Invasive species: the battle to beat the bugs

Asian longhorned beetle / Alamy

Biosecurity ‘weaponry’ is helping to halt the global spread of non-native species, from rampaging caterpillars to giant hornets

Select an insect from the cards placed throughout the story. Too invasive? Swat them away or use the button in the lower right
Illustrations by Daniel Long

The best time to annihilate oak processionary caterpillars is when they are young, just a few millimetres long and still high up in the trees. At this stage, their appetite for oak is rapacious, so dousing the leaves in a biocontrol agent like Bacillus thuringiensis (Bt) is one way to get the caterpillars to ingest it. When Bt toxins dissolve in the caterpillar gut they become active, puncturing the stomach and killing the insect in several days.

Craig Harrison, of the UK’s Forestry Commission, explains this to me one bright spring morning in a south London wood of bluebells, brambles and oak. We have come to the front line of UK biosecurity control to watch two contractors in biohazard overalls manoeuvre a quad bike with a big red blower on the back around the wood. Every so often they park the quad, aim the blower at the trees and spray. “Money-wise and in terms of resources, oak processionary is the biggie,” Harrison says. A mist of certain caterpillar death wafts through the canopy.

Oak processionary

Oak processionary moths (OPM) and their caterpillars have no natural place in an English wood. They are an invasive non-native species from southern Europe unwittingly introduced to the UK in 2005 — and one in thousands of species worldwide that have been scattered by humans around the globe to disastrous ecological effect.

Ecologists liken the introduction of a non-native species to Russian roulette. Most often, an insect transported in a tourist’s suitcase to an alien environment will not survive — perhaps it can’t find food or gets eaten itself. Occasionally, though, a beetle stowed in wooden pallets shipped from China will face few of the predators that had once kept it in check. It will find plentiful food and a mate and become “established”. And then it will spread.

In recent decades, as global trade and mass transit have bloomed, the great global reshuffling of flora and fauna has accelerated. It is estimated in any 24 hours 10,000 different species, such as molluscs, crustaceans, seaweed and plankton, are moved around the world in ballast water. “We’re talking about the widespread redistribution of life on earth — there’s no precedent in the fossil record,” says Anthony Ricciardi, a biological invasions specialist at McGill University in Montreal. The International Union for the Conservation of Nature calculates that invasive species are the second most significant threat to biodiversity after habitat loss.

If we have been slow to grasp the destruction that mass species dislocation can reap, we are at least getting better at handling it. Our advancing understanding of biosecurity “weaponry” — from tree genetics and biocontrol to GM technology — is helping to both manage existing incursions and prepare for future threats.

There is now little chance the UK will be rid of OPM completely; the bug has established itself in west London and is fanning out through the south-east. The caterpillars are noticeable from May until July when they grow defensive, toxic hairs and crawl in nose-to-tail “processions”. As well as damaging trees through defoliation, they threaten human health as when touched, the hairs cause rashes.

Harlequin ladybird

The history of biocontrol — where predators, parasites or pathogens are released into a habitat to bring an invasive under control — is potted with high-profile failure and ecological calamity. When Australia introduced cane toads to control sugarcane beetles in Queensland in the 1930s, the toads swarmed the country, wiping out indigenous fauna. Harlequin ladybirds, a native of Asia introduced in Europe to control aphids, are thought to have reduced the UK native population of two-spot ladybirds by 44 per cent in the past decade. (As well as being voracious aphid-eaters, harlequins are cannibals.)

Biocontrol agents today are more specialised. Vertebrates are rarely used because their diets are broad and they may “hit” more than the intended target. Unlike insecticides, biocontrols don’t release chemicals into the soil. They also usually self-perpetuate. “If you want to have a long-term suppression of pest numbers, you have to have something that continues to work without continued human effort,” says Roy Van Driesche, of the department of environmental conservation at the University of Massachusetts.

Emerald ash borer

Van Driesche is researching parasitoids — organisms akin to parasites but with the distinction of killing their host — that might be used as agents against the emerald ash borer, a beetle from Asia that will cost the US an estimated $10bn to replace lost ash trees. When the beetle’s larvae hatch under ash bark, they bore into the trunk through the phloem, cutting off vital supplies of nutrients and water. One of Van Driesche’s parasitoids works by growing inside the borer grub. “[The grub] would look like a sock stuffed with boiled eggs,” he says. At some point, the “sock” ruptures, the parasitoid pupates and the grisly cycle repeats.

A tale of two insects
New Zealand flatworm

Not all introduced species are undesirable. Europeans brought honey bees to the US in the 1600s, and earthworms that now help maintain healthy soil in North America are also European imports. Plants from abroad have enriched gardens for centuries. And if some invasive species are not exactly welcome, their impact on the new habitat is tolerable. New Zealand flatworms have been quietly invading Scotland for 50 years, yet Defra considers them to be established and not worth the fight — though tell that to a native earthworm who, on meeting a flatworm, is coated with a digestive mucus and slurped up like soup.

Asian longhorned beetle

Yet invasions can sometimes so devastate local ecosystems, species and crops that ecologists equate them to natural disasters. They can be more costly than earthquakes or floods. The Asian longhorned beetle, under federal quarantine in three states in the US, would threaten forest valued at billions of dollars if it managed to escape. The EU spends €12bn a year on controlling the region’s 12,000 alien species and repairing the harm they do.

Yellow crazy ant

The environmental toll can also be great. On Christmas Island, in the Indian Ocean, yellow crazy ants — so-named because of their eccentric walk — have halved the population of red crabs, a species found nowhere else on earth. Now they have established “super colonies”, the ants are a threat to the survival of a species that is both a tourist attraction and key to the island’s ecology.

With introduced diseases, the game of Russian roulette is much the same. “When an entirely new pathogen shows up, it’s like bringing a gun to a knife fight,” writes Elizabeth Kolbert in The Sixth Extinction: An Unnatural History. In the UK, ash trees have few natural defences against ash dieback, an often-fatal fungal disease first found in the country in 2012. Ash makes up about 30 per cent of British woods, meaning tens of millions of trees are expected to perish.

The recent discovery of a healthy tree named Betty in a dieback-ravaged Norfolk wood is raising hopes that the battle isn’t lost completely. Researchers at the University of York and the John Innes Centre in Norwich have identified genetic markers that allow them to predict a tree’s tolerance. Betty, they think, has the making of a rare survivor tree. “Now we understand the genetic basis we can actually start to do genetic selections,” says Nicola Spence, the UK’s chief plant health officer. “By selecting tolerance-tested seed from trees like Betty and putting those into a selection programme we can start to regenerate tolerant trees quite rapidly.”

Asian hornet

“Quite rapidly” in tree talk means within a decade; the replacement of an obliterated tree population, meanwhile, could take the best part of a century. With some invasive species, such as the Asian hornet that is spreading through France, it might be preferable to move a bit quicker — or, in other words, to have a more nimble weapon in our armoury.

“Insecticides are very good at reducing large number of insects to lower numbers,” says Neil Morrison, a research and development group leader at Oxitec. “Approaches like ours can reduce pest numbers to much lower levels.”

Oxitec, an Oxford university spin-off bought in 2015 by Intrexon for $160m, is developing genetically modified insects as a weapon against infectious disease such as Zika virus and agricultural pests. In the US the company is conducting trials on the diamondback moth, an invasive species that feeds on broccoli and cabbage estimated to cost farmers worldwide $5bn a year.

Oxitec’s technology is inspired by sterile insect technique, a method where male insects are zapped with radiation to stem their reproductive ability. Oxitec inserts a lethal, “self-limiting” gene into male insects that allows them to reproduce but prevents their offspring maturing. In the Oxitec trials, diamondback populations in greenhouses were controlled within 10 weeks of GM moth release.

Could GM insects become a future biosecurity tool? In December the House of Lords’ science committee concluded that while GM insects would not provide a “silver bullet”, the UK had a “moral duty” to at least test the technology.

The Agriculture and Horticulture Development Board (AHDB) also suggests GM insects might be of use. If left unchecked the spotted wing drosophila (SWD) — an invasive soft fruit pest that has no biocontrols and limited chemical controls at present — could cost the UK fruit industry up to £120m. “Given that SWD is an alien species,” the AHDB says, “its eradication would restore the correct ecological equilibrium rather than disturb it.”

The idea of “restoring” balance to the natural order is something that unites proponents of biosecurity. Not that there is a clear, static iteration of how nature should be; species have always shuffled about to a degree. Yet many ecologists think that our species has so altered the global biota by shunting insects, plants and microbes faster and farther than they could have moved alone that we have a duty, as Ricciardi puts it, “to clean up our own mess”.

A few weeks after witnessing the OPM death spray, I head to London’s Richmond Park on a hairy caterpillar hunt. Accompanying me is Andrew Hoppit, OPM project manager at the Forestry Commission. Since the park is home to protected species of butterflies, the use of insecticide and biocontrols here has been tricky. If it weren’t for fear of harming other species, says Hoppit, “I could control all the OPM in London tomorrow”.

We traipse through the park, past dog walkers and children on trikes, until we get to an oak tree in full leaf. Though thousands of the estimated 40,000 oak trees in the park have OPM nests, Hoppit warns me that at this early stage the caterpillars might be difficult to see. “As they get bigger, they’ll start to coalesce and form nests,” he says. “They form a webbing like a spider and when it’s saturated with caterpillars they start to drip out of the tree, like a science fiction movie.” He reaches into the canopy and pulls down a branch. It takes a minute to find them, but eventually we spot dozens of small furry caterpillars clustered in the leaves, having their lunch.