Red fire ants

Red fire ants are one of a number of highly invasive ant species which have infiltrated countries around the globe, causing havoc to a number of ecosystems. In the U.S.A., they are commonly called red imported fire ants to distinguish them from native fire ants.

The Misplaced

Solenopsis invicta, the red fire ant, is a species of ant native to South America. According to AntWiki, it is classified as highly invasive.

As with many ants species, three forms of the ant - workers, fertile males and queens are present. The worker ants are wingless unlike the queens and the fertile males. The ants range in size from 2 to 6 millmetres. Note the scale on the image below. The workers are divided into major and minor types. Minor workers tend to the eggs and larvae. Major workers forage for food. All workers are reddish brown in colour with a darker abdomen.

Queens are up to 9 mm long.

Diet and Tolerances

Feeding

Red fire ants are omnivorous. They will eat plants, fungi, other arthropods and vertebrates in early vulnerable stages e.g. bird nestlings. They will also eat corpses.

These ants also "farm" aphids in order to consume the sweet exudate the aphids produce from plant sugars.

For details of their diet, a good description can be found at What do fire ants eat?

Environmental conditions

The highest habitat temperature tolerable for this species is around 49.0C, and foraging was possible 18 hours a day in summers.

It is thought that a minimum rainfall of 510 mm per year is needed to sustain populations, but if water is available via irrigation, then the species range can be extended.

Ideal fire ant locations have dry, sunny and flat conditions. Loose earth is preferable for mound construction.

Reproduction and population growth

The lifespan of red imported fire ant workers depends on their size. Minor workers may live 30 to 60 days, major workers 90 to 180 days, and queens may live two to six years. Complete lifecycle from egg to adult takes between 22 and 38 days (Hedges 1997)

There are two colony forms – colonies with a single egg-laying queen (monogyne) and those with multiple reproductive queens (polygyne). The multi-queen colonies (sometimes with several hundred queens) reach higher densities than single-queen colonies – up to 50 million ants per hectare. In polygyne colonies, a new queen mates within the nest and then sets up a new nest just metres away. In the monogyne form, the virgin queens and the males mate in the air. Queens have been reported flying as far as 30km (with an assisting  wind) to build a new nest.

Dispersal

Fire ants have the reproductive capacity and behavior patterns to spread fast once they arrive in new territory.

Comment on speed of invasion over a 5 year span in China:

The South American insect has spread to at least 435 counties in 12 provinces, around double the number of counties where it was found in 2016, according to the ministry’s latest figures.((https://www.caixinglobal.com/2021-04-01/chinas-farmers-battle-surging-invasions-of-crop-munching-fire-ants-101684818.html))

Red fire ants are able to increase their distribution from the invasion point in a number of ways.

• They actively roam looking for new territory to take over. They can travel above ground and underground.
• Red fire ants will also nest under logs, garden materials, or in hay or mulch.
• They readily use home gardens and sometimes enter houses where they are attracted to electricity, so they will crawl inside computers, air conditioning units and electrical switch units. The ants also gather inside outdoor infrastructure such as meters and pumps.
• Fire ants will hitch rides with  any vehicle to which they have access e.g. freight haulers, garden supply trucks, agricultural machinery
• Flooding is disastrous for many invertebrate species, but fire ants can form rafts of ants and debris and float to new territories.

Where is the invasion?

Red fire ants are native to South America, but have spread to the United States, China, Taiwan, Japan, the Philippines and Australia. In 2023, about 90 colonies were found in Italy. They are spread by stowing away on cargo boats and in air freight.

Current distribution

Potential distribution in the absence of control measures

The Consequences

The Displaced

The ants' generalist diet and their near constant territory expansion make them a formidable ecosystem disruptor, as does their reproductive capacity. Fire ants are aggressive and their ability to kill by stinging repeatedly allow them to quickly eliminate other species in areas where their numbers re not checked by natural enemies.

The damage done

Fire ants have greater ecological impacts than most ants because they reach extremely high densities. An assessment of their likely impact on 123 animals in southeast Queensland predicted population declines in about 45% of birds, 38% of mammals, 69% of reptiles and 95% of frogs. They swarm prey victims and overwhelm them with multiple stings from large numbers of ants.

These ants damage crops, rob beehives and kill newborn livestock. During dry times, they dominate the margins of dams and livestock cannot reach water without being seriously stung.

As well as being an environmental and health hazard, they cost US industry and agriculture a shocking $7 billion a year and if left unchecked, fire ants would have similar impacts in Australia.¹

They encourage the spread of sooty mold through their aphid farming.

They injure humans and their stings have resulted in deaths in some cases. In Australia, they could cause an extra 140,000 medical consultations and 3000 anaphylactic reactions a year.((Help stop fire ants in their tracks))

Removing the invaders

Competitors, predators, parasites and pathogens keep populations of red fire ants in check in the ecosystems in which they evolved.

It is only when a species is misplaced by human activity that problems arise.

In the past, attempted control of red fire ants has been carried out via poisoning, which has not been particularly selective, and requires  on-site treatments.

Australian chemical pest control case study

A chemical called Fipronil acts as an insect nerve agent and has been injected into 35,539 fire ant nests in south-east Queensland and northern NSW since January 2023 as part of the national red imported fire ant eradication program. This chemical is banned in the EU, California and parts of Asia because it is not selective and kills off many types of organisms in the infected ecosystems. Australian scientists have warned that this control method is just creating a second problem because as well as being non-selective, Fipronil can persist in the environment for up to 3 years. There is currently a parliamentary enquiry into it use, and accusations have been made by scientists and community representatives that "the government program tasked with their elimination is an “absolute shambles” and that an independent eradication body is urgently needed."((https://www.theguardian.com/environment/2024/mar/04/australian-program-to-eradicate-red-fire-ants-is-a-shambles-senate-inquiry-told))

Newer methods have examined the ways in which the are controlled in their native territory. Biological control is, if done properly, very selective and self sustaining.

Natural enemies of Solenopsis invicta include

• Several viruses attack red fire ants and research is determining which of these might be the candidate for use in biocontrol efforts.
• Some types of microsporidia fungi are known to attack Solenopsis.
• Certain species of flies, belonging to the genus Pseudacteon, are known to attack fire ants and parasitise them
• Predators such as armadillos, antlions, spiders, birds, and some lizards eat red fire ants. It is not likely that these animals would be able to carve out niches for themselves in the new environment.
• The tawny crazy ant is one of the few competitors that can match/beat the red fire ant for competitiveness and aggression.

For detailed information about this topic, see Natural Enemies of Fire Ants

Dig Deeper

• Predicting future invaders and future invasions
• Help stop fire ants in their tracks
• Solenopsis invicta
• Natural Enemies Close In on Fire Ants
• ‘Wildly toxic’ poison used on fire ants is killing native Australian animals, experts warn Senate inquiry
• Hedges SA. 1997. Handbook of Pest Control, 8th Ed. (Moreland D, editor) pp. 531-535. Mallis Handbook and Technical Training Company.

There are 4 ants on the list of the 100 most invasive species in the world.

More than 520 ant species have been found in places where they shouldn’t be — 60% of these naturalise², wreaking havoc on local ecosystems. Small organisms but big problems. Invasive ants are a bigger threat than we thought