where you’ll cross into Brood XXIII, which has been close to the interstate ever since you passed Rend Lake. If you head south on I-24 instead of I-57, you’ll stay in Brood XIX almost to the Ohio River. Nowhere along the way will you encounter double- densities or anything at all to tip you off that you are in a co-emergence year or that you just witnessed something unsual. Plenty of times along the way, you’ll encounter gaps or treeless areas without cicadas… But nothing obvious will indicate to you that that one gap south of Kankakee was special, because that’s where the broods swapped over.
“Subtle” is not a word usually associated with periodical cicadas– but the specialness of the 2024 co-emergence was subtle.
How many cicadas were there?
Although there are a lot of periodical cicadas, they are not generally considered to be insect pests and there is no reason to use pesticides on them. Instead, see our FAQ for other means of management.
It turns out that it is extremely hard to estimate the population sizes of periodical cicadas, for any number of reasons. The oft-quoted figure of densities that can exceed a million per acre comes from a census taken during the 1956 emergence of Brood XIII in Raccoon Grove, IL (Dybas and Davis 1962).
If we accept a rough estimate of one million cicadas per acre… should that be surprising? How many ants are there per acre? How many mayflies? How many fruit flies? Insects often come in large numbers. What’s special about Magicicada is not the large numbers per se, but the periodicity– the predictable, synchronous emergence of large numbers of adults and their near-total absence in the years between (see the “straggler” page for more information).
The state of Delaware is roughly 1.5 million acres in size. If we accept an estimate of a million cicadas per acre and if the total combined area of a periodical cicada emergence is roughly the size of Delaware, then more than a trillion cicadas will be involved. For 2024, since cicadas emerged from Maryland to Oklahoma, Illinois to Alabama, clearly, trillions of adult cicadas were present– but not all in the same place at the same time.
Are cicadas affected by climate change?
Yes… but they always have been. It’s complicated…
What would happen if 13- and 17-year cicadas interbred*?
Lloyd, Kritsky, and Simon (1983) proposed a single locus Mendelian model to explain periodical cicada life cycles, with an allele for 17 year cycles dominant to an allele for 13-year cycles**. Sota (2022) proposed a more complex polygenic model in which life cycle length is governed by variations in growth rates and the action of developmental “gates” that allow individuals to move on to the next developmental stage only if they meet or exceed certain size criteria. Regardless of which model better represents Magicicada life cycle architecture, both predict that the offspring of hybridization events between co-emerging 13- and 17- year broods will have life cycles of 13 or 17 years rather than cycles of intermediate length. Thus, following a co-emergence, if hybrid offspring were produced, then they would emerge alongside the next generation of one or both of their parental broods and be indistinguishable from cicadas that were not hybrids. What would happen in generations after that? Stay tuned for forthcoming publications…
*Hybrid offspring have been produced experimentally between species belonging to different species groups (White 1973; Fontaine et al. 2007) and between species differing in life cycle (Cooley et al. 2006).
**But see Marshall (2001).
Why weren’t there areas of “double densities” during the 2024 co-emergence?
First, as noted, Brood XIII and XIX do not overlap to any significant extent. Second, there is no immediate reason to expect that any woods in which the broods did overlap could support more cicadas than any other woods. Competition for resources (e.g., food, space, or ovipisition sites) is expected to impose an upper limit on cicada densities.
As with most things related to periodical cicadas, the story is a little more complicated. Simon et al. (1981) studied Broods I and XIV on Long Island, which have the same life cycle (17) and are always separated by a temporal gap of no less than four years (Brood XIV emerges, and then Brood I emerges four years later, and the pattern repeats 13 years later). Simon et al. (1981) found that these broods overlapped slightly and that woods with both broods supported greater densities of cicadas than woods with only Brood XIV. There is a reasonable argument to be made that greater densities are possible in this circumstance, since these broods are consistently separated by temporal gaps of four (or more) years; this temporal gap ensures that whatever aboveground resources adults of each brood might compete for can replenish and that the nymphs of each brood are at different enough states of development that competition for resources belowground is reduced.
But a 13 and 17 year brood do not have a consistent temporal separation; in some years (such as 2024), the broods co-emerge, suggesting maximal competition, while in other parts of the 221-year cycle, the two broods are as separated in time as possible, suggeting minimal competition. Thus, there is no reason to expect that the increased densities noted by Simon et al. (1981) for overlapping broods of the same cycle would also be expected where broods of different cycles overlap.
Table of upcoming co-emergences (highlighted rows show adjacent broods):
| 13-year brood | 17-year brood | Year |
|---|---|---|
| XIX | XIII | 2024 |
| XIX | IX | 2037 |
| XXIII | XIII | 2041 |
| XIX | V | 2050 |
| XXII | VIII | 2053 |
| XXIII | IX | 2054 |
| XIX | I | 2063 |
| XXII | IV | 2066 |
| XXIII | V | 2067 |
| XIX | XIV | 2076 |