What to expect from this year's rare double brood of cicadas

Two broods of cicadas have started emerging simultaneously for the first time in 200 years. It's loud, messy and very interesting.

The last time an event like this happened, Thomas Jefferson was President of the United States. It's deafening.

Trillions of periodical cicadas have started emerging across the Midwest and Southeast of America, after spending more than a decade burrowed underground. This year, two broods of flying cicadas have appeared at the same time, and it is the first time they have emerged simultaneously since 1803.

Periodical cicadas have an incredibly long life cycle, unlike their non-periodical brethren which mature each summer. After hatching, the immature periodical cicadas, called nymphs, spend either 13 or 17 years underground, feeding on roots, before squirming their way above ground and transforming into adult cicadas.

The 17-year Brood XIII emerged in Northern Illinois, and the 13-year Brood XIX has appeared in parts of the southeastern US. Both events are due to begin in late April. And for those in the right spot, there is a small area where the broods may potentially overlap. According to researchers at the University of Connecticut, the greatest likelihood of contact between the two broods is in small patches of woodland around Springfield, Illinois.

"This is not a common event," says Gene Kritsky, a cicada expert and professor emeritus of biology at Mount St Joseph University in Ohio. His love of these insects began 50 years ago when he first learned about periodical cicadas, and realised there was much to discover by using historical data to create maps of their distribution patterns. He describes himself as a "frustrated historian who is also an entomologist".

And it is not just Kritsky who is fascinated by these musical insects, which belong to the stink bug family, and have species names ranging from "common cactus dodger" and "scissor grinder" to "masked devil" and "whiskey drinker". His passion has inspired other Americans to document cicadas – half a million videos and photographs have been uploaded to Kritsky's citizen science app Cicada Safari, which he launched in 2019.

The assorted species of Magicicada have earned themselves a "worldwide following", observed one paper on the ecology of periodical cicadas, due to their "recklessly theatrical" emergences in tremendous numbers.

Although, not everyone enjoys it – a number of Americans don anti-cicada outfits, and even plan trips away to escape the trillions of bugs that descend all at once. The aftermath can also get a little messy, with homeowners having to sweep their driveways of the tiny corpses that pile up, while their little rotting bodies create quite a stink. But cicadas do not bite, sting or carry diseases – and cannot be effectively controlled by pesticides.

With all those insects emerging at once, it can also get loud – really loud. Male cicadas produce their distinctive clicking sound by vibrating an organ near the base of their wings called the tympanic membrane. The females also produce a similar, quieter sound with their wings. Cicadas from Brood XIX were recorded producing calls of up to 75 decibels during an emergence in 1998. That is equivalent to standing next to a vacuum cleaner or a hair dryer.

Cicadas emit a cacophony of otherworldly sounds, including high-pitched whines, ticks and buzzing, which combine to produce the chorus we hear. And something interesting seems to happen when different species overlap.

Brood XIX, also known as the "Great Southern Brood", is in fact made up of several different species of cicadas, including one called Magicicada neotredecim. Usually different species in a brood emerge in distinct areas, but occasionally they emerge together in "contact zones".

Magicicada neotredecim males have been found to subtly lower the frequency of their calls when they encounter another Brood XIX species known as Magicicada tredecim. To the untrained eye, however, they are virtually impossible to tell apart – one has a mostly orange abdomen and the other has a black and orange abdomen. DNA analysis really provides the only sure way of distinguishing one from another.

Across the US, there are 12 broods of 17-year cicadas, and three broods of 13-year cicadas.

Groups that share the same emergence years are called broods and are labelled by Roman numerals. Broods are complex groups of different species that emerge in different parts of the country at the same time. Exactly why they become synchronised in this way, however, is still the subject of great scientific debate. Some believe it may be due to "climate shocks" – sudden, extreme changes in the weather – can lead to parts of a brood to break out of their usual schedule, creating new broods.

The sign of an impending cicada emergence is earth riddled with fingertip-sized holes. There can be several dual emergences during a 221-year period, and this year's dual emergence is the fifth since the year 2000 – it is the combination of broods that makes it a rare event.

"The Northern Illinois Brood has been reported to be a pretty dense brood," says Mike Raupp, a professor emeritus at the University of Maryland's entomology department. "And the Great Southern Brood is very widespread and reaches high densities in many locations. So, he adds, in the area of overlap in Illinois, the density of cicadas could be immense. 

Although they can be harmful to young trees, when females lay eggs in the tree's new growth, they can be beneficial to the ecology of the region. They provide a source of food – and in this case, an ample banquet – to predators. "Birds and small mammals will have a feast which will result in increased reproduction and survival of their offspring," explains Raupp. "From an evolutionary standpoint, it could be very interesting."

There are more than 3,390 species of cicadas around the world, but only seven in North America are known to be periodical. The rest emerge annually. Although they are classed as distinct species, many species of periodical cicadas are able to interbreed, producing hybrids.

It means that where the broods cross over this year, three species of Brood XIII cicadas will have the opportunity to interbreed with four species of Brood XIX cicadas, Raupp explains. "The outcome of this will produce hybrids and only the cicadas and Mother Nature know what the outcome will be," he says.

When periodical cicadas emerge, they bring great benefits to the environment where they live. The nymphs aerate the soil as they tunnel to the surface, improving water infiltration to the ground and encouraging root growth. When they die and decompose, they add nutrients to the soil.

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But like most other creatures, cicadas' behaviours are changing. They are emerging earlier in the spring than they did a century ago, says Kristky. "And there have been more broods emerging four years ahead of schedule." Destruction of forests threatens populations too – in 1954 the entirety of Brood XI went extinct, due to forest clearing to make way for agriculture and urbanisation.

What draws entomologists like Raupp and Kritsky back is the mystery of the bugs – and the fact that it's nature putting on a show. "There could be more noise, more fear for entomophobes, more fun for bug geeks like me," says Raupp. "And yes, it is a wicked cool and interesting event that happens nowhere else on Earth."

The next time Broods XIX and XII emerge together it will be 2245. The question is: what kind of world will they be coming out to?

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