「经济学人」Flying bugs

The ancestors of these two neighbouring broods last met in 1803

All across the eastern states of America, it is cicada season. These small winged insects roughly the size of a paperclip emerge at regular intervals in vast hordes known as broods, many millions or billions of individuals strong. They spend a few frantic weeks breeding before the females lay eggs in slits carved into tree branches. When these hatch, around six weeks later, the juveniles head underground to find a root into which they can plunge their feeding tubes. Many years later, the cycle repeats.

Fifteen such broods of the genus Magicicada exist in North America. Some, like Brood VII in New York and Brood XIII in the Midwest, live on a 17-year cycle. Others, like Brood XXII in the South, emerge every 13 years. This year, for the first time this century, two broods with contiguous territory are emerging simultaneously. The exact number of insects expected to fill the air from Georgia to Wisconsin, though disputed, is high. “Maybe a trillion’s not out of the question,” says John Lill at George Washington University. “It’s going to be pretty extraordinary.”

The American periodical cicada has long intrigued observers owing to the unusual prime-number life cycle of its broods. One widespread explanation suggests that this is an evolutionary adaptation to outsmart any predator that lives on a shorter life cycle. As prime-number cycles synchronise only with multiples of themselves, any such predator would get lucky only rarely.

But, says Martha Weiss of Georgetown University, “The prime number thing turns out to be kind of a red herring.” More important than exactly how long the cicadas wait to reach full maturity may simply be that they do wait a very long time. No other insect is known to spend so much of its life cycle—upwards of 99%—waiting to reach maturity. By the time it emerges, in other words, the world (including any predators) will have probably forgotten about its existence. And when the insects emerge in bulk, as all periodical cicadas do, the odds of the species’ survival are raised even further. Similar advantages accrue to cicadas in India and Fiji, which have non-prime-number life cycles of four and eight years, respectively. Besides, prime-number cycles are not enough to outsmart everything. Massospora cicadina, a cicada-eating fungus, has the same lifespan as its host. “They’ve figured it out,” says Dr Lill.

The insects themselves are thought to count the passage of time in two ways. The years are tracked by tallying the starting and stopping of sap flow in the roots on which they feed, and the season is tracked by waiting for the temperature 20 centimetres underground to approach 18°C. This normally happens in late May.

The two broods of cicadas emerging this summer will have ancestors that last met in 1803. This year’s glut will, therefore, provide entomologists with a rare opportunity. The most pressing questions concern what happens when individuals from different broods meet, as is expected to happen at the boundary between their territories, in central Illinois. Should mating occur, for example, what cycle will any viable offspring follow, and over which territories will they subsequently spread?

Speculation abounds. Some biologists believe that the 17-year brood will slowly evolve into a 13-year one. (All 13-year broods are thought to have arisen in this fashion.) As 17-year cicada broods come with a contingent misleadingly referred to as “stragglers” that emerges four years early, it is possible that early stragglers will breed further generations of early stragglers. This year’s convergence may shed light on that process.

Scientists are also keen to understand how climate change is affecting the cicada. When the insects are in their juvenile stage underground, they feed off water-carrying tissue in tree roots known as xylem. As xylem flow begins and ends with a tree’s annual growing season, warmer temperatures mean cicadas could feed for longer each year, potentially accelerating their growth. That, in turn, could increase the number of early stragglers in future broods. Warmer temperatures will also let broods expand northwards, to territories previously too cold to colonise. “Overall,” says Dr Lill, “they’re pretty buffered.”

This article appeared in the Science & technology section of the print edition under the headline “Bug out”(May 28th 2024)

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