Yellow crazy ants disrupt entire ecosystems
The Misplaced
There is still uncertainty about the origin region of the the yellow crazy ant, Anoplolepis gracilipes.
It is one of the larger of the so called tramp ants, being usually 4 to 5 mm long. It has remarkably long legs and long antennae which it waves wildly if disturbed. It is yellowish in colour. It has few predators as its habit of spraying formic acid when under attack makes them unpalatable.
Diet and Tolerances
A. gracilipes has a broad diet which is a typical characteristic of a tramp ant. It will scavenge dead material as well as attack a wide variety of invertebrate prey such as isopods, myriapods, molluscs, arachnids, land crabs and insects. It will also attack vertebrates such as small lizards and young birds. And it has a third method of obtaining food - farming. This process encourages the breeding of honeydew-producing sap sucking insects, particularly scale insects and actively protects these insects from predators, much like a farmer would protect their livestock. Yellow crazy ants feed on the sweet sticky liquid that these insects exude.
Its ability to forage throughout the day and night and over a wide range of temperatures allows it to rapidly alter invaded ecosystems. High temperatures (such as those that occur around midday) and surface ground temperatures of 44°C may prevent workers from foraging. Ant activity begins to decline from around 25°C and foraging may be limited by heavy rain and strong winds. Researchers have reported an increase in both foraging activity and nest size in the dry season.1
Reproduction and population growth
| Stage | Duration | Notes |
|---|---|---|
| Time for egg hatch | 18 - 20 days | Eggs hatch to larval form |
| Time from hatching to functional worker | 16 - 20 days | Larvae pupate |
| Time from hatching to functional queen | 30 - 34 days | Larvae pupate |
| Worker life span | approx 6 months | |
| Queen life span | several years | Each queen can lay 700 eggs per year. Hives can have many queens in residence |
Social behaviour
In most ant colonies, aggressive neighbours limit the size of any one given colony, and members of one colony are never accepted into another. Yellow crazy ants, are not aggressive to their own species no matter what colonies are involved. This allows them to spend more time and energy finding food and extending territory.
Yellow crazy ants are included in the list of the World's 100 Worst Invasive Alien Species as determined by the ISSG, IUCN, Species Survival Commision and Bionet.
The Displaced
Yellow crazy ants can cause widespread devastation in the areas they infest. Hence it is possible that they will cause the displacement of many species. They spit formic acid as a weapon for both protection and hunting. Although individuals are small, they can build their population numbers very quickly and overwhelm larger creatures by massing in huge numbers. The image of seabirds on the right (Photo credit: Sheldon Plentovich, Pacific Islands Fish and Wildlife Office) shows the life threatening damage that can be inflicted on prey many, many times bigger than the ants themselves. Birds who have received injuries in this way are not likely to be able to sustain themselves in the wild as they will be at a disadvantage when it comes to catching fish successfully and avoiding predators.
The ants' generalist diet and their near constant territory expansion make them a formidable enemy, as does their reproductive capacity.
The Consequences
Where is the invasion?
This destructive species is widespread through Oceania and Asia. It has also spread to parts of Africa and South America. Any tropical forest in the world could potentially support them.
Christmas Island
The introduction of yellow crazy ants to Christmas Island in the Indian Ocean is a well documented example of the effect of introducing Anoplolepis gracilipes into a new environment. This island has an area of 135 square kilometres and lies near the southern edge of the equatorial region. It has a tropical monsoon climate.
A. gracilipes arrived on the island sometime in the period 1920s to 1930s. It took decades for them to build up their number and this was helped by the introduction of a foreign scale insect that the ants could farm for sugar dew. Supercolonies of the ant emerged. A supercolony can contain more than 7 billion ants with multiple queens present.
The invader ants displaced native ants due to aggressive attacks and sheer weight of numbers.
The ants become a formidable pest and eliminated millions of native land crabs on the island which caused extremes changes to the local flora. The crabs were eating the leaf litter in the forest and their removal meant that the profile of the forest was changed dramatically. The understory was no longer kept clear, its microclimate was changed and movement by larger animals was now blocked. A further habit of the ants also had a powerful impact by farming scale insects for their nectar exudation. The scale insects suck sap from the plants the infest and weaken their vitality and viability. The ants also attacked local invertebrates, amphibians and small reptiles which destabilised food chains at a number of levels. They even attacked nesting birds as shown above which further disrupted the local food web and systems further afield as many of these birds were migratory species.
Removing the invaders
For a number of years, the main method of combating the ant invasion on Christmas Island was spraying insecticide.
Biocontrol is now regarded as a more effective treatment. Given that the yellow crazy ants depend on the introduced scale insects for their sugar input, scientists decided to target the scale insects to bring the ant numbers down. A tiny wasp that lays its eggs in the scale insects' bodies was introduced. Unlike many other failed attempts at biocontrol, research was carried out over a 5 year period to ensure that this wasp would be specific in its target species and would not cause a new set of problems. Unfortunately, this solution is only effective in this one situation due to the single species of scale insect present. Other infestation sites need to develop their own biocontrol pathways.