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A Time article about the upcoming emergence of Brood X cicadas in the U.S. has the following quote from a biologist:

“It’s not something you can ignore,” says John Cooley, a biologist at the University of Connecticut. “When they come out it’ll be millions per acre.”

I found another article about the Brood X cicadas with this quote from a professor:

“Oh, it’s deafening,” said Matt Kasson, an associate professor of plant pathology and mycology at West Virginia University. [...] “You'll get tens of millions emerging per acre,” Kasson said.

Cicadas are 1-2 inches long, so let's say the average cicada is 1.5 inches long. Based on photos, their width is about one-third of their length, so let's say the average cicada is 0.5 inches wide. That means the average cicada would take up an area of about 0.75 square inches.

Since one acre is equivalent to 6,272,640 square inches, about 8,363,520 cicadas could fit in one acre if they were all side by side next to each other. So if there were millions of cicadas per acre, the ground would practically be covered with them if they were all sitting on the ground. If there were tens of millions of cicadas per acre, there would be multiple layers of them completely covering the ground.

Of course, when the cicadas emerge, they won't all be sitting on the ground - they'll spend a lot of time at various heights in trees. But still, I am skeptical of the claims that there will be millions or tens of millions of them per acre. Is that really true?

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    The emergence holes are about 1 cm in diameter. At this early stage of the emergence, there are parts of my yard where there are more than one per square inch, so anecdotally this seems plausible.
    – antlersoft
    Commented Apr 28, 2021 at 14:03
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    I don't think it's clear these professors meant "simultaneously," is there any more context that they might mean "millions emerging per acre every day?"
    – Azor Ahai -him-
    Commented Apr 29, 2021 at 16:56
  • Do cicadas emerge at their full adult size?
    – njzk2
    Commented Apr 29, 2021 at 20:38
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    @njzk2: They come out at pretty close to the full adult size, climb a tree, shed their old exoskeleton, shake out their new wings and let their new shell harden, then fly off like drunken toddlers with jetpacks until they randomly collide with something. The 17 year cicadas are on the smaller side relative to the annual cicadas, but the nymph shed exoskeletons you find on trees are still pretty close to an inch long (and they're curled, where the cicadas that emerge lie flatter/longer).
    – ShadowRanger
    Commented Apr 29, 2021 at 23:22
  • Since cicadas don't actually remain on the ground, I think the more relevant back-of-the-envelope calculation would use volume. If a cicada is 1/4 cubic inch in volume, then 10 billion of them could occupy the thirty feet above a single acre. You only need a fraction of a percent of this volume to be usable tree real estate to match these numbers.
    – Kevin Carlson
    Commented Apr 30, 2021 at 15:36

3 Answers 3

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According to Periodical cicada nymphs impose periodical oak tree wood accumulation Nature volume 287, pages 326–327 (1980):

Densities of cicadas underground are very great — Dybas and Davis report emergence densities of over 300 nymphs per square yard or about 1,500,000 per acre.

Where "Dybas and Davis" is A Population Census of Seventeen‐Year Periodical Cicadas (Homoptera: Cicadidae: Magicicada) Ecology 43, 432 (1962).

On the other hand, this article ESTIMATING NYMPHAL POPULATIONS OF 17-YEAR CICADAS IN EASTERN OHIO, 1968 finds in the hundreds of thousands per acre (this is not Brood X though).

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    Correct me if I am wrong, but I think "emergence density" means that 1,500,000 nymphs will emerge in an acre, but that doesn't necessarily mean they will all be present in that acre at the same time? Some might emerge and fly away, followed by others emerging. Hence the OPs skepticism of the cicada "layering". Am I right?
    – DJClayworth
    Commented Apr 27, 2021 at 16:37
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    @DJClayworth It means that for 17 years they have all been present in the acre and emerge almost simultaneously, but sure then can move around after they emerge. Birds will eat them so soon there will be less because of being eaten. But if you have an extensive forest with thousands of acres the number entering the acre should be similar to the number leaving the acre, other than being eaten.
    – DavePhD
    Commented Apr 27, 2021 at 16:45
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    Are these the peak densities at specific sites, or average densities for suitable habitats, or average densities across the whole regions considered? Those are three very different things.
    – PLL
    Commented Apr 28, 2021 at 11:23
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    Importantly, the idea that "millions" would emerge from a single acre, but not all stay present in that acre after emergence and so not all be present at the same moment in a single acre, isn't inconsistent with the quotes provided. This sounds like a game of backwards telephone, where the words stay the same but the meaning gets modified through successive generations. "Tens of millions" sounds like an exaggeration, though, unless there's something specific about these cicadas that makes that a possibility.
    – Jason
    Commented Apr 28, 2021 at 13:00
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    This "disturbed earth kills cicadas" thing is actually pretty noticeable. My mother's neighborhood, which was largely built in the '50s and had minimal rebuilding through the '80s, had tons of cicadas in the 1987 emergence. In the '90s, the older working class residents began leaving and a younger professional class moved in, tearing down the old houses and building McMansions 3x the size in their place. As a result, the 2004 emergence was pretty paltry in her neighborhood; you could drive a few miles to less recently redeveloped neighborhoods and get swarmed, but levels were low in her area.
    – ShadowRanger
    Commented Apr 29, 2021 at 23:30
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"...the ground would practically be covered with them". Yes, this will happen. This cicada segment from the April 27th PBS News Hour at minute 3:42 shows the ground covered with cicadas (from the last time?) Then the property owner says there were so many moving around on the ground that he got sea-sick. Earlier it has a nice map of the Eastern US states where they live.

But, from personal experience they're not going to pop out of every lawn that way, In a semi-urban area it will merely be buggier and noisier than usual.

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    This doesn't answer the question. Will there be millions per acre?
    – Oddthinking ♦
    Commented Apr 28, 2021 at 7:35
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    @Oddthinking Hmm...my thinking was: "will there be millions" was already answered,; "but won't the ground be covered with them" is a specifically asked and unanswered part of the Q; and partial answers are fine (complete answers to only part of the question).
    – Owen Reynolds
    Commented Apr 28, 2021 at 13:21
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    That depends on the acre. Cicada distribution isn't uniform. Also, it's a rate. There are about 6.2 million square inches in an acre. Put 5 cicadas on a standard 3x5 index card, and you have achieved "millions per acre" density. The claim is not that a typical one-acre lot will have millions of cicadas, though there will certainly be some wooded areas that can achieve that. It's not as think as you might think; they won't be covering the ground, but spread vertically throughout the trees.
    – chepner
    Commented Apr 28, 2021 at 14:26
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    Thy won't be popping out of your lawn. Unless maybe your location was wooded 17 years ago, and the tree roots are still under there..
    – GEdgar
    Commented Apr 28, 2021 at 16:13
  • @chepner Watch the clip. The guy lives where cicadas hatch and has seen it. Watch the first part where he finds sleeping cicadas all over this big lot. In the right places, before they learn to fly and get eaten much, the ground will be covered with millions.
    – Owen Reynolds
    Commented Apr 28, 2021 at 16:16
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This is in wooded areas. When they emerge from the ground, they don't stay there. They go up in the trees and make their loud buzzing noises. Here is a picture from the web...

Invertibrate Mathematicians
SOURCE

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    This refutes the OP's back of the envelope calcilations, but doesn't support the actual claim.
    – Oddthinking ♦
    Commented Apr 27, 2021 at 22:08
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    @Oddthinking this does not refute my calculations, since we don't know how tall those trees are, how many trees are in that area, and how many cicadas are sitting in the grass.
    – pacoverflow
    Commented Apr 28, 2021 at 0:53
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    @pacoverflow: You have used a simplistic set of assumptions in your model that the cicadas will be spread evenly over the ground. This undermines that assumption, making your calculations less useful. Meanwhile the title question remains unanswered.
    – Oddthinking ♦
    Commented Apr 28, 2021 at 7:37
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    The OP already mentioned they'd fly to trees and such. The question is will 6 to 8 million emerge per acre like the links say
    – sevensevens
    Commented Apr 28, 2021 at 15:07
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    They emerge from the ground as nymphs. They climb the nearest tree, if possible, and molt into an adult. An adult will avoid the ground if possible. Anecdotal, was camping in the woods when 1970 X brood was emerging.
    – Mattman944
    Commented Apr 28, 2021 at 22:53

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