Ants

Ants are ubiquitous, occurring throughout most of the world, including most oceanic islands. The only places that they do not exist is above the tree line in mountainous areas and in the Antarctic. Most groups of ants have species that occur worldwide. However, some groups and subgroups are restricted to specific areas; the acacia ants (pseudomyrmecines) and ponerines, for example, occur primarily in the tropics, and bull-dog ants are found only in Australia. The primarily tropical army ants have a limited distribution in the United States, with two species occurring as far north as Iowa.

The only ants of significant medical-veterinary concern in the United States are the fire ants (Solenopsis and Wasmannia spp.) and harvester ants (Pogonomyrmex spp.). Most other North American ants rarely sting people, or they are so small that they are incapable of piercing human sldn. The carpenter ants (Camponotus spp.), which are commonly destructive wood pests, lack a sting, as do members of the subfamilies Formicinae and Dolichoderinae. Other ants such as^o»m»ei(Ponerinae) and army ants (Ecitoninae) are a concern in tropical regions. However, there is one ponerine species occurring in the southeastern United States, Odontomachus haema-toda, that can cause painful stings to humans. This peculiar ant possesses elongated mandibles that are held open at 180°, and these snap shut quickly to impale prey or enemies on their sharp teeth. It also uses its mandibles to snip and to jump away when threatened.

The life cycle of ants is highly varied. In some species, such as the army ants, whose queens are wingless throughout their life, colony initiation occurs by budding, a process whereby a colony divides into two colonies. In contrast, many ant species have winged reproductives, and colony initiation is typically by a single winged queen. At certain times of the year mature colonies produce an abundance of winged males and queens that leave the nest en masse on a nuptial flight. After mating, the males die and the inseminated queens lose their wings before searching for a suitable nest site. The queen lays eggs in the new nest site and feeds the developing larvae from her food reserves stored as fat and flight muscle. The emerging brood become the workers and take over nest maintenance, foraging, and nursing activities. The small colony grows slowly at first and may take a few years to become mature and produce its own reproductives. A single queen is the rule in some ant species, whereas multiple queens occur in others. Some species, such as the imported fire ants, have both monogynous and polygynous colonies.

Ants nest in a variety of situations. In the case of army ants, there is no physical nest, but only a bivouac formed from the ants themselves holding on to one another by their legs to form a large mass. Carpenter ants excavate wood to form cavities for their nest, while many ants make aerial nests of carton, a material formed from soil or plant fibers and saliva. Some Formica species build large nests up to 1 m high consisting of a mound of small twigs, hence the name "thatching ants." Most ants establish their nests in soil, where they excavate extensive galleries and tunnels. Soil is an ideal nesting material as it moderates temperature extremes, holds moisture, and can be easily shaped by the ants into brood- or food-holding chambers. The various groups of ants have definite preferences for specific types of soil.

Ants can have tremendously populous colonies. Primitive ants tend to have only a few hundred workers (e.g., some ponerines), while other ants may have up to 10,000 workers in their colonies (e.g., harvester ants). The enormous populations of 1 —2 million for army ants

FIGURE 19.5 Red imported fire ant (Solenopsis invicta) worker, srin ging human skin. The ant seizes the skin between its mandibles to provide leverage as it flexes the tip of its abdomen forward to penetrate the skin with its sting. (Original by Blair Sampson.)

and 22 million for driver ants (Dorylinae) are impressive. However, colonies of these species are small compared to megacoknies of some Formica species, which may contain 300 million workers and nearly 1 million queens occupying an area of a few square kilometers. Such polygynous colonies are usually very tolerant of nonnestmate ants, and it is very difficult to determine whether such colonies are separate units or indeed one giant ant colony.

Fire ants (Solenopsis species,)

Most ant stinging problems in North America are due to the two species of imported fire ants in the southern United States: Solenopsis invicta (Fig. 19.5) and S. richteri (Fig. 19.6). Less frequent stinging problems are caused by the two native fire ants in the southern United States, S. geminata and 5. xyloni, and the little fire ant Wasmannia auropunctata.

The black imported fire ant (S. richteri) was introduced from South America into the United States at

FIGURE 19.6 Black imported fire ant (Solenopsis richteri), worker. (Courtesy of US Department of Agriculture.)

Mobile, AL, in the early 1900s, followed by the introduction of the red imported fire ant (S. invicta) at the same port in the late 1930s. Since that time few other stinging insects have created more controversy, generated more research, or received more publicity than these two species of ants. They now inhabit a major portion of 12 southern states (Fig. 19.7), with S. invicta occupying 95% of the infested area; S. richteri occurs only in parts of Mississippi, Alabama, and Tennessee. The distributions of these ants probably have reached their northernmost limits where they can survive in the central and eastern United States. There is, however, a possibility that these ants will become established in the western coastal states as far north as Washington. Climatic conditions on the West Coast are suitable, and the westward spread of these ants has been impeded only by the arid regions of western Texas. The red imported fire ant was reported in Nevada in 1993 and in Arizona in 1994. More recently, it has invaded California and Oklahoma.

Fire ants are omnivores and opportunistic feeders. They feed mostly on insects and other arthropods. Fire ants readily feed on seeds of some plants and can affect local plant assemblages by transporting viable seeds of other plant species. They also feed on germinating plants, as well as on fruits and roots, and can cause further damage by girdling tree seedlings.

Fire ants are soil nesters, with most colonies being initiated by a single inseminated queen after a nuptial flight during April—August. A queen makes a burrow 3—12 cm deep and within 24 hr lays her first eggs in a chamber at the end of the burrow. As many as 2,500 colonies can be initiated per hectare, but few of these incipient colonies survive the next winter. Colony growth is rapid and often produces more than 10,000 workers within a year. Some colonies contain as many as tens of thousands to hundreds of thousands of workers within a few years. Polygynous colonies are fairly common. Broods may be produced year-round in the southernmost distributions of the ants, but brood production ceases during the winter months north of 30° N latitude. The size and texture of the above-ground mound is variable depending on soil type, moisture, and vegetation (Fig. 19.8). Mounds in sandy areas are generally flat, whereas those occurring in clay soil may be up to 1 m tall by 1 m in diameter. Large colonies often construct several interconnected mounds.

Fire ants quickly respond to disturbances of their nests and attack intruders in force. When a colony is disturbed, the ants swarm over the intruder until the first worker stings and alarm pheromones are released, This triggers stinging behavior in other workers. The workers grasp the skin with their mandibles

houses and infest clothing, beds, and food. Laborers sometimes refuse to work in cropland where these ants are abundant.

For a comprehensive review of fire ants in the United States, see Taber (2000).

Harvester ants (Pogonomyrmex species)

As their common name implies, these ants all regularly use seeds as part of their diet. In addition, Pogonomyrmex workers scavenge for dead arthropods. Although there are a number of genera in the subfamilies Ponerinae, Myrmicinae, and Formicinae that comprise the harvester ants, only Pqgonomyrmex species are of particular stinging concern in North America. Several species occur in North America, but probably only seven species constitute a stinging hazard (Cole, 1968). The more common ones are the western harvester ant (Pogonomyrmex occidentalis), the red harvester ant (P. barbatus), the California harvester ant (P. calif amicus), and the Florida harvester ant (P. badius).

Pogonomyrmex workers (Fig. 19.9) are large, up to 10 mm in length. Most are light red or brown, although the gaster of some species may be dark brown to black (Cole, 1968). These ants are identified by the presence of a psammophore, a fringe of hairs on the underside of the head. These "beards" are used in nest excavation to push material from the nest much like the blade of a bulldozer. Harvester ants are usually slow-moving and cannot walk up slippery vertical surfaces such as glass. Dense concentrations of colonies are common in the western United States, where most species occur.

Harvester ants construct their nests in dry, sandy to hard soils. The entrance to the nests is often marked by

FIGURE 19.9 Harvester ant (Pogonomyrmex sp ), workers gathering food. (Photo by R D, Akre.)

a crater or a cone in the center of a slight mound. A pile of small stones usually surrounds the entrance. Some species in hot deserts lack a mound. The nest can be 1— 10 m in diameter, with tunnels extending down to 5 m or more. The area around the nest is usually completely devoid of vegetation. Colonies of some species have up to 10,000 workers. Individual colonies often survive for 14—50 years, reaching maximum densities of 80 or more nests per hectare. Foraging trails from individual nests may extend out 60 m. Where nest densities are high, large expanses of ground may have little vegetation. Because of this habit of harvesting seeds and reducing vegetation, they can damage rangeland used for cattle grazing and sometimes become significant pests locally (MacKay, 1990). Nests invariably occur in sunny locations; if the nest becomes shaded by vegetation or human activity, the ants generally move.

Harvester ants sting readily and can inflict intense pain. The incidence of stings is low, however, because their relatively large size and conspicuous nests cause most people to avoid them. Also, their colonies are relatively small compared to some other ant species. An additional source of stings has been ant farms sold in stores throughout the United States. Ant-farm kits usually contain coupons to be exchanged for live ants. In some cases, Pogonomyrmex species have been shipped for this purpose. Children have been stung severely when the plastic ant farms containing these ants have been dropped or otherwise broken open.

Pavement ant (Tetramorium caespitum)

This ant (Fig. 19.10) was introduced to North America from Europe. At one time it was especially common only along the Atlantic seacoast, but now it is very common throughout North America. Nests are usually located in soil, but also commonly occur in houses. The pavement ant is omnivorous, being particularly fond of meats and fatty substances. Although it can cause extensive damage to many cultivated plants, it is more of a nuisance than a stinging problem for homeowners.

FIGURE 19.10 Pavement ant (Tetramorium caespitum), worker. (Courtesy of the US Department of Agriculture.)
FIGURE 19.11 Pharaoh ant (Monomorium phantoms), worker. (Courtesy of the US Department of Agriculture.)

This small ant (3—3.5 mm long) usually is not capable of penetrating human skin with its sting. Nevertheless, stings have been reported to cause a skin rash in children, but only rarely have they stimulated serious allergic reactions.

Pharaoh ant (Monomorium pbaraonis)

This tiny ant (Fig. 19.11) is only about 2 mm long and nests in every conceivable habitat: in soil, in houses, between sheets of paper or linen, and in trunks of clothing, to mention only a few. The pharaoh ant probably is native to Africa and has been widely disseminated by commerce so that it is now found throughout most of the world. It occurs in nearly all cities in the United States. It forms huge, poiygynous colonies of more than a million workers and produces brood year-round. This ant is omnivorous, feeding on sugary materials, dead insects, breads, and many other foodstuffs, and, like the pavement ant, it prefers meats and fats. It is among the more difficult of all ants to control.

In addition to being a concern to human health because it stings, the pharaoh ant has been known to infest surgical dressings and intravenous units in hospitals and will attack the delicate tissues of newborn babies, especially the eyelids and navel. Because this ant has been found in bedpans, drains, and washbasins, it can come into contact with disease organisms and has been implicated in the transmission of pathogenic organisms in some hospitals (Edwards, 1986). In tropical regions other ant species have also been implicated as vectors of pathogens in hospitals (Fowler et al1993).

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