U.K. Braces for a Massive Explosion of Flies

The United Kingdom may soon face an invasion from below: a massive outburst of harmless, long-legged flies that could number in the billions.

Each autumn the U.K. sees an explosion of the European crane fly (Tipula paludosa), a gangly species about half an inch long referred to as a “daddy long-legs.” (In the United States, that nickname most often refers to harvestmen, a type of arachnid.) This year, though, a damp winter has let the flies’ larvae thrive in the soil, setting up a major outburst, and warm September weather stands to let the flies spread farther north than ever before.

The looming spread poses no safety concerns. Though they look like large mosquitoes, the flies are harmless and do not bite humans.

Named for their slender, dangling legs, crane flies live life in an amorous whirlwind. Adults emerge from the soil from mid-August to mid-September, and within a day, they mate and lay up to 300 eggs at a time in grassy fields.

The resulting larvae—called leatherjackets for their tough outer layers—live in the soil through the following spring, nibbling on plant materials before beginning their metamorphosis and starting the cycle anew.

Crane flies have long been considered agricultural pests, since leatherjackets feed on the root systems of turf grasses and cereal crops. In fact, the New York Invasive Species Information database reports that T. paludosa is invasive to the United States and Canada and poses a persistent threat to golf courses.

In the United Kingdom, pesticides and drainage have reduced the flies’ agricultural threat, rendering their annual maturation a nuisance to some—and to others, a natural wonder.

“When there is a mass emergence of the adult flies from the pupae in the ground, they can sometimes get blown against say, a wall, [and] can look quite spectacular,” writes John Kramer, the co-organizer of the U.K.’s National Cranefly Recording Scheme and a member of the Dipterists Forum, an academic society devoted to studying flies.

The flies have been bursting from U.K. soils for a while, having colonized the island from northwestern Europe before the English Channel opened some 9,000 years ago, after the end of the last ice age. “They have been hanging on in refuges ever since,” Kramer writes, to impressive effect: Britain is home to some 326 crane fly species in all, including T. paludosa.

Kramer notes that crane flies’ larvae thrive in damp soil, and in the U.K., it’s been an exceptionally damp year—potentially leading to an unusually large outburst.

December 2015 was the wettest December and calendar month for the country since 1910, according to the Met Office, the United Kingdom’s weather service. And the precipitation didn’t stop there. January, February, April, June, and July also brought rainfall levels above the 1981-2010 average, with some areas in England and Wales occasionally receiving more than twice the normal amount.

“If there is a population explosion, perhaps the wet winter would account for it,” writes Kramer.

This year may prove to be exceptionally buggy, but when and how future crane flies burst from the earth remains uncertain, thanks to climate change.

The Met Office and the Committee on Climate Change, an independent U.K. advisory body, report that in coming decades, the country's summers will get hotter and drier on average—potentially worsening soil conditions for crane fly larvae.

“If the soil got drier, you would expect a reduction in some species, especially those living in drier sandy soils,” writes Kramer. “However, there could be complex effects of an increase in atmospheric energy.”

“Increased energy produces increased evaporation and therefore rainfall,” he adds. “We have already seen a number of exceptional weather events such as torrential rain and flooding.”

Kramer also notes that rainfall stoked by climate change might benefit crane fly species whose larvae are aquatic or semiaquatic. But even then, extreme weather may bring downsides.

“[Larvae] may get washed out to sea in rivers, or move to the soil surface to get more oxygen, and therefore get eaten by birds,” he writes. “Prediction is difficult, and research (e.g. on the tolerances of the larvae) would be important to feed into any model [of climate change’s impacts].”