Christmas Island Yellow Crazy Ant Control Program Moving from Chemical Control to a Biological Control Future Background The unique fauna and the ecological role of red crabs on Christmas Island The terrestrial landmass of Christmas Island is the top 361m of a 5km high seamount. It is a very remote island located 350km south of the island of Java, Indonesia and 2,600km north‐west of Perth. Christmas Island is 135km2 in area with 80km of coastline. It is markedly terraced from the coastal cliffs up to a central plateau and covered in thick forest in undisturbed regions. The climate is tropical with distinct wet and dry seasons. Image: Geoscience Australia. Christmas Island, along with the neighbouring Cocos (Keeling) Islands, make up the Indian Ocean Territories of Australia. They are governed by the federal Department of Infrastructure and Regional Development. Importantly, 63 per cent of Christmas Island is gazetted as a national park and managed by Parks Australia, a division of the federal Department of the Environment. Like many oceanic islands around the world, Christmas Island has evolved a unique flora and fauna during its many millions of years of undisturbed and remote existence. This special assortment of organisms extends from plants to birds, mammals, fish, reptiles and insects and of course to the famous land crabs for which the island is internationally renowned. Geographic location of Christmas Island in the north‐ eastern Indian Ocean. Image: Director of National Parks. The most iconic of these are the red and robber crabs, but they are just two of more than 20 species of land crabs on the island. Because of their huge numbers, red crabs especially provide critical ecosystem services in the rainforest, eating their way through tons of leaf litter every year and returning vital nutrients to the soil. Their activities as seed and seedling consumers create a uniquely open understory in rainforest on the island. Where land crabs are abundant they can also reduce the impacts of introduced species on the island’s ecology. Unfortunately, the land crabs of Christmas Island Christmas Island are under attack from a highly damaging invasive species, the yellow crazy ant. On Christmas Island some trees, like this coral tree, grow much larger than in other parts of their range because of the phosphate rich soils. 1 Image: Director of National Parks. What are yellow crazy ants? Life history The scientific name of the yellow crazy ant is Anoplolepis gracilipes. Gracile means slender and lightly built, a reference to the relatively skinny body and long legs of this species. When disturbed they move around in a frantic motion, hence the name ‘crazy’ ants. They are aggressive and competitive to other ants and insects and this enables them to out‐compete and displace other species and dominate food resources. The yellow crazy ant is an extremely successful and resourceful species, and considered to be one of the worst invasive species on earth. The home range of yellow crazy ants is not known specifically but they have spread through tropical and sub‐tropical zones of much of the world. This ant is extremely adept at ‘hitching’ of rides with human produce and materials. Yellow crazy ants have spread far across the indo‐pacific region often using human shipping and air traffic to migrate into new areas. Image: Wetterer (2005). It is thought that they were accidently introduced to Christmas Island through shipping. Yellow crazy ants are polygynous (multi‐queened) and unicolonial (they don’t attack each other) which means that multiple nests can support more than one queen, and quite often 100s or even 1000s of queens and tens of thousands of workers. In the absence of natural control mechanisms, as the situation is on Christmas Island, these traits enable the ants to form into large populations known as ‘super‐colonies’. The largest of these super‐colonies was recorded on Christmas Island in 2001 and was 750 hectares in size! Yellow crazy ants form nests in every possible niche within the forest and in supercolonies, it impossible to tell where one nest ends and another begins. They feed on a range of animals to access protein but also they also obtain carbohydrates from plant nectar and honeydew, which is produced by an introduced insect called the yellow lac scale. The ants forage 24 hours a day and there can be more than 1000 ants every square metre. Consider that very large supercolony: 1,000 (ants per m2) × 10,000 (m2 per hectare) × 750 (hectares) = 7,500,000,000 ants. That’s 7.5 billion ants in just one supercolony. And that’s just an estimate for how many ants were on the ground – there were probably just as many ants in the canopy visiting scale insects! This species is known as a fomicine ant because they actually don’t sting but spray formic acid from a small nozzle at the tip of their abdomen as a defence mechanise and also to subdue prey. Formic acid is one of nature’s most powerful acids. The acid is a big problem for insects and land crabs which share the same habitat as the ants and it can also cause irritation in humans. These traits and others enable the yellow crazy ant to have a significant impact on the ecosystems into which it has been introduced. At 1000 ants per square metre, yellow crazy ants spray enough formic acid to wipe out local populations of land crabs. 2 The rise of the ants Since Christmas Island was settled more than 120 years ago, its ecology has been influenced by an assortment of plants and animals introduced either intentionally or accidentally by humans. While most of these have had little if any detectable impacts, others have been disastrous for the island. The most prominent and damaging animal introductions include rats, cats, wolf snakes, centipedes and yellow crazy ants. Much of Christmas Island’s rich ecology was still considered intact prior to the introduction then spread of yellow crazy ants. The yellow crazy ant was accidentally introduced some time between 1915 and 1934. The species is recognised worldwide as a significant ecological pest that can negatively affect intact ecosystems. Unfortunately, the most well known case of this invasive potential is here on Christmas Island where since the late 1990s yellow crazy ants have killed tens of millions of land crabs, the most notable of these being the iconic red crab. The yellow crazy ants also directly compete with and prey on native vertebrate and invertebrate species, indirectly cause the dieback of trees, reduce soil health, alter forest composition and also facilitate the invasion of other introduced species into super‐colony areas. Interestingly, yellow crazy ants were present on Christmas Island for decades before having an obvious ecological impact. The formation of damaging super‐colonies is a recent phenomenon, with the first one found in 1989 in scrubby forest on a rocky terrace high above The Grotto. That supercolony died out, and the current phase of expansion started around the mid to late 1990s when more supercolonies were found near The Dales and Greta Beach. By 2001 yellow crazy ants had formed supercolonies in 2,500 hectares of the island’s forest. Most super‐colonies at this time were located in the national park with a preference for the coastal terraces. A control program was initiated in the late 1990s to suppress the spread and damage caused by yellow crazy ants and this control program is ongoing. On Christmas Island, a combination of factors has enabled the species to form large, ecologically‐ damaging super‐colonies. Principal among these is a mutualistic relationship with another group of introduced species, scale insects. Many of these are sap suckers, living on trees where they suck sap straight out of the stems. Their sugar‐rich waste product is called honeydew, and is avidly collected by the yellow crazy ants as a food source. The ants farm and protect these scale insects so that the sugary food source is maintained. Several species of honeydew‐ producing scale insects are common in A yellow crazy ant worker feeding from yellow lac scale insects. supercolonies, but the yellow lac scale insect Image: Director of National Parks. Tachardina aurantiaca is thought to be the main contributor to the yellow crazy ant’s honeydew diet. In super‐colonies, this and other honeydew‐producing scale insects occur at outbreak densities, and yellow lac scales be so dense as to sheath the twigs they settle on. 3 Clearly the mutualism between the ants and scales is key to supercolony formation, but like the ants, honeydew‐producing scale insects have also been on the island for a long time. Why have supercolonies formed only relatively recently? What may have changed about the mutualism that allowed rapid and extensive population buildup of both partners? El Nino climate events may have been important. El Nino causes longer, drier dry seasons on Christmas Island, and drought stresses trees. This can have the effect of making plant sap relatively more concentrated, promoting population increases of scale insects. More scale insects, more honey‐dew, and more honeydew means more ants. Because ants tend scale insects, more ants means more scale insects! In other words, the extent to which both partners facilitate each other in the mutualism could have been promoted by the severe El Nino that occurred in the late 1990s. It’s a plausible idea, but unfortunately supporting evidence has proved elusive. The impact of yellow crazy ants on Christmas Island biodiversity A typical undisturbed patch of Christmas Island forest will experience multiple changes following the foundation and spread of a super‐colony.