Biological Control: A Christmas Miracle?

Christmas Island

Hearing the name Christmas Island may conjure up images of Santa Clause, snow and a factory full of elves, but the real Christmas Island is actually quite the opposite. Situated in the Indian ocean, this tropical island is home to the largest variety of land crabs in the world. The most famous of these is the red crab, whose annual migration to the sea to breed has been hailed as ‘one of the greatest natural spectacles on earth’ by Sir David Attenborough (Image 1). These red crabs are keystone species, and by eating the leaf litter and controlling seedling recruitment they engineer the environment.

christmas-island-red-crab

Image 1 – Red crabs on their annual migration. Source: public domain

Since the 1990’s this unique ecosystem has been under attack by supercolonies of the invasive yellow crazy ant (yes, that is its real name!) who kill red crabs and change the balance of the environment. The Australian government is now planning on releasing a micro-wasp from Malaysia as a biological control for this invasive take-over.

Alien Invasion

The yellow crazy ant (YCA) invaded Christmas island in the early 1900’s having been brought to the island by humans on ships. For roughly 80 years the YCA lived on the island in relatively low numbers and with relatively little impact on the native inhabitants. Then, in the 1990’s ant supercolonies formed and now occupy over 30% of the rainforest in Christmas island (1).

The ants are able to form these supercolonies, where densities can reach over 2000 ants per m2, due to a mutualistic relationship with another Christmas island invader – the yellow lac scale insect (1). The scale insect is a parasite of trees, feeding on the products of photosynthesis as they travel in the phloem. This insect also produces a sugary secretion known as honeydew which is then harvested by the YCA (Image 2). This nutritious honeydew enables the ants to reach such high densities. In return, the ants protect the scale insect and its larvae against predation. This relationship allows both the YCA and the yellow lac scale insect to have unnaturally high populations.

Yellow crazy ant receive nutrition from scale insect

Image 2 – Yellow crazy ant tending some yellow lac scale insects. Source: public domain

High populations of both of these invaders is bad news for the native flora and fauna of Christmas Island. It is estimated that the ants have killed between 1/4 and 1/3 of the entire population of red crabs on the island (2). The crazy ants kill the crabs by spraying formic acid into their eyes and mouthparts, blinding the crab and eventually leading to its death by dehydration (2) (Image 3). The crabs are now excluded from areas where the supercolonies reign but large numbers are still killed during their migration to the ocean as this passes through the YCA territory (2). The exclusion of the crabs has led to other introduced species becoming prevalent, such as the giant African land snail and cockroaches and millipedes who can live in the leaf litter now that the crabs aren’t there to consume it (3).

crazy-ants-attacking-red-crab

Image 3 – Red crab being attacked by yellow crazy ants. Source: public domain

The scale insect has equally devastating effects on the Christmas Island ecosystem. As they feed on the carbohydrates made by trees they cause them to dieback, creating gaps in the canopy that then allow invasive weeds to multiply. Not only this, but the honeydew that they produce allows sooty moulds to grow over the trees, reducing photosynthesis and exacerbating the die-back and death of the forest (3).

Chemical Control

The Australian government recognised that these two invasive species are a threat to the diversity of Christmas Island. They started aerially baiting the crazy ant supercolonies, first in 2002 and then in 2009 and 2012 (Image 4). An insecticide (FipronilTM) was dropped from helicopters into the supercolonies where it reduced crazy ant densities by 99.4% (4). Due to their close relationship, death of the ants also has been shown to lead to a 100% decline in density of the scale insects in certain circumstances (3).

Although baiting is effective, it is not seen as a long term solution to the YCA problem. This is because it is a poison, which ideally the Australian government doesn’t want to be using. It is also an expensive form of control (costing 1.2million AUD per treatment year) and can only target huge supercolonies, not small populations of the ants. A study in 2014 also showed that although most non-target species are not affected by the use of FipronilTM, the Christmas Island imperial pigeon (classed as Near Threatened by the IUCN) had a significantly lower abundance in aerially baited sites (5). As this is the case it isn’t an ideal way to control the spread of the YCA.

chris-hooking-up-a-load-of-ant-bait-2

Image 4 – Aerial baiting on Christmas Island. Source: public domain

Biological Control

In December 2016 Parks Australia announced that it has agreed to release a 2mm micro-wasp onto Christmas Island to control the YCA infestation. This biological control works by targeting the yellow lac scale insect and therefore cutting off the crazy ants food supply (4). This will stop the ants from being able to form supercolonies and hopefully return the ecosystem back to how it was pre 1990’s when the ant populations exploded.

The micro-wasp is a parasitoid wasp that lays its eggs in adult female scale insects. The wasp larvae then grow to adult-hood inside the insect, killing it. This process means that more wasps are produced whilst reducing the population of the scale insect. Biological controls have been known to go wrong in the past, most notably with the cane toad in Australia. La Trobe University has spent years studying this wasp to make sure that it is safe to release on Christmas Island. They have shown that it will only target one species of scale insect, doesn’t form hives or colonies and cannot sting humans or animals, making it the perfect biological control.

The Future 

We can only wait and see if a tiny wasp from Malaysia will perform a Christmas miracle and help to control the crazy ant invasion, but all signs look positive so far. If it does, maybe we can re-name it the Rudolph wasp, the saviour of Christmas Island.

23a14c0800000578-2855889-an_estimated_120_million_red_crabs_live_on_the_52_square_mile_is-24_1417450599408

Image 5 – Will the red crabs soon be able to get back to their annual migration without the fear of crazy ants? Source: public domain 

References

  1. Abbott KL. Spatial dynamics of supercolonies of the invasive yellow crazy ant, Anoplolepis gracilipes, on Christmas Island, Indian Ocean. Divers Distrib. 2006;12(1):101–10.
  2. O’Dowd DJ, Green PT, Lake PS. Invasional “meltdown” on an oceanic island. Ecol Lett. 2003;6(9):812–7.
  3. Abbott KL, Green PT. Collapse of an ant-scale mutualism in a rainforest on Christmas Island. Oikos. 2007;116(7):1238–46.
  4. Misso M, West J. Conservation management of the terrestrial biodiversity of Christmas Island: Challenges and perspectives. Raffles Bull Zool. 2014;Supplement(30):17–23.
  5. Stork NE, Kitching RL, Davis NE, Abbott KL. The impact of aerial baiting for control of the yellow crazy ant, Anoplolepis gracilipes, on canopy-dwelling arthropods and selected vertebrates on Christmas Island (Indian Ocean). Raffles Bull Zool. 2014;7600(30):81–92.

 

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