Waiting for Brood X

Periodical cicadas? Every magazine and newspaper has an article. And they all use the term “invasion” – as if the writers welcome the chance to use this forbidden word so openly. These creatures, the beasts of Brood X, will irritate only the irritable; children will be delighted, they say. They will not even really hurt the trees, though for some of the smaller ones, when the cicadas finally die it will be a bit of a re-leaf. We are therefore looking forward to a fairly painless invasion, an un-plague of proportions not so much biblical as Unitarian: if you want it to be a blessing, it’s a blessing. Fire up the grill: these suckers are good eatin’!

Their strategy for staying ahead of predators partakes heavily of the economies of scale. As an article in The EconomistThe Invasion of the Brood – observes,

Most biologists believe that the odd lifestyle of periodical cicadas is an example of a survival strategy called ‘predator satiation’: the insects emerge in such prodigious quantities that predators cannot possibly eat them all. And their curious prime-numbered lifecycles may be another anti-predator strategy.

Glenn Webb, a mathematician at Vanderbilt University in Nashville, Tennessee, has demonstrated mathematically that prime-numbered lifecycles could help cicadas avoid damaging ‘resonances’ with the two- and three-year population fluctuations of their predators. These would result in lots of predators being around in years when there were lots of prey. Dr Webb’s model shows that, over a 200-year period, average predator populations during hypothetical outbreaks of 14- and 15-year cicadas would be up to 2% higher than during outbreaks of 13- and 17-year cicadas. That may not sound like much, but it is enough to drive natural selection towards a prime-numbered life-cycle.

As with most insects, little is known about the habitat preferences of periodical cicadas. Fortunately, a long-term study is underway:

Dr Clay’s research builds on data that generations of Indiana’s entomologists have been gathering at 17-year intervals for over a century. He estimates, though, that he will need results from at least three more Brood X outbreaks to draw firm conclusions about cicadas’ habitat preferences. Like his forward-looking predecessors, he will have to rely on future generations of entomologists to ensure that his labours bear fruit. Many entomologists in the American mid-west, it seems, are also now on a 17-year cycle.

This is not the stuff of a 2-year master’s thesis – or even a PhD. In fact, I venture to suggest that the “publish or perish” strategy for survival differs quite strongly from “predator satiation,” and is more proper to a higher trophic level.

There is something refreshing about the cicada ethos: unlike the mentality of a horde of locusts, say, or American shoppers intent on mass consumption, the cicadas aspire only to molt, metamorphose, sing, mate and lay eggs. There’s no mistaking the mechanical quality in a male cicada’s noontime trill: this is a sex machine. His real life as a larva burrowing in the dark, sucking sap from tree roots – that’s all over. This is the afterlife; he’s in cicada heaven now. And according to the preliminary findings of Dr. Clay and his colleagues, that heaven looks a whole lot like a sprawling suburban subdivision in the American Middle West.

This post also appears on the PA Wildlands news blog.

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Dave Bonta (bio) crowd-sources his problems by following his gut, which he shares with 100 trillion of his closest microbial friends — a close-knit, symbiotic community comprising several thousand species of bacteria, fungi, and protozoa. In a similarly collaborative fashion, all of Dave's writing is available for reuse and creative remix under a Creative Commons Attribution-ShareAlike 3.0 United States License. For attribution in printed material, his name (Dave Bonta) will suffice, but for web use, please link back to the original. Contact him for permission to waive the "share alike" provision (e.g. for use in a conventionally copyrighted work).

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