Parks Australia and La Trobe University have developed a new way to try and control invasive yellow crazy ants on Christmas Island.

The method uses indirect biological control, or ‘biocontrol’, to target one of the crazy ants’ key food sources.

What are yellow crazy ants?

Yellow crazy ants (Anoplolepis gracilipes) are one of the worst invasive species on Earth. Their original habitat is not known, but they have spread throughout many of the world’s tropical and subtropical regions. They are extremely aggressive and usually out-compete other insects to dominate food resources.

Crazy ants were accidentally introduced to Christmas Island during the first half of the 20th century. Thanks to a lack of natural predators on the island, they have been able to form super-colonies covering hundreds of hectares and supporting billions of ants.

Why are yellow crazy ants such a problem for Christmas Island?

Crazy ants defend themselves by spraying formic acid, one of nature’s most powerful acids. Formic acid can kill land crabs, including our precious red crabs.

A supercolony of yellow crazy ants might have up to 1000 ants per square metre. The formic acid that the ants produce at this density can wipe out entire local populations of endemic red crabs. Since the late 1990s, the invasive ants have killed tens of millions of land crabs.

That’s terrible news for our crabs, but it’s bad for many other species too. Red crabs make an important contribution to the health of Christmas Island’s rainforests by eating their way through tons of leaf litter and returning vital nutrients to the soil.

When crab populations die, the whole island ecosystem suffers. Controlling the yellow crazy ant is vital to the preservation of Christmas Island’s unique flora and fauna.

How has Parks Australia been tackling the crazy ant problem so far?

Until recently, the only way to stop crazy ants wiping out Christmas Island’s wildlife was to use poison bait.

Rangers from Christmas Island National Park had to lay the bait in super colonies swarming with billions of crazy ants, using a mix of hand delivery and aerial baiting from a helicopter.

While baiting is effective, it is only a temporary solution. The crazy ants move back in from remote and inaccessible parts of the island, and within a few years more baiting is needed. This is expensive and labour intensive.

What is biocontrol?

In this case, biocontrol means using another insect to target the crazy ants’ food source. This will hopefully cut down their numbers and stop them causing so much environmental damage on Christmas Island.

In this case, the biological control agent that we are using is a tiny insect called a micro-wasp.

If the project is successful, it will reduce the need for poison baiting to control the ants.

How the biocontrol project works

The micro-wasp (Tachardiaephagus somervillei) is a tiny Malaysian insect about 2 mm long.

These wasps don’t sting or build nests, nor do they harm humans, native wildlife or horticulture. In fact, many similar micro-wasps already live on the island.

Scientific studies indicate that this particular micro-wasp will stop crazy ants by cutting down their food supply, honeydew. Honeydew is produced by another non-native species, the yellow lac scale insect. A micro-wasp kills a lac scale insect by laying eggs inside it.

These micro-wasps are extremely ‘host-specific’ – they prey only on one particular species of lac scale insect and do not harm anything else.

Rangers and scientists are rearing the micro-wasps and releasing them into super-colonies of crazy ants.

So far we’ve seen the number of lac scale insects on the island reduced – good news for our plants. But at this stage we don’t know for sure what impact this is having on crazy ant super-colonies.

Isn’t introducing a new species risky?

Micro-wasps are already used for biological control on mainland Australia, so this approach is fairly common.

We’re a long way from the days of the cane toad, which was released in Australia with very little scientific testing. These days, biological control is carefully planned and managed.

The Australian Government’s Department of Agriculture and Water Resources carried out a risk analysis for the release of Tachardiaephagus somervillei on Christmas Island. After a thorough risk analysis the Department of the Environment and Energy recommended that the Minister add the control agent to the list of approved live imports.