Red Imported Fire Ants

Brief:

The red imported fire ant, Solenopsis invicta, or simply RIFA, is one of over 280 species in the widespread genus Solenopsis. Although the red imported fire ant is native to South America, it has become a pest in the southern United States, Australia, Thailand, Taiwan, the Philippines, Hong Kong, and the southern Chinese provinces of Guangdong, Guangxi and Fujian. There are also reports of anthills in Macau, the former Portuguese enclave that borders the province of Guangdong.

RIFAs compete successfully against other ants, and have been enlarging their range. By about 2013 though, colonies of old-world crazy ants (also known as rasberry crazy ant) have been introduced in the same ranges as RIFAs. These Crazy Ants are ecologically dominant against fire ants.

RIFAs were the first clear-cut case discovered of a green-beard gene, by which natural selection can favour altruistic behaviour.

Red imported fire ants have both a pedicel and postpediole. In other words, they belong to a group of ants that have two humps between the thorax and abdomen. The workers have ten antennal segments terminating in a two-segmented club. It is often difficult to distinguish between the red imported fire ant and some other species in the genus. A number of characters are used, but are not always consistent between the black imported fire ant (Solenopsis richteri) and hybrids between the two species.

Like other insects RIFAs breathe through a system of gas filled tubes called tracheae connected to the external environment through spiracles. The terminal tracheal branches (tracheoles) make direct contact with internal organs and tissue. The transport of oxygen to cells (and carbon dioxide out of cells) occurs through diffusion of gasses between the tracheoles and the surrounding tissue and is assisted by discontinuous gas exchange. As with other insects, the direct communication between the tracheal system and tissues eliminates the need for a circulating fluid network to transport O2. Thus, RIFAs and other arthropods can have a modest circulatory system even though they have highly expensive metabolic demands.

RIFAs face many respiratory challenges due to varying physical properties of its environment including increased desiccation, hypoxia, and hypercapnia. Hot, humid climates promote an increase in heart rate and respiration, which potentially increases water loss. Hypoxia and hypercapnia can result from RIFAs colonies living in poorly ventilated thermoregulatory mounds and underground nests.

Habits

Studies have been conducted on the sex ratios exhibited within colonies of RIFAs. More specifically, it was observed that the queen actually predicts the sex ratios. In an experiment, 24 field colonies were selected with highly biased sex ratios in a monogyne population. Eleven of these colonies were male specialists, and 13 were female specialists exchanging queens, twenty-two of the 24 colonies accepted the foreign queen, and 21 of these colonies produced a new batch of reproductive 5 weeks later.

It has been observed that RIFAs workers not only tend to queens indiscriminately, but they also indiscriminately attack them. After temporary cooperation associations end between queens, a queen who produced more workers gained no advantage over the less productive queens. Queens producing diploid males reared fewer offspring but were as likely to survive as queens producing only workers. It would have been assumed that if workers controlled queen mortality, they would be expected to discriminate in favour of their mother, therefore increasing their inclusive fitness.

Habitat

Red imported fire ants are extremely resilient, and have adapted to contend with both flooding and drought conditions. If the ants sense increased water levels in their nests, they will come together and form a huge ball or raft that is able to float, with the workers on the outside and the queen inside. Once the ball hits a tree or other stationary object, the ants swarm onto it and wait for the water levels to recede. To contend with drought conditions, their nest structure includes a network of underground foraging tunnels that extends down to the water table. Also, although they do not hibernate during the winter, colonies can survive temperatures as low as -9 °C.

Threats

Red imported fire ants are known to have a strong, painful, and persistent irritating sting that often leaves a pustule on the skin. An animal, including humans, typically encounters them by inadvertently stepping into one of their mounds, which causes the ants to swarm up the legs, attacking en masse. The ants respond to pheromones released by the first ant to attack and sting in concert, often killing smaller animals by overloading their immune systems.

Fire ants are excellent natural predators and are biological controls for pests such as the sugarcane borer, the rice stink bug, the striped earwig, aphids, the boll weevil, the soybean looper, the cotton leaf worm, the hornfly, and many other pests harmful to crops. However, they also kill beneficial pollinators, such as ground-nesting bee species. Seeds, fruits, leaves, roots, bark, nectar, sap, fungi, and carrion are all fire ant prey, and they are not shy about creating their own carrion, either. They are proficient enough at overwhelming intruders that they can virtually clear an area of invertebrates, lizards, and ground-dwelling birds.

They are a pest, not only because of the physical pain they can inflict, but also because their mound-building activity can damage plant roots, lead to loss of crops, and interfere with mechanical cultivation. It is not uncommon for several fire ant mounds to appear suddenly in a suburban yard or a farmer's field, seemingly overnight. Their stings are rarely life-threatening to humans and other large animals, causing only 80 documented deaths as of 2006. However, they often kill smaller animals, such as birds. They sometimes kill newborn calves if they do not get on their feet quickly enough. The sting of the RIFA has venom composed of a necrotizing alkaloid, which causes both pain and the formation of white pustules that appear one day after the sting.

An outbreak of the RIFAs in Queensland, Australia, was discovered on 22 February 2001. The ants were believed to be present in shipping containers arriving at the Port of Brisbane from the United States. The initial emergency response was followed by the formation of the Fire Ant Control Centre in September 2001. Joint state and federal funding of A$175 million was granted for a six-year eradication program involving the employment of more than 600 staff and the broad-scale baiting of approximately 678.9 km2 between 8 and 12 times, followed by two years of surveillance. Following the completion of the fourth year of the eradication program, the Fire Ant Control Centre estimated eradication rates of greater than 99% from previously infested properties.

Prevention

The key to fire ant control is to locate all mounds and treat them and then prevent their reoccurrence. Mounds can be treated by drenching with liquid insecticides or by baits containing Fipronyl. Fipronyl is carried by back to the ant mounds by the workers and then fed to the ant population. Fipronyl is then spread throughout the colony by means of food sharing by the worker ants. This slowly wipes out the entire population including the queens. Fipronyl is non-repellent to the ants, which means that they cannot sense that there is a pesticide, or toxicant present, which greatly encourages feeding and the consumption of the bait. An outdoor perimeter application of a liquid insecticide helps to prevent their entry into homes.

Others

Some photos and information are provided by WR Gay Pest Control Pty Ltd and Bayer Environmental Science.

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