What makes these ants so successful?

Social organisation is one of the key factors that contributes to the success of ants as invaders. Their advanced social system has gifted them a number of unique ways that enhance survival and success.

These benefits includes group protection against predators, organised resource exploitation, aggressive colony defence and buffering against environmental changes. 

In addition to sociality, invasive ants share a suite of other features which set them apart from non-invasive ant species.  These characteristics enhance their ability to survive and thrive in new habitats.

Colonies have many queensColonies spread by buddingAssociation with people and tradeInvasive ants form supercolonies : Aggression towards other ants : Helpful relationships with plant pests

These traits all combine together to enable invasive ants to reach super-high abundances. Typically the abundance of invasive ants is what makes them such a problem for agriculture, the environment and economies. 

Colonies have many queens

Typically, an ant colony consists of a single fertilized female (the queen) attended by her daughters, which are sterile female workers.

From time to time, winged (alate) males and females are produced. These males and females normally leave the nest to mate and start new colonies.

Invasive ants often have multiple queens per colony (polygyny), which increases reproductive output and leads to larger colony size.

Most ant species do not have multiple queens but almost all invasive ants do.

Queen ants are responsible for all reproduction in the colony. The number of queens is an important factor in the problems invasive species cause, because usually the more queens there are, the more workers there are and the worse the problem is.

Also, if a queen dies the colony continues to be productive.

 

Colonies can have many queens. This tiny colony fragment of Pharaoh ants alone has five queens in it! (© Quah, Termites and Ants of Malaysia) Little fire ants can have as many as 55 queens per square metre!

The number of queens also has important implications for management of these ants - all the queens have to be killed to eradicate the colony, and the more queens there are, the more difficult this will be.

An extreme example of polygyny is the little fire ant.  In its invaded range, this species can be present in extraordinary densities of more than 220,000,000 individuals per hectare. And as many as 550,000 queens per hectare or 55 queens per square meter!

High queen densities make it very difficult to control this species.  Survival of even a small portion of Little Fire Ant queens allows the colony to recover from treatment in a few weeks.

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Colonies spread by budding

What is budding? Budding is when a queen takes a bunch of workers and walks off to start a new nest or colony.

Ants have many ways of starting a new colony, but simply, this either involves queens flying to start a new colony after they have mated with a male. Substantial risks are associated with this activity as the queens are exposed to predators, parasites and attack from enemies in the search for a suitable habitat and their subsequent attempt at producing the first clutch of workers.

A queen (bottom centre) Argentine ant surrounded by workers on the move to a new location (© Phil Lester)
  The other way is by budding. Reproduction in invasive ants often happens by budding. Budding results in extremely high densities of nests and large numbers of workers, close to each other because all the reproductive effort of the new queen gets added to the original colony.

Budding has a far lower risk of failure, because many of the causes of mortality, are eliminated. There is a high likelihood that suitable nest sites will be found close to the original nest. The invasion can expand and fuse to form a dense population and a continuous invasion front.

However, this can make management of ants easier than for ants that fly, as budding makes it difficult to spread far from the original nest.

All of the "five worst" ant species spread predominantly by budding (except monogyne colonies of the red imported fire ant - these ants spread by winged queens flying up to 2 km).

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Association with people and trade

Invasive ants are often closely associated in a number of different ways.

For spreading distances of more than a few metres, invasive ants rely on accidental transport by people, also called jump-dispersal. These species often hitchhike on personal possessions, commercial goods or produce to establish colonies in new locations.

Long range spread of invasive ants is often promoted by trade.

Polygyny and budding both increase the chances of ants being spread to new locations by humans because colony fragments are more likely to have a queen in them.

Colony fragments containing a queen and enough workers to start a colony are carried to new locations with cargo or in shipping containers.

This type of movement started as early as the 1600s when Spanish galleons traded between the Philippines and Mexico, bringing with them tropical fire ants.

The Hawaiian archipelago contains no native ants, yet over 50 ant species have arrived and established since European contact.

 
Invasive ants can easily be accidentally moved to new locations by people (© Jerry Schappert, The Bug Doctor)

The African big-headed ant was carried throughout the Pacific region in the 1800s by ships carrying produce. It is no surprise that this species established so readily: as few as ten workers and a single queen can start a new invasion.  

It only takes a single queen to start a new colony, so checking for the presence of ants before items are moved (especially when moving things from an infested site to an uninfested site) is very important!

In fact, certain species, such as the Argentine ant, don't even need a queen to be present to begin a new colony- the workers can raise a new queen from brood (eggs or larvae) they are carrying. This makes good biosecurity protocols critical for preventing the unintentional spread of these ants.

Invasive ants also often like living in rubbish dumps and and among things that people leave behind. These areas often provide resources for the ants to nest and find food in. Making sure rubbish is treated properly is an important part of controlling invasive ants.

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Invasive ants form supercolonies

Perhaps most important to the success of invasive ants is that they form supercolonies. This trait is also called unicoloniality, meaning the ants all form one (uni) colony. 

Supercolonies have incredibly high numbers of workers. The orange in the images is thousands upon thousands of yellow crazy ants gathering by a stream (© Frank Tedo)
  In supercolonies there is no aggression between worker from different colonies. Thus, energy that would have been spent defending territorial boundaries can be used for foraging and reproduction.

The loss of aggression in supercolonies often lead to invasive ants forming massive supercolonies with enormous numbers of workers.

It is these enormous worker numbers that ultimately allow these ants to dominate an area. One European supercolony of Argentine ants stretches over 6,000 km, from Italy to Spain!

Unicoloniality is closely associated with the trait of polygyny and the lack of aggression exhibited by worker ants towards neighbouring colonies. Unicoloniality offers many advantages to invading ants. 

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Aggression toward other ants

Invasive ants behave very aggressively towards other ants and other animals within their foraging territory. For example, Argentine ants frequently attack newly mated queens of other ant species and stop them forming new nests.

However, the most important conflicts occur at or near food resources. Argentine ants actively stop other ant species (and other animals) getting to food resources. As a result they are generally able to take the majority of resources to the detriment of competitors.

Some other ant species permanently relocate following  conflicts with invasive ants. This behaviour reduces competition for resources and as a result, makes it easier for the invasive ants to dominate.

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Smaller Argentine ants attack a larger native New Zealand ant (© Phil Lester)

Helpful relationships with plant pests

Many invasive ants benefit from helpful relationships, called mutualisms, with plant pests. Often Hemipteran insects (scales, mealybugs, whiteflies, aphids and planthoppers) are serious plant pests. The insects feed on the sap of plants.

Some ant species “farm” these plant pests, protecting them from predators and parasites, and moving them to good feeding locations.  In return, the ants receive honeydew, a liquid rich in sugars. This is why they referred to as sugar devils. The combination of ants and other insects can be devastating to plant health.

Bicoloured pennant ant farming mealybugs on breadfruit in Fakaofo, Tokelau (© Monica Gruber, Pacific Biosecurity)
  These helpful relationships are one reason that invasive ants are able to build huge populations. By caring for these insects, the ants are able to make use of new sources of energy.

The red imported fire ant does not often look for food on plant foliage, so it does not have relationships with insects above ground. But it is often associated with root mealybugs, using the sugars they produce underground.

The yellow crazy ant is able to completely change the structure of forests it has invaded. On Christmas Island in the Indian Ocean these helpful relationships leads to a “meltdown” of the entire ecosystem.

The little fire ant often tends common plant pests in native forests and agriculture.  This provides the considerable energy needed to sustain large population densities, and results in reduced agricultural yields. 

Without the presence of suitable "friends", Argentine ants cannot invade some habitats, suggesting that these mutualisms are vital for the invader.

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Information sources

Additional Information kindly collated by Cas Vanderwoude, Hawai`i Ant Lab

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Ulloa-Chacon, Cherix. 1990. The little fire ant Wasmannia auropunctata (R. ) (Hymenoptera: Formicidae). In Applied Myrmecology A World Perspective (eds Vander meer, Jaffe, Cedeno), pp 281-289. Westview Press

content reviewed by Souad Boudjelas, Pacific Invasives Initiative, November 2016

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