Solenopsis geminata
(tropical fire ant)

Relative to other invasive fire ant species such as Solenopsis invicta, the tropical fire ant, S. geminata, has a greater world-wide distribution. Where introduced, it has often become one of the dominant pest ant species, affecting fauna and flora (Hoffmann et al., 1999; Geetha Viswanathan and Ajay Narendra, 2000).

This species is native to the New World, including the Caribbean islands such as Puerto Rico. It has spread world-wide, in some cases to environments where the conditions are unfavourable for this species to survive outdoors such as Winnipeg, Canada (Ayre, 1977).

The phytosanitary risk is high for areas that do not already have this species and are located in warmer climates.

S. geminata generally nests in the soil of suitable environments.

S. geminata is omnivorous, but among the fire ants is known more as a seed-feeding species. The morphologically distinctive major worker ants are specialized in collecting and feeding on the seeds of many plant species, including weeds (Wilson 1978; Carroll and Risch, 1984; Tennant and Porter 1991; Sangal and Neena Bakshi, 1994). Although this ant is reported to occur in a number of crops (avocado, banana, cabbage, citrus, cocoa, corn, cotton, coffee, cucumber, eggplant, mango, okra, papaya, pastureland, pineapple, sorghum, soybeans, strawberries, sugarcane, tobacco, tomato, rice and vegetables) and in cultivated land, it is seldom reported to be a direct pest of these crops.

Seed loss to crops such as cabbage, corn, sorghum and tomato can be significant. However, this species also girdles plants, imbibes sap, and bites branches, shoots, buds, flowers and fruits (Hill, 1975). The ants in nests that are built at the base of citrus tree trunks gnaw through the bark (Essig, 1926; Wolcott, 1933). Indirectly, S. geminata can negatively affect plant health by tending sucking insects (Homoptera), or in one case, prevent pollination (Carroll and Risch, 1983). However, because these ants sting, they can affect field workers that are sensitive to arthropod venom, and can affect equipment such as drip irrigation systems (Ota and Chang, 1981) and electrical equipment (Prins, 1985). In addition, they occur in urban areas (such as in turf and occasionally enter buildings), wildlife areas, forests and other sites suitable for the species.

The research of S. geminata is fairly extensive, particularly from locations where it is an introduced exotic species. In these locations, S. geminata often becomes one of the more dominant ant species, causing many problems similar to those caused by Solenopsis invicta (Porter et al., 1997).
The biology of S. geminata is similar to other closely related fire ant species. Wilson (1978) described caste behaviour. The head capsules of the major worker ants are distinctly larger and lobed relative to those of minor workers because of their specialized seed foraging and milling behaviour.

The colonies of S. geminata can either be monogyne, each containing a single queen ant, or polygyne, containing many reproductively-active queen ants (Adams et al., 1976; MacKay et al., 1990; Williams and Whelan, 1991).

Genetics

Ross et al. (1987) has evaluated the genetics of S. geminata and other fire ant species. This species has been known to hybridize with other closely related fire ant species; Morrison (2000) discusses the hybrid Solenopsis geminata x Solenopsis xyloni.

Physiology and Phenology

The pheromones known for S. geminata are discussed by Blum (1982) and Jaffe et al. (1985), and considerable work has been carried out to characterize the ant's venom (e.g. Battenfield et al., 1982).

Reproductive Biology

The winged reproductive male and female S. geminata leave the nests on mating or nuptial flights, late in the day or at dusk on days of or following rains during warmer periods of the year (Travis, 1941; Hung et al., 1977; Trager, 1991). Two forms of queen ants are recognized and have different mating strategies (McInnes and Tschinkel, 1995). Following mating, the female reproductive ants or queens drop their wings and dig a burrow to start a new colony. The queens begin laying eggs and can lay up to 1123 eggs per day (Travis, 1941). Development from egg to adult can take from 3 to 4 weeks and up to 2 months (Wheeler and Wheeler, 1955; Vargo, 1993). Egg hatch occurs in 14 to 17 days, larval development in 24 days to 6 weeks and the pupal stages last up to 19 days (Veeresh, 1990a). The worker ants include minor workers (2.6 mm larvae), major workers (5.2 mm larvae) and intermediate sized workers. Colony maturity occurs in 1 to 2 years (Wilson and Taylor, 1967). The colonies have been reported to contain up to 100,000 ants (Way et al., 1998), but other estimates are lower, generally ranging from 10,000 to 12,000 (Veeresh, 1990a, b).

S. geminata, like other ants, are social insects. This species generally builds nests in the soil in open, sunny areas, although they can also occur in the shade and inside structures. The nests are formed as craters or mounds in which the ants seek optimum temperatures for reproduction. At certain times of the year, the nests have multiple entrance holes (16 to 20 per nest) in mounds up to 2 ft wide and 12 inches tall (Van Pelt, 1958), although most mounds are not as tall or even crater-like. The subterranean foraging tunnels (up to 3 inches deep) extend from the nest in all directions and may be 100 ft long, with vertical tunnels extending up to 6 ft deep in the ground (Wheeler, 1910). The density of colonies in areas varies greatly and can range from 10 mounds per acre to 80 mounds per acre in native habitats (McInnes and Tschinkel, 1995), but in some areas the polygyne forms of the ant can reach up to 1000 mounds per acre (MacKay et al., 1990). In India, mound densities have been reported as 1500 mounds per acre (Veeresh, 1990a). The colonies frequently migrate to new locations (Lakshmikantha et al., 1996), migrating over 100 ft from the original nest locations.

Environmental Requirements

S. geminata often colonizes disturbed habitats (Risch, 1981; Perfecto, 1991). It is capable of colonizing most types of soils and media. It occurs in shaded orchards and woods (Essig, 1926; Wilson and Brown, 1958), as well as open areas. Habitat types vary greatly (Wheeler, 1910; Travis, 1941; Creighton, 1950; Van Pelt, 1958; Whitcomb et al., 1972; Buren et al., 1974; Moody et al., 1981; Moody and Franke, 1982; Carroll and Risch, 1984; Tschinkel, 1988; Verhaagh, 1991; Muniappan and Marutani, 1992; McInnes and Tshinkel, 1995; Way et al., 1998). This species prefers low to mid-elevations below 1500 ft (Forel, 1993; Perfecto, 1994), but has been reported to occur at 3000 ft (Smith, 1936). It prefers mild winter temperatures and high humidity (Snelling, 1963). Cokendolpher and Francke (1985) described the temperature preferences of S. geminata. The influence of microhabitats on distribution is discussed by Torres (1984). Foraging occurs in the temperature range of 77 to 90°F, with extreme temperature limits that prevent foraging below 36°F and above 122°F (George and Narendran, 1987).

Associations

Although S. geminata is reported to be associated with a wide variety of crop and animal commodities in the field, the impact of the ants is often not reported to be negative, nor is injury always caused to the agricultural commodity. In many cases, the foraging ants prey on pest arthropods such as ticks, caterpillars and beetle life stages. They are associated with sucking insects such as aphids, leafhoppers and other Homoptera, which they 'tend', to feed on the sugary honeydew secretions (Tennant and Porter, 1991).

Suspect-ants should be collected in a vial of alcohol using forceps, cotton swabs or other suitable collecting methods that prevent the collector from being stung. Five to twenty-five specimens, particularly of the larger, major worker ants, should be collected and submitted to an identification specialist for species verification.

The methods used for detecting and inspecting other fire ant species, such as Solenopsis invicta, can be used for S. geminata. Both species build nests in the soil and create mounds with honey-comb-shaped galleries in which ants reside. These can be observed on the soil surface, particularly after rainy periods. Any sites where the soil appears to be 'worked' and formed into small granulate particles should be inspected for the presence of ants. This is particularly important during the inspection of articles of trade stacked on pallets or containing suitable media for ant colony nesting. However, baiting methods are more suitable for detecting ants in transported articles, or in locations or seasons when the ants do not construct visible mounds. Bait stations to monitor S. geminata, using attractive substances such as tuna fish, have been evaluated (Nestel and Dickschen, 1990; Islam et al., 2000).

The Pacific Invasive Ant Key (PIAKey) manual Pacific Invasive Ants Taxonomy Workshop Manual can both be used in identifying invasive ants in the Pacific region.

S. geminata is similar to the red imported fire ant, Solenopsis invicta (see data sheet on Solenopsis invicta).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

The efforts to develop controls for this pest in southern USA were reported as early as the 1930s (Travis, 1938, 1939), suggesting that this species of fire ant was considered pestiferous even before the arrival of the imported species. However, relative to the control programmes developed for the red imported fire ant, Solenopsis invicta, fewer management programmes have been specifically developed for S. geminata. Certainly, because these are both fire ants (closely related and similar in biology) the control programmes developed for S. invicta should also be useful for control of S. geminata. In areas of the world where S. geminata has been imported, their populations may be greater and control justified (George and Narendran, 1987; Porter et al., 1997). In India, management options are presented by Veeresh (1990b).

Phytosanitary Measures

The inspection of articles of trade that can potentially harbour this species (e.g. nursery stock, sod, hay, field-working equipment or articles of trade associated with soil or other suitable nesting media) should be routine and can prevent the introduction and spread of this species.

Cultural Control and Sanitary Methods

Ota and Chang (1979, 1981) and Heinz and Carlson (1984) developed drip irrigation in Hawaiian sugarcane fields and assessed it as resistant to damage by foraging S. geminata.

Host-Plant Resistance

None has been reported.

Biological Control

Although numerous organisms have been identified as natural enemies of S. geminata, few of these have been used as a method of control.

Chemical Control

Neem extract (azadiractin) has recently been assessed as a treatment for S. geminata (Geetha Viswanathan et al., 2002).

Field Monitoring/Economic Threshold Levels

No economic threshold levels have been developed for S. geminata.

IPM Programmes

As a species of 'fire ant', pest management programmes developed for S. invicta can be applied to this species where it occurs and causes problems that justify suppression.