A fairy fly

Fairy wasps unleashed to protect Eucalypts

By John Upton

Eucalyptus trees are the scraggly kings of Australian landscapes, growing hard and fast, resilient to fire and sundry other stresses. After their crowned heads were plucked from native wildlands and thrust into monoculture plantations in continents far afield, though, pests began sucking the antipodean puissance out of the botanical emperors.

Cue scientific tinkling and hopes for a tiny-winged salvation.

A healthy Eucalyptus plantation in Hawaii. Photo by Forest and Kim Starr.
A healthy Eucalyptus plantation in Hawaii. Photo by Forest and Kim Starr.

Natural forests and other ecosystems are being cleared the world over to make space for Eucalyptus plantations. They sprawl over millions of acres, from the American Southeast to Africa to New Zealand.

The trees are largely being grown to be pulped for paper and, more recently, to be burned to produce energy. Sometimes they’re just planted along paths and roads and as forests because they’re easy to grow, and they look nice.

Amid this upheaval, a biological chink has been gouged from the trees’ armors of hitherto resilience. Across the globe, Eucalypts in plantations and neighborhoods alike are being attacked by tiny sap-sucking bugs.

The culprits are called bronze bugs — because their victims’ hues change from green to bronze as their leaves dry out. As the sap is sucked from the trees, their growth is crippled. The heaviest of attacks can leave the trees dead.

Bronze bugs
Bronze bugs on a Eucalyptus leaf. Photo by Simon Lawson.

To protect hulking gum tree plantations from bronze bugs, scientists are starting to release even tinier critters. Their newest weapon is a species so small that it lays its eggs inside the eggs of the marauding pests, which hatch to feast on the meat of an egg that was laid for another, killing the unborn.

Eucalyptus trees, the bronze bugs that steal their sap, and the fairy wasps that hijack the bronze bugs’ eggs are all Australian natives. But until the turn of the century, few people had given the bronze bugs any thought. That’s when they started attacking trees in Sydney — possibly infesting tree species that had been transplanted outside their native ranges.

“There were very few records of it until it started outbreaking in Sydney in the early 2000s,” said Simon Lawson, a University of the Sunshine Coast entomologist who studies Eucalyptus pests.

From Sydney, the bronze bugs spread, hitchhiking with world trade to South America and South Africa, where the invasive populations made themselves at home amid their native prey. More recently, they’ve have been spreading through Europe and the Middle East. They’re also in New Zealand.

A fairy fly
A fairy fly. Illustrated by Perry Shirley.

The bronze bug outbreaks have coincided with a substantial rise since the 1990s in the spread of exotic pests in general — and, more recently, with a rise in the spread of Eucalyptus pests.

“Just in the last ten to 15 years or so, there’s been a real increase in the number of Australian-origin Eucalyptus insects that have been moving around the world into Eucalyptus plantations,” Lawson said.

To try to relieve the problem, Lawson and other researchers across the planet are turning to the pests’ natural predators. The main predator tested in laboratories and dispatched in the wild so far has been Cleruchoides noackae. C. noackae are from a family of wasp and ant relatives called fairyflies — or fairy wasps. As the name suggests, the family includes some of the tiniest insects ever discovered.

C. noackae
C. noackae. Photo by Samantha Bush, University of Pretoria.

Fairy wasps are often used as biological controls — as sentient insecticides.  They’re all parasitoids. That’s similar to a parasite, but dialed to a different equilibrium: parasites generally let their hosts live; parasitoids do not.

Following quarantine and tests that convinced them C. noackae was safe for native bugs, Brazilian agriculture officials released swarms of  them in the state of Minas Gerais in 2011. Two years later, field research found that about half the bronze bug eggs in local Eucalyptus plantations had been parasitized by the fairy wasps.

The results, which will be detailed in an upcoming scientific paper that’s still being finalized by Brazilian agriculture officials, are “quite a bit better than what we’ve seen in the native populations in Sydney that they’re derived from,” Lawson said.

Similar releases are planned or already underway in other South American countries and in South Africa.

Cracking the bronze bug problem, which was set off when Eucalypts were introduced to exotic environments, might mean doubling down on the number of species that are introduced to patch the problem over.

Ongoing research to identify alternative biological control agents, such as other species of fairy wasps, will also be critical for controlling the pests, Lawson said. “You’re better off having more than one agent.”

Bronze bug eggs on an infested leaf. Photo by Simon Lawson.
Bronze bug eggs on an infested leaf. Photo by Simon Lawson.