Periodical cicadas spend 13 or 17 years underground and then emerge together in numbers that can exceed a million per hectare. The synchrony works by giving predators more than they can possibly eat; the famous explanation for the prime numbers is a hypothesis, not a finding.
A periodical cicada spends almost its entire life underground as a nymph, drinking dilute sap from tree roots, and then — after exactly 13 or exactly 17 years — emerges within a few weeks of every other cicada in its brood. The densities are hard to convey: over a million animals per hectare in good sites, enough that the sound is measured in the same range as a chainsaw and the dead bodies noticeably fertilise the soil afterwards. The strategy underneath is called predator satiation and it is well supported. Nothing that eats cicadas can maintain a population between emergences, because there is nothing to eat for over a decade; so when the emergence comes, every predator in the forest eats until it cannot eat more, and the fixed number they take is trivial against the number that emerged. Being common is the defence. Where the story overreaches is the primality. The claim that 13 and 17 were selected *because* they are prime — minimising overlap with shorter predator or parasite cycles — is a genuinely elegant hypothesis that has proved close to untestable on an organism that reproduces every 17 years, and it circulates with far more confidence than the evidence carries. What actually counts the years underground is also unknown.
Developed record · 86% complete · reviewed 2026-08-11
What this page covers
Seven Magicicada species in eastern North America, on 13- and 17-year cycles. Annual cicadas elsewhere in the family are a separate matter and appear every year.
Often confused with: Annual cicadas, which have multi-year lifecycles that are not synchronised; Locusts, which are grasshoppers and unrelated
An adult periodical cicada lives about a month. The nymph that preceded it lived thirteen or seventeen years, a few centimetres to a metre down, with its mouthparts in a tree root drinking xylem — the dilute, nutrient-poor sap moving up from the roots, which is such thin fare that slow growth is less a strategy than a consequence.
Eggs are laid in slits cut into thin branches; the nymphs drop to the ground and burrow.
Five nymphal stages underground, feeding on xylem the whole time.
Emergence when the soil at about 20 cm reaches roughly 18°C — after the right number of years, not instead of it.
Nymphs climb, moult a final time, and the adults live four to six weeks.
Males sing in aggregations, females respond with a wing-flick, and both die shortly after breeding.
The two-part cue is the neat bit. Soil temperature decides the night; the year is decided by something else entirely, and what that something is remains unknown. Nymphs appear to track annual cycles — plausibly through the seasonal rhythm of the sap they drink — but no mechanism has been demonstrated. Trees experimentally forced to flower twice in a year have produced cicadas emerging a year early, which is a strong hint and not a demonstration.
Stragglers emerge four years early or late with some regularity, and four is not incidental: it is the difference between 13 and 17. That is one of the better pieces of evidence that the two cycles are evolutionarily related rather than independent.
Words used here
Brood
All the periodical cicadas emerging in a given year across a mapped region. Numbered with Roman numerals.
Xylem
The tissue carrying water and dissolved minerals up from the roots. Very dilute food, which is why growth takes so long.
Emerging all at once works by giving predators more than they can eat
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
Periodical cicadas of the genus Magicicada spend 13 or 17 years underground feeding on root xylem, then emerge in synchronised broods at densities that can exceed a million individuals per hectare. Predators are generalists whose populations cannot track a food source absent for over a decade, so a fixed number are consumed and the remainder breed regardless of predation pressure. Emergence within a brood is cued by soil temperature once the requisite number of years has passed.
Who this applies to
the North American periodical cicadas
Studied in
Magicicada
You may have heard
“Cicadas emerge on prime-numbered years to avoid predators”
Two claims travel together and only one is established. That mass synchrony works by swamping predators is well supported and quantitative. That the *primality* of 13 and 17 is the reason — avoiding overlap with shorter predator cycles — is a plausible hypothesis about events on a 17-year timescale that nobody has been able to test in a way that could fail. It is repeated as the finding, and it is the speculation attached to it.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
Emergence densities and predator responses have been measured across multiple broods and decades, and satiation makes a quantitative prediction that survives it.
How far it can be extended
Documented across Magicicada species and broods. Other long-cycle cicadas elsewhere are barely studied.
Caveats
Predator satiation explains the synchrony, not the specific cycle lengths.
Stragglers emerging years early or late complicate the picture of discrete broods.
Almost all evidence is North American.
Still unanswered
Why 13 and 17 specifically, rather than any other long interval?
Local cicada density manipulated across field plots, with survival to reproduction measured — the direct test of the satiation prediction that individual survival should rise with density.
A cicada nymph counts the tree’s years, not its own
Emerging evidence
Real findings exist, but too few or too recent to be settled.
Periodical cicada nymphs transplanted onto trees experimentally forced to flower twice within one calendar year emerged a year early, indicating that the underground count is driven by an annual signal in the host plant rather than by an internal clock running independently of the environment. Emergence in the correct year is then converted into an emergence night by soil temperature.
Who this applies to
North American periodical cicadas
Studied in
Magicicada
You may have heard
“Cicadas have a 17-year internal clock”
They have a counter, and it appears to be counting something outside themselves. Nymphs on trees tricked into flowering twice in one year came out a year early — so the clock is the tree’s, read through the sap, and the insect is keeping a tally rather than running a timer. That is a cheaper mechanism than an internal seventeen-year clock, and a more fragile one.
Why we rate it this way, and what the caveats are
Emerging evidenceModerate confidence
A direct manipulation with a clean predicted result, and a single small experiment on an organism that reproduces once every seventeen years. It is unlikely to be replicated soon, which is a reason for caution rather than for discounting it.
How far it can be extended
Demonstrated in a single manipulation on Magicicada; the mechanism is expected across the genus and has not been retested, the experiment being close to unrepeatable on this timescale.
Caveats
Shows that host cycling can shift emergence, not that it is the sole input.
Small numbers, one manipulation, no replication.
Explains how the count is kept, and not why the count is 13 or 17.
Still unanswered
What in the sap changes annually, and how is it accumulated over seventeen cycles?
Why do stragglers appear specifically four years off — the difference between the two cycles?
The double-flowering manipulation and the resulting year-early emergence.
How we know
Tricking a tree into two years, and watching the cicadas believe it
A cicada nymph emerges after exactly seventeen years underground. Is it running an internal clock, or counting something outside itself?
The obstacle is obvious: the organism reproduces once every seventeen years, so the ordinary approach of manipulating and waiting is close to impossible. The design gets around it by manipulating the environment rather than the animal. Nymphs were transplanted onto peach trees whose flowering was forced so that the tree completed two flowering cycles within one calendar year. If the nymph runs its own clock, that changes nothing. If it counts annual cycles in the sap it drinks, the tree has just given it two years in one, and it should emerge a year early.
What happened
Nymphs on the double-flowering trees emerged a year ahead of those on normal trees.
What it shows
That the count is environmental rather than endogenous: the nymph tracks an annual signal in the host plant and tallies it, instead of running a seventeen-year timer of its own. That is a much cheaper mechanism than an internal clock of that duration, and it makes a testable prediction about what should disrupt it.
What it does not show
It shows the host cycle *can* shift emergence, not that it is the only input, and it does not identify what in the sap changes annually. It says nothing whatever about why the count is seventeen — the harder and more famous question, which this leaves exactly where it was. Small numbers, one manipulation, and an experiment nobody has been in a position to repeat.
The controls — what makes this evidence rather than a story
Nymphs on unmanipulated trees of the same species, so the comparison is the tree’s schedule and not the transplanting.
A prediction made in advance with a specific direction and size — one year early, not merely "different" — which is what makes the result interpretable.
Host trees rather than the cicadas themselves manipulated, avoiding any physiological intervention in the animal being measured.
Testing whether safety really does come in numbers
Are periodical cicadas safer simply because there are so many of them?
Predator satiation is an appealing explanation and an easy one to assert. It makes a sharp prediction, though: if the strategy is to overwhelm predators rather than to evade them, then an individual cicada should do *better* the more competitors it has around it — the opposite of what usually happens as density rises. Cicada densities were manipulated across field plots during an emergence, some plots stocked far above natural density and others below, and survival to reproduction measured in each.
What happened
Individual survival rose sharply with local density. At high densities, predators took a smaller proportion of the emergence and per-capita reproductive success went up. At low densities, cicadas were heavily eaten.
What it shows
That the strategy works exactly as the satiation hypothesis predicts, and it explains a set of traits that otherwise look like a catalogue of design errors. Periodical cicadas are slow, edible, defenceless and enormously conspicuous — because none of that matters if the local predators are physically unable to eat a meaningful fraction of you.
What it does not show
It explains synchrony without explaining the particular intervals: satiation gives no reason for thirteen and seventeen specifically, and the prime-number arguments are separate and much weaker. Field plots also cannot fully control predators moving between them, which if anything makes the measured effect an underestimate.
The controls — what makes this evidence rather than a story
Density varied experimentally rather than compared between naturally different sites, which would confound density with habitat.
Plots at natural density as a reference point.
Survival to reproduction measured rather than survival alone, since the strategy is about breeding not living.
Plots distributed so that local predator communities were broadly comparable.
Periodical cicadas make no attempt to escape. They are slow, loud, conspicuous, harmless and easy to catch, and birds, mammals, reptiles, fish and domestic pets all gorge on them. That is not a failure of defence — it is the defence, executed at the level of the population rather than the individual.
The reason it works is that no predator can prepare. A specialist that ate periodical cicadas would starve for sixteen years, so the predators are generalists whose numbers are set by their ordinary food supply. Their appetite is therefore fixed and finite when the emergence arrives. Emerge in numbers that exceed it by an order of magnitude and the survivors are not the lucky ones — they are simply the ones there was no room left to eat.
The prediction that follows is testable and has been tested: cicadas emerging outside the main window, or at low density at the edge of a brood, are eaten at far higher rates and often disappear entirely. Being early is fatal in a way that has nothing to do with being early and everything to do with being alone.
The effect ripples outward. Bird populations that feed on the emergence rise in the following year, which measurably reduces predation on other insects, and the decomposing cicada bodies deliver a nitrogen pulse that shows up in forest plant growth.
Words used here
Predator satiation
Surviving by appearing in numbers greater than predators can consume. Only works if it happens all at once.
Both cycle lengths are prime, and the coincidence is too neat to ignore. The standard explanation is that a prime cycle minimises how often the cicadas coincide with predators or parasites on shorter cycles: a 17-year cicada meets a hypothetical 5-year predator every 85 years, where a 16-year cicada would meet it every 80 and a 15-year one every 15. It is a lovely argument and it appears in a great many places as the reason.
The difficulty is that it has never been tested in a way that could have failed. There is no known predator or parasite on a multi-year cycle that periodical cicadas would need to avoid. The hypothesis was constructed to explain the numbers rather than derived from an observed threat, and the organism reproduces once every seventeen years, which places experimental work beyond reach.
A second explanation has better support and less charm: primality may minimise interbreeding between broods on different cycles. Two populations on 13 and 17 years coincide only every 221 years, so hybrids — which would emerge on some intermediate schedule and lose the protection of the crowd — are rare. This at least concerns something known to exist, since the 13- and 17-year forms are closely related and do occasionally overlap.
NatureHQ’s position is that the synchrony is explained and the primality is not. That distinction is worth keeping, because the prime-number story is repeated with the confidence of the satiation finding and rests on nothing like the same evidence.
Almost everything remarkable about cicadas is true of a tiny minority of them.
Almost everything famous about cicadas is true of seven species out of about three thousand
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
The Cicadidae comprise roughly 3,000 described species worldwide. Periodical cicadas — synchronised thirteen- and seventeen-year emergences in geographically defined broods — comprise seven species of the single genus Magicicada, restricted to eastern North America. The remainder are annual or protoperiodical: individuals take several years to develop but generations overlap, so adults appear every year and emergence is not synchronised. Cicadas are not locusts, which are grasshoppers, and the two are not closely related.
Who this applies to
cicadas worldwide, contrasted with the periodical genus
Studied in
Cicadidae, Magicicada
You may have heard
“Cicadas come out every seventeen years”
Seven species do, in one genus, in eastern North America. There are around three thousand cicada species, and the rest appear every summer — their generations overlap, so there is no synchronised emergence to notice. The whole spectacular reputation of the family belongs to a tiny and geographically restricted minority. They are also not locusts, which are grasshoppers and not related; the name stuck from early settlers reaching for a biblical comparison.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
Basic taxonomy and distribution, uncontroversial except for species boundaries, which affect the count rather than the pattern.
How far it can be extended
Species counts and distributions are catalogued taxonomically; the restriction of periodicity to Magicicada is not in dispute.
Caveats
Described species counts rise steadily and the figure is approximate.
Some non-Magicicada species are protoperiodical, showing partial synchrony, so the division is not perfectly clean.
Williams and Simon, 1995 · Annual Review of Entomology
Defines the periodical genus and its restriction, and contrasts it with the non-periodical majority.
There are roughly three thousand described cicada species worldwide. Seven of them — all in the single genus Magicicada, all in eastern North America — are the periodical cicadas, with the thirteen- and seventeen-year synchronised broods. Everything else in the family is annual or protoperiodical: individuals still take several years to develop, but the generations overlap, so adults appear every summer and there is no synchronised emergence to notice.
That is worth stating plainly because the reputation of the whole family has been built from the exceptional minority. A cicada singing in an Australian summer, or a Mediterranean one, is not on a seventeen-year clock and never was. It is doing the ordinary cicada thing, which is to spend a few years underground on tree-root sap and then a few weeks above it being extremely loud.
Seventeen years underground is how long it takes to grow on xylem sap
Well supported
Good evidence backs this, though some details remain open.
Periodical cicada nymphs feed on xylem fluid from woody tree roots — the dilute upward stream of water and dissolved minerals — rather than on the sugar-rich phloem sap most sap-feeding insects exploit. Xylem fluid is extremely nutrient-poor, and growth on it is correspondingly slow, with nymphs passing through successive instars and moving between feeding sites over the underground period.
Who this applies to
periodical cicadas, with xylem feeding general among cicadas
Studied in
Magicicada, Cicadidae
You may have heard
“Cicadas hibernate underground for seventeen years”
They are not dormant for a moment of it. They are feeding, moulting through successive stages and moving between roots — the whole time. What takes seventeen years is growth, because the food is xylem fluid, which is essentially water with traces of minerals in it and the worst thing available to eat in a tree. The animal is not waiting out the interval; it is spending it, slowly, building an insect out of nearly nothing.
Why we rate it this way, and what the caveats are
Well supportedModerate confidence
The feeding site and tissue are directly established by excavation and examination. That the diet is the principal constraint on development time is a strong and widely accepted inference rather than a direct measurement, since ingestion rate underground cannot be measured.
How far it can be extended
Xylem feeding is characteristic of cicadas as a family; the extreme development time is specific to the periodical species.
Caveats
Excavation disturbs the feeding relationship being observed.
Ingestion rate underground cannot be measured directly, so the causal link to development time is inferential.
Annual cicadas also feed on xylem and develop in far less time, so diet alone does not determine the interval.
Still unanswered
If xylem feeding is general to cicadas, why do only a few lineages take over a decade?
Williams and Simon, 1995 · Annual Review of Entomology
The underground developmental sequence and its duration across instars.
The diet also supplies the piece most accounts of the seventeen years leave out. Predator satiation explains why emergences are synchronised; it does not explain why they are so far apart. Cicada nymphs feed on xylem fluid — the dilute upward stream of water and dissolved minerals, which is essentially the worst thing available to eat in a tree. Excavating nymphs and examining their feeding sites confirmed it is xylem rather than the sugar-rich phloem most sap-feeding insects target.
An insect growing on that grows very slowly. So the animal is not waiting out seventeen years underground: it is spending them, building an insect out of nearly nothing. That reframes the whole life cycle from an act of patience into an act of arithmetic.
Words used here
Protoperiodical
Showing partial emergence synchrony without a fixed multi-year cycle. Several non-Magicicada cicadas are like this, which blurs the neat annual/periodical division.
Phloem
The tissue carrying sugars around a plant. Far richer than xylem, and what most sap-feeding insects target — cicadas do not.
A cicada is mostly a loudspeaker, and it does not rub anything together
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
Cicada sound is produced by the tymbal, a ribbed cuticular membrane on each side of the abdomen that buckles inward rib by rib under muscle contraction and springs out on release, each buckling producing an acoustic pulse. The largely hollow abdomen functions as a resonator tuned to the pulse frequency and radiates the sound efficiently, producing among the highest sound outputs of any insect for its size. Hearing is sharply tuned to the species' own song band, and males reduce the sensitivity of their own auditory organs while singing.
Who this applies to
cicadas; tymbal mechanics vary in detail across the family
Studied in
Cicadidae, Cyclochila australasiae, Magicicada
You may have heard
“Cicadas make their noise by rubbing their legs or wings together”
That is crickets and grasshoppers, and cicadas do nothing of the sort. A cicada has a ribbed membrane on each side of the abdomen that snaps inward like the lid of a jam jar, hundreds of times a second, inside a body that is largely hollow and acts as a resonator. It is why so small an animal is so extraordinarily loud, and why the abdomen is mostly empty. The animal is close to being a loudspeaker with an insect attached — including a volume control on its own ears, which it turns down while singing.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
Simultaneous recording of tymbal movement and acoustic output with experimental manipulation of the resonator, complemented by independent electrophysiological measurement of hearing.
How far it can be extended
The tymbal is a defining feature of cicadas, with mechanics measured directly in several species and the auditory tuning measured separately in periodical cicadas.
Caveats
Tymbal mechanics were measured in a large Australian species chosen for convenient size; details vary across the family.
Manipulating the resonator necessarily alters the structure being measured.
Older electrophysiology, so absolute hearing thresholds should be read with caution.
Still unanswered
How do several cicada species chorusing in the same wood avoid masking one another beyond simple frequency separation?
Auditory tuning matched to conspecific song, and the reduction of a male's own hearing sensitivity during singing.
Almost every account describes cicada noise as the insect rubbing something together. That is crickets and grasshoppers, and cicadas do nothing of the sort. A cicada carries a tymbal on each side of the abdomen — a ribbed sheet of cuticle that a muscle pulls inward until it buckles, rib by rib, each buckling producing a click, and which springs out again on release. Several hundred times a second.
The reason so small an animal is so extraordinarily loud is the rest of the body. The abdomen is largely hollow and acts as a resonator tuned to the pulse frequency, radiating the sound efficiently outward. A singing cicada is close to being a loudspeaker with an insect attached, and the hollow abdomen — which looks like an animal missing its insides — is the amplifier.
Cicada hearing is sharply tuned to the species' own song band, and males reduce the sensitivity of their own hearing organs while singing. An animal producing that much sound a few millimetres from its own ears has a problem, and it solves it with a volume control.
The narrow tuning does a second job. A chorus of one species is close to inaudible to another, which is part of how several cicada species can sing over each other in the same wood without any of them losing their own signal in the noise.
Words used here
Tymbal
A ribbed membrane on each side of the abdomen that buckles inward under muscle contraction to produce sound. Unique to cicadas and a few relatives.
Stridulation
Producing sound by rubbing two body parts together. What crickets and grasshoppers do, and what cicadas emphatically do not.
The observation is remarkable. The mechanism everybody reports has never been shown.
The fungus really does replace the abdomen and change the signalling — how is unknown
Contested
Researchers actively disagree, and the disagreement is substantive.
Massospora infection of periodical cicadas replaces the posterior abdomen with a mass of fungal spores while the animal remains active. Infected males have been observed producing the female-typical timed wing-flick response to the calls of other males, in addition to calling; healthy males approach and attempt to mate with them, transferring spores. Infected animals of both sexes continue attempting to mate despite the loss of the genitalia. Cathinone has been detected in cicadas infected with Massospora cicadina and psilocybin in a different Massospora species. No study has established that these compounds cause the behavioural changes, at what concentration they reach cicada tissue, or that the fungus acts on the nervous system.
Who this applies to
Massospora infections of periodical and some annual cicadas
“A fungus drugs cicadas and turns them into zombies under its control”
Several separate things get welded into one sentence, and only some are established. The abdomen really is replaced by a plug of spores while the animal flies around, which is stranger than the framing. Infected males really do give the female signal, drawing in other males and spreading spores to both sexes. An amphetamine really was detected in one species and psilocybin in another. What has never been shown is any link between those last two facts: no concentration at a site of action, no dose–response, no demonstrated effect on insect behaviour, nothing indicating the fungus acts on the nervous system. "Mind control" names a mechanism nobody has evidence for.
Why we rate it this way, and what the caveats are
ContestedModerate confidence
The physical replacement of the abdomen and the continued mating behaviour are directly and unambiguously observed. Whether the fungus manipulates behaviour, and by what means, is unresolved: the alkaloid finding is suggestive and every causal step between detection and behaviour is missing.
Where researchers disagree
The behavioural change may be adaptive manipulation by the fungus, or a by-product of extensive physical destruction of the abdomen and its associated signalling apparatus. No study distinguishes these.
The alkaloids were detected but their concentration at any site of action is unknown, no dose–response relationship has been established, and insect nervous systems differ from vertebrate ones in the receptors these compounds are known to act on.
The biosynthetic route in Massospora was not identified, so whether the fungus makes these compounds or acquires them is unresolved.
Still unanswered
Does Massospora act on the cicada nervous system at all, or is the behavioural change a consequence of tissue destruction?
Do the detected alkaloids reach concentrations at which they affect insect behaviour?
Field observation of infected males producing the female wing-flick signal and of healthy males responding, with spore transfer during mating attempts.
Detection of cathinone and psilocybin in infected cicadas, proposed as contributing to behavioural change but not shown to do so.
Massospora is a fungus that infects cicadas, and what it does is not in doubt. The posterior third of the animal — abdomen, genitalia and all — is destroyed and replaced by a chalky plug of spores, while the cicada continues to fly, walk and attempt to mate. Anything that touches it, or that it lands on, gets spores.
The behavioural finding is stranger still, and it is measurable because periodical cicada courtship is a precisely timed exchange rather than a vague display: a male calls, and a receptive female answers with a wing flick at a specific interval. Infected males were observed producing that female response to the calls of other males — while also calling themselves. Healthy males approached and attempted to mate with them. A single infected male therefore reaches both sexes.
Then chemistry arrived and the story escaped. Analysis of infected animals detected cathinone — an amphetamine — in cicadas infected with the periodical-cicada species of Massospora, and psilocybin in a different Massospora infecting annual cicadas. Neither compound had been reported from these lineages, and in the psilocybin case the fungus is unrelated to the mushrooms that normally make it. That is a genuinely surprising chemical result.
Established: the abdomen is replaced by a spore mass while the animal remains active.
Established: infected males produce the female signal, and healthy males respond.
Established: cathinone and psilocybin are detectable in infected cicadas.
Not established: that those compounds reach a concentration at which they affect anything.
Not established: any dose–response relationship, or any effect on insect behaviour.
Not established: that the fungus acts on the nervous system at all.
The gap between the third line and the fourth is where "zombie fungus drugs cicadas" comes from, and nothing bridges it. Nobody has measured how much of either compound reaches cicada tissue, nobody has shown that these compounds change insect behaviour at those concentrations, and insect nervous systems differ from vertebrate ones in the receptors these compounds are known to act on. The behavioural change may equally be a consequence of the extensive physical destruction of the signalling apparatus. Both are live hypotheses; neither has been tested against the other.
NatureHQ will therefore describe what the fungus does in full — which is remarkable enough that it needs no help — and decline the word "zombie". Manipulation is a reasonable hypothesis that fits the observations. It is not the same as having been demonstrated, and the comparison people reach for, the ant-controlling Ophiocordyceps, has decades of work behind its mechanism that this does not.
Words used here
Active host transmission
A parasite spreading through the host's own continued activity rather than by killing it and waiting. What Massospora does.
Cathinone
An amphetamine, normally associated with the khat plant. Detected in Massospora-infected periodical cicadas; its effect on cicadas is unknown.
A large emergence is genuinely loud — chorusing aggregations reach levels comparable to power tools, sustained through daylight hours for weeks. Beyond that, the disruption is mostly aesthetic. Periodical cicadas do not bite, do not sting, carry no disease, and do not eat leaves: the adults barely feed at all.
The one real damage is mechanical and specific. Females cut slits in thin branches to lay eggs, which can kill the branch tip — noticeable on young or newly planted trees, and of no consequence to a mature one. Netting young trees during an emergence is the entire mitigation; insecticide is both ineffective at these densities and destructive to everything else.
Calling them locusts is a historical error worth correcting, because it carries an implication of crop destruction that is simply false. European settlers encountering a mass emergence reached for the biblical plague; a locust is a grasshopper, chews leaves, and is a different order of insect entirely.
Why it matters: A nymph underground must track thirteen or seventeen annual cycles with enough accuracy to emerge within weeks of its siblings. Nothing is known about how it does this.
What would settle it: Identifying the seasonal signal in xylem sap that nymphs respond to, and showing that manipulating it shifts emergence.
Why prime numbers?
Why it matters: The standard explanation invokes predator cycles that are not known to exist. The brood-hybridisation explanation is better grounded and less famous, and neither has been tested.
How did 13- and 17-year cycles arise from each other?
Why it matters: The four-year straggler pattern suggests the two are related by a switch rather than independently evolved, and the genetics of that switch is unresolved.
What will warming do to a 17-year clock?
Why it matters: Emergence is cued by soil temperature after a fixed count of years. Warming shifts the trigger without shifting the count, and the consequences for synchrony are unexamined.