How Tiny Wasps Could Save the Christmas Island Red Crab From Invasive Crazy Ants

Christmas Island red crab
Christmas Island red crabs (Gecarcoidea natalis) live alone most of the year, typically in the island's rain forests. But they swarm to the sea for breeding season, often spawning in November or December. (Photo: John Tann/Flickr)

Christmas Island is known as "the Galapagos of the Indian Ocean," a reference to its small size, remote location and array of native wildlife. One of its most famous residents is the Christmas Island red crab, renowned for a yearly migration in which tens of millions of crabs scurry across the island to lay eggs in the ocean.

Lately, however, these crabs have been decimated by yellow crazy ants, an invasive species introduced to Christmas Island last century. The ants form supercolonies with billions of individuals, and their taste for red crabs poses a grave threat. Even crabs that live in areas without crazy ants are often killed during the annual trek, thus never returning to their off-season forests. The crabs play important roles in the island's unique ecosystems, so a population decline could trigger dangerous ripple effects.

Yet there's still hope. After years of trying to control the ants directly, researchers with Parks Australia and La Trobe University now hope to save the crabs by instead targeting a different invasive insect. And as Parks Australia explains in the animated video above, that involves releasing yet another non-native insect.

It may sound crazy, and it is sort of an ecological Rube Goldberg machine. But unlike many infamous plots to fight exotic species by adding new exotic species, this plan has been carefully researched — and it might just be crazy enough to work.

The yellow crazy ants' conquest of Christmas Island was enabled by the yellow lac scale insect, which supports the ants' supercolonies by producing a sweet, sticky substance called honeydew. This mutualism has helped both invaders reach monstrous population densities, a concept known as "invasional meltdown."

To break it up, researchers are releasing a Malaysian micro-wasp with a wingspan of just 3 millimeters. The wasps lay eggs inside scale insects, killing them and producing more wasps that go on to kill more scale insects. "This wasp (and other predators) are so effective," the researchers wrote earlier this month, "that the yellow lac scale insect is rare in its native habitat." Recreating that effect on Christmas Island could keep crazy ants in check, they add, citing an experiment in which four weeks without scale insects led to a 95 percent drop in ant activity on the ground.

Wasps are already used in similar ways to control invasive insects in other parts of the world. But this kind of strategy has gone wrong in the past — as with mongooses in Hawaii, or cane toads in Australia — so a lot of research was needed to make sure the wasps wouldn't just cause new problems on Christmas Island.

Scientists tested the idea by exposing the wasps to eight closely related species of scale insects, none of which were harmed. They also exposed the wasps to yellow lac scale insects while they were being tended by yellow crazy ants, demonstrating that the ants aren't an effective deterrent against wasp attacks. (And these wasps don't build big colonies or sting humans, adding to their appeal.)

"We believe that this is the most closely scrutinised biological control project in Australia," La Trobe researchers Susan Lawler and Peter Green wrote in early December. "When the wasps arrive on Christmas Island in a few weeks, we are confident that this will set an example for best-practice conservation."

The wasps may not have an immediate effect, but if their arrival really does help the red crabs recover, it could be just the kind of miracle Christmas Island needs.