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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