A tiny wasp could save Christmas Island’s beautiful red crabs from crazy ants

Have you heard the story about wasps killing bugs, which feed ants, which in turn kill the crabs that keep the forests of Christmas Island healthy?

If not, that’s because it hasn’t happened yet, but it’s a story worth telling.

Parks Australia will release a 2mm-sized wasp on Christmas Island in the coming weeks to control an infestation of yellow crazy ants, a major threat to the island’s wildlife, including its famous red crabs.

Biological control – the use of one species to control another – is notorious for causing the Australian cane toad invasion. So how do we know it works?

Christmas Island and its crabs

Christmas Island is a unique natural habitat with many endemic species. The National Park covers two thirds of the island, known as the Galapagos of the Indian Ocean.

Many people know Red Crab Their massive migration to the ocean is hailed as one of the wonders of nature.

There are many other species of crabs on Christmas Island, including the impressive Robber CrabThey are probably the largest terrestrial arthropods (the group to which insects and crustaceans belong) on ​​Earth.

These abundant land crabs work together to clean leaf litter from the forest and maintain burrows that prevent soil compaction, creating an open and diverse forest.

But this thriving natural system was disrupted when an invasive ant species emerged in large numbers on the island.

Ant

Early 20th Century, Yellow Crazy Ant （Slender-legged golden scale insect) arrived on Christmas Island. The ants now form super colonies, with billions of them spread over hundreds of hectares.

The crazy ants spray formic acid into the crab’s eyes and leg joints, immobilizing the crab. The crab soon dies and becomes food for the ants.

In some cases, crabs living in areas without crazy ants are killed during their annual migration and therefore never return to their original forests. This creates crab-free zones even in areas without the ants.

As crab numbers declined, forests became less diverse, with dense understory vegetation and compacted soils due to the collapse of crab burrows. Other invasive species, such as the giant African snail, became common where crab numbers declined.

Australian Parks Many different methods have been tried to reverse the effects of yellow crazy ants on red crabs, from aerial drops to hand baiting.

Because the impacts were so severe, a chemical control program for the super crab population began in 2001. The program slowed the crab population’s decline, but it was costly and time-consuming, so researchers began studying other options, including the use of other species.

Bugs: scale insects

Supercolonies of yellow crazy ants need a reliable food source, and another invasive species provides that: the yellow lac scale insect (Orange taro).

Scale insects (a The real mistake) suck the sap of trees and produce a sweet secretion called honeydew from their anal holes, which the ants then harvest.

It seems that without the carbohydrate-rich honeydew provided by the abundance of scale insects in a forest, these supercolonies of crazy ants cannot survive.

There is evidence that scale insects increase ant reproduction, making them more likely to attack other species. A large field experiment showed that if we prevented ants from accessing scale insects, ant activity above ground dropped by 95% in just four weeks.

Scale insects may need ants as much as ants do. Some ants protect scale insects in the same way that humans protect livestock, by keeping other predators away.

The interaction between the two invasive species has led to extremely high population densities, which is called Invasion Crash.

The good news is that scale insects, unlike ants, are amenable to biological control. For example, the Australian ladybird beetle has been used with surprising success to control scale insects. Cotton pad scales In North America.

Wasp

We began searching for a species that could control scale insects on Christmas Island. We found it: a species called Somerville Carpa pest that attacks the yellow lac scale insect native to Southeast Asia.

The wasp lays its eggs inside mature female scale insects, then kills them from the inside, producing more wasps, which then lay their eggs inside more females. The wasp (and other predators) are so aggressive that the yellow lac scale insect is very rare in its native habitat.

Obviously, we had to test that this wasp wouldn’t attack other species. The researchers tested it in the wild in Malaysia, an unusual approach that yielded excellent results. The scientists exposed eight closely related species of scale insects to the wasp, and none were harmed.

This demonstrates that other scale insect populations on Christmas Island would not be threatened if this wasp were introduced, with the possible exception of another scale insect that is itself a pest.

The researchers also found that when scale insects were being watched over by crazy yellow ants, the wasps still worked – and they still attacked. After years of research, there is great excitement about the imminent release of the wasps on Christmas Island.

Postscript: Toad

We all know stories of failed biological control efforts. In Australia, a plan to introduce cane toads to control sea beetles backfired. In Hawaii, a plan to introduce mongooses to control rats also failed because mongooses are active during the day and rats are nocturnal. In both cases, the species were introduced without adequate research.

But these examples have changed the rules and laws regarding introduced species. Today, governments are more aware of the risks of invasive species. Any introduction must be subject to rigorous testing and risk assessment.

In this case, La Trobe University researchers worked closely with Parks Australia and the Forest Research Institute of Malaysia to collect enough data to meet the Australian Government’s requirements.

We believe this is the most rigorously scrutinized biological control program in Australia. When the wasps arrive on Christmas Island in a few weeks, we believe it will set an example for best practice conservation.

Fewer ants means more crabs, healthier trees, fewer African snails, and more fertile soil. And it will save on costly conservation efforts in future years.

Parks Australia has produced a special animation about the program, which can be viewed here http://www.parksaustralia.gov.au/christmas/news/biocontrol.html.