Sugar devils

Browsing ant collecting honeydew from aphids (© Dawidi, CC BY-SA 3.0) 

Many invasive ants love sugar.

Sometimes the ants get these sugars directly from plants - from flowers and from what are known as extra-floral nectaries (EFNs). EFNs are sources of nectar outside the flowers. Some plants that have these EFNs also have helpful relationships with ants. The ants take the sugar from the EFNs and they protect the plants from pests. This is called a mutualism: both species benefit from the relationship. 

In Samoa, yellow crazy ants benefit from noni / nonu (Morinda citrifolia), which bears extra-floral nectaries. If you want to know if yellow crazy ants are in your area - check out the noni!

Ants also have mutualisms with plant pests, and this helps them get sugar from plants indirectly. Often hemipteran insects (scales, mealybugs, whiteflies, aphids and planthoppers) are serious plant pests. The insects feed on the sap of plants. They excrete left over sugars and this is what the ants love.

Some ants even “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.  The combination of ants and other insects can be devastating to plant health.

These helpful relationships are one reason that invasive ants are able to build huge populations. By caring for these plants and insects, the ants are able to make use of new sources of energy which fuel higher worker abundance and a high rate of activity and aggression.

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 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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Ant harvesting honeydew from a leafhopper nymph (© Fir0002/Flagstaffotos, CC BY-NC)

The noni / nonu plant is widespread across the Pacific and a favourite of the yellow crazy ant, seen collecting nectar from it here (© Monica Gruber)

If mutualisms are helping to drive the problems ants are causing, dealing with these pests is a critical part of the control programme. PestNet is an email network that helps people worldwide obtain rapid advice and information on crop protection. Topics include pest identifications from digital images, pest outbreak alerts, pest management (biological, cultural, and chemical), and quarantine interceptions. Subscribe to PestNet to ask questions.

PestNet also has a comprehensive range of resources for managing pests, including a range of useful factsheets that offer solutions to controlling these types of mutualists (e.g. mealybugs, scale insects) in the Pacific

Yellow crazy ants on Christmas Island, Indian Ocean

An extreme example of the consequences of an invasive ant-insect mutualism is the yellow crazy ant and yellow lac scale insect (Tachardina aurantiaca) on Christmas Island.

The yellow crazy ant was present on Christmas Island for 50 years before it began causing problems. Some time in the early 90’s the population exploded and soon yellow crazy ant supercolonies covered 25% of the rainforest on the island. In the midst of a supercolony, population densities can be over 2000 workers per square metre!

Yellow crazy ants tending yellow lac scale insects (© Pete Green)

Red land crabs are abundant in areas not invaded by yellow crazy ants (© Monica Gruber)

This population explosion coincided with the arrival of a scale insect from southern Asia, which produces lots of honeydew. These scale insects provide the yellow crazy ants with a steady source of carbohydrate resources allowing them to achieve huge worker abundances.

The yellow crazy ant also benefits the yellow lac scale by protecting it from predators in return for honeydew. Abundances of scales can be so great they completely coat tree branches. 

One of the yellow lac's favourite host trees is the Tahitian chestnut (Inocarpus fagifer), a tree highly valued in the Pacific for its food value, medicinal properties and cultural values. The high ant densities are very hard on the trees and cause these canopy trees to die.

Researchers from La Trobe University have investigated how yellow crazy ants and yellow lac scale depend upon each other. They found that they could stop yellow crazy ants from harvesting honeydew by blocking their access to the canopy where the scale lives.

Yellow crazy ant numbers dropped by 80% in only a few weeks compared to control areas where the ants could collect honeydew freely. In another experiment, yellow crazy ants were removed from areas using pesticide bait and the scale population densities monitored. Within 12 months all the scales in areas where yellow crazy ants had been removed were dead.

The yellow crazy ant population explosion has been a disaster for the rainforest ecosystem on Christmas Island. Yellow crazy ants prey on the native red land crab, and completely eliminate them. Between the late 1990s and early 2000s it’s estimated that these ants killed between 10-15 million land crabs, or between a quarter and a third of the total population.

As red land crabs are the dominant consumer on the forest floor, the ant is also indirectly changing forest structure on the island. Usually the red land crabs keep the forest floor clear by eating most new seedlings. Without them, the forest becomes very dense, and more introduced tree species survive and thrive. The absence of the crab and increased leaf litter has also helped another invader, the giant African land snail, which has increased 250-fold in yellow crazy ant invaded areas.

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A site without yellow crazy ants (© Pete Green)
A site infested with yellow crazy ants. Without the red land crabs the vegetation grows unchecked (© Pete Green)
Red land crabs can not survive in areas with yellow crazy ants (© Director of National Parks)

Currently a management programme is in place on Christmas Island to control yellow crazy ant numbers. This programme has prevented further spread of yellow crazy ants and has allowed the red crab population to slowly recover.

A biological control programme has also been started to control the yellow lac, in the hope that this will also control the yellow crazy ant and, in turn, the African snail. Hopefully the environment will return to a more natural state. In 2018 the biological control efforts were starting to show results, and were controlling the lac insects. 

Cases like Christmas Island show the importance of mutualisms to invasive ants. These mutualisms should be considered when doing biosecurity risk analyses, and when protecting your borders. For example, if your country has yellow crazy ants at low abundance, but no yellow lac scale, then special care should be taken to watch out for the scale, as its introduction could be a catastrophe, causing population explosions of both species.

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Information sources

Many thanks to Pete Green for additional information.

Abbott. 2005. Supercolonies of the invasive yellow crazy ant, Anoplolepis gracilipes, on an oceanic island: forager activity patterns, density and biomass. Insectes Sociaux 52: 266-273

Abbott, Green. 2007. Collapse of an ant-scale mutualism in a rainforest on Christmas Island. Oikos 116: 1238-1246

Boland, Smith, Maple, Tiernan, Barr, Reeves, Napier. 2011. Heli-baiting using low concentration fipronil to control invasive yellow crazy ant supercolonies on Christmas Island, Indian Ocean. In: Island invasives: eradication and management (eds Veitch, Clout, Towns), pp 152-156. Gland, IUCN

Davidson. 1997. The role of resource imbalances in the evolutionary ecology of tropical arboreal ants. Biological Journal of the Linnean Society 61: 153-181

de Souza, Delabie, Fowler. 1998. Wasmannia spp. (Hym. Formicidae) and insect damages to cocoa in Brazilian farms. Journal of Applied Entomology 122: 339-341

Delabie, Cazorla. 1991. Damages caused by Planococcus citri Risso (Homoptera pseudococcidae) to the production of cocoa tree. Agrotropica 3(1): 53-57

Fasi, Brodie, Vanderwoude. 2013. Increases in crop pests caused by Wasmannia auropunctata in Solomon Islands subsistence gardens. Journal of Applied Entomology 137: 580-588

Green and O’Dowd 2009. Management of invasive invertebrates: lessons from the management of an invasive alien ant. In: Invasive species management: a handbook of principles and techniques (eds Clout, Williams) pp. 153–172. Oxford, UK: Oxford University Press

Green, O'Dowd, Abbott, Jefery, Retallick, Mac Nally. 2011. Invasional meltdown: invader-invader mutualism facilitates a secondary invasion. Ecology 92(9): 1758-1768

Helms, Vinson. 2002. Widespread association of the invasive ant Solenopsis invicta with an invasive mealybug. Ecology 83(9): 2425-2438

Holway, Lach, Suarez, Tsutsui and Case. 2002. The causes and consequences of ant invasions. Annual Review of Ecology, Evolution, and Systematics 33: 181-233

Invasive species council,

Maple, O'Dowd, Green, Neumann, Wittman. Christmas Island yellow crazy ant control program: moving from chemical control to a biological control future. Brochure from Parks Australia, Christmas Island National Park, La Trobe University

Natural Resources Defense Council (NRDC),

O’Dowd, Green, Lake. 2003. Invasional 'meltdown' on an oceanic island. Ecology Letters 6: 812-817

O'Dowd, Green, Lake. 1999. Status, impact, and recommendations for research and management of exotic invasive ants in Christmas Island National Park. Report to Environment Australia

Rowles, Silverman. 2009. Carbohydrate supply limits invasion of natural communities by Argentine ants. Oecologia 161: 161-171

Savage, Johnson, Whitney, Rudgers. 2011. Do invasive ants respond more strongly to carbohydrate availability than co-occurring non-invasive ants? A test along an active Anoplolepis gracilipes invasion front. Austral Ecology 36: 310-319