A study in North America has found that periodical cicadas, insect species that only emerge every 13 or 17 years, face extinction due to the loss of old-growth forests.
Researchers from Iowa University have concluded that cicadas are unable to adapt to the change of habitat caused by removal of established woodland, even with replacement growth. The dietary dependence of the larvae upon root structures and increased vulnerability of emerging adults to predation, mean that periodical cicadas could vanish from much of their traditional range.
Cicadas are a family of winged insects that are found on every continent, except for Antarctica, and are renowned for mass emergences that are characterised by their loud, buzzing song.
Part of the order Hemiptera, more than 3,000 species comprise Cicadoidea, the majority of which exist within an annual life cycle, emerging once per year.
In North America, there are 7 species known as periodical cicadas, which remain underground in larval form for far longer. Four of these species have 13-year life cycles while the remaining 3 species live within 17-year cycles. Each colony of cicada maintains extraordinary synchronicity, emerging in huge numbers to complete a metamorphosis into adult form.
“I think that cicadas are really cool because they have this amazing biology. But that interesting biology also has doomed them,” explained Andrew Forbes, Professor of Biology at Iowa University. “Their viability is connected with human land use. Forests get cut down, and they regrow, but these animals are living underground for 17 years, and when you cut down the old trees, the cicadas lose their food source, they die out, and it might take them hundreds of years to get back.”
An adult female cicada measures up to 33mm long, and once fertile, will cut slits into the bark of a tree in which to lay eggs. They hatch into larvae which drop to the ground and then use their front legs to burrow into the soil. They remain at a depth of up to 60cm, feeding upon the juices of the tree roots before emerging en masse up to 17 years later.
The researchers found that a colony will expand by an average of 50 yards (45 metres) per emergence, limiting expansion where trees are removed. Forbes also led a relocation project which saw 20,000 Brood XIII cicadas introduced into a new area. With no defence against predators other than sheer number, those relocated insects were all eaten within a week.
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The observational study, first published in the journal Ecology, does offer some hope in its conclusion. If the forests where periodical cicada remain are preserved then the species can endure.
“Those can be places where they can survive and disperse slowly,” Forbes says. “In order to expand their range, you also would need new forests to remain intact and not be cut down.”
Find out more about the study.
Top image: Periodical cicadas. Credit: Don Cornett/Getty Images
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