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S. richteri (Order: Hymenoptera, Family: Formicidae), is a social insect that lives in colonies, usually associated with a mound. Most individuals are sterile female workers that perform a variety of functions, including care of the queen and brood, foraging, defense and nest building. The worker caste is polymorphic, ranging from small (minor) through intermediate (media) to large (major) individuals. Additionally, immature stages (eggs, larvae and pupae, or brood), winged reproductives and at least one queen will be present.
Colonies take approximately 2 years to mature and, on average contain, 200,000-400,000 individuals. Mature S. richteri colonies produce conspicuous mounds similar to those of S. invicta, averaging 30-50 cm in height and width, but they may be larger, reaching 90 x 90 cm. In hot dry conditions of late summer, S. richteri mounds may flatten out or disappear as the colony moves entirely underground. Mound building activity is stimulated by rainfall (Rhoades and Davis, 1967), and outbreaks have been found to be correlated with heavy precipitation, due to the queen’s needs for moist soil to excavate a nest (Green, 1962;Lofgren et al., 1975). Foraging worker ants enter and exit the colony through tunnels radiating up to 5-10m away from the mound. Colonies extend into the ground below the mound as interconnecting galleries, as much as 30-40 cm below ground level. In the USA, S. richteri colonies are usually found in open areas associated with some type of disturbance, e.g., lawns, hayfields, pastures, roadsides and highway medians, athletics fields, school grounds, etc. In their native Argentina, ideal habitats for S. richteri include the Pampas grasslands, as well as pastures of varying water content and seasonally waterlogged grassland (Taber, 2000). The disturbance of mounds results in a rapid defensive response by the worker ants, which will climb vertical objects in large numbers to bite and sting.

Recoginition


Methods for detection of S. richteri are the same as those for S. invicta (see datasheet on Solenopsis invicta).
Visual inspection
Soil that is associated with any articles of trade or shipping equipment from areas known to be infested with S. richteri should be carefully inspected for the presence of ants. This could include various types of produce, turf and other nursery materials, honey bee equipment, hay, etc.
Foraging surveys
Baits are commonly used to survey for foraging activities of fire ant workers. A variety of food materials can be used, including sugar water, hot dogs, cookies, tuna, moistened pet food, etc. Baits are placed on or in such containers as petri dishes, plastic vials or test tubes, cardboard or laminated paper squares, etc. Under optimum conditions, fire ant workers will quickly find the baits and recruit other workers to them via trail pheromones. Baiting may be used by researchers to study ant behaviour, document impact of fire ants on other ant species, determine effectiveness of different control methods, time control applications, etc.
Monitoring fire ant mounds
Estimating the density of fire ant mounds in a given area is an easy way to quantify populations and monitor changes in population size in response to suppression measures. In addition to numbers, mound sizes and brood presence/absence can be used to further assess populations (e.g., see USDA mound rating system, Harlan et al., 1981). Some limitations to these methods include disappearance of mound structure in hot, dry weather, making detection more difficult ease of missing small, young colonies, location of fire ant colonies in areas not associated with a mound or hard to observe (e.g., tree stumps, hay bales), etc. Changes in populations through the year with changes in season usually necessitate sampling more than once to obtain reasonably accurate information.

Related invasive species

  • Solenopsis richteri

Related Farm Practice

  • Pastures
  • Harvesting
  • Drought
  • Pests
  • Outbreaks
  • Activity
  • Hosts
  • Predation
  • Soil

Related location

  • Argentina
  • Brazil
Impact

S. richteri is native to southeastern Brazil, central Argentina and parts of Uruguay. After its accidental introduction into the USA around 1918, it expanded its range into much of the southeastern USA and became a ubiquitous presence in a variety of urban and agricultural settings, as well as an important economic and environmental pest. However, the red imported fire ant, S. invicta, after its introduction through Mobile around 1930, gradually took over most of the range of S. richteri, and now occupies around 1,100,000 km 2, primarily in the coastal plains from N. Carolina to Texas (Porter and Briano, 2000). Currently, S. richteri is restricted to approximately 30,000 km 2 in northwestern Alabama, northeastern Mississippi and in parts of southern Tennessee, including a relatively recent introduction into Memphis (Jones et al., 1997). A broad band of hybridization zone between S.richteri and S. invicta exists between the two populations, occupying around 130,000 km 2 (Shoemaker et al., 1994). Comprehensive reviews of imported fire ants can be found in Lofgren et al. (1975), Taber (2000) and Tschinkel (2006). S. richteri is apparently more cold-hardy than S. invicta and thus has some potential to expand farther north, including possibly the southern Great Plains of the USA, which are similar to the South American Pampas, to which S. richteri is native. However, given human control efforts combined with intrusions of S. invicta and the S.richteri / S. invicta hybrid, it does not seem likely S. richteri will expand its range significantly in the future in the USA (Taber, 2000).

Has Cabi datasheet ID
50571
Symptons


Information on crop hosts and feeding by S. richteri is limited, although S. richteri is known as a potato pest in Brazil (Taber, 2000). It is reasonable to expect similarities to S. invicta. S. invicta is omnivorous and foraging fire ants may be found in or on plants when they are preying on phytophagous arthropods associated with those crops. Plant feeding appears to be aggravated by dry or drought conditions. On other plants, the ants seem attracted to oil-containing plant parts such as the embryo portion of maize and sorghum seeds. Foraging workers on plants can become a hazard to field workers and tall, hardened mounds harbouring ant colonies in certain crops such as hay pastures or soyabeans can interfere with mechanized cutting and harvesting operations.
Affected plant stages include flowering stage, fruiting stage, post-harvest, pre-emergence, seedling stage and vegetative growing stage.
There is little or no specific information available on symptoms occurring in crops as a result of S. richteri feeding. In S. invicta, the following types of damage may be observed:
Fruits/pods: internal feeding;external feeding.
Leaves: wilting.
Roots: internal feeding;external feeding.
Seeds: internal feeding;external feeding.
Vegetative organs: internal feeding;external feeding.
Whole plant: plant dead;dieback;uprooted or toppled;internal feeding;external feeding
Biology and Ecology
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Genetics
In all ants, sex is determined by fertilization;unfertilized eggs produce males and fertilized eggs become females. Males occur only in the reproductive form, while females may become sterile workers or fertile reproductives (incipient queens). Whether a female becomes a worker or reproductive depends on its feeding and chemical (juvenile hormone and pheromones) environment (Tschinkel, 2006).
Two social forms are recognized in fire ants: monogyne colonies have a single functional (reproductively-active) queen, while polygyne colonies have multiple functional queens, ranging from 2-20,000 (Taber, 2000). Worker ants in monogyne colonies display territorial behaviour toward neighbouring colonies, whereas polygyne colony worker ants do not. As a result, polygyne colonies may have several-fold the number of ant mounds in a given area, sometimes reaching densities of several hundred mounds per hectare, 2-4 times the densities seen in monogyne areas (Porter et al., 1991;Porter, 1992;Porter et al., 1992;Fritz and Vander Meer, 2003). Polygyne fire ants are thus considered a greater economic and environmental threat than the monogyne form, although not as widespread. Polygyny is widespread only in S. invicta in the USA.
Polygyny exists in S.richteri in South America and is widespread there (Calcaterra et al., 1999;Briano et al., 1995). Apparently, only the monogyne form exists in the USA, although polygyny has been discovered in S. richteri / S. invicta hybrid populations (Glancey et al., 1989).
In the USA, S. richteri and S. invicta hybridize, resulting in an intermediate form that can produce fertile offspring. Polygyny has been reported in hybrids, but does not seem to be widespread. Currently, a broad band of hybridization between S. richteri and S. invicta exists from the Mississippi River to Atlanta, Georgia, occupying approximately 130,000 km 2 (Shoemaker et al., 1994).
Reproductive Biology
The S. richteri life cycle is similar to that of S. invicta (Taber, 2000). Winged reproductives form mating swarms and mating occurs in the air, after which the queen lands and sheds her wings;males die soon after mating. Several hundred virgin males and females may leave a colony at any one time. Mating flights can occur year round, especially in the native range in South America, but in North America often occur between April and August, usually on a warm, sunny day following rain. Following wing removal, queens establish colonies and start laying eggs. S. richteri queens establish their nests within approximately 3 cm of the soil surface, which is shallower than for S. invicta queens (Lofgren et al., 1975);however, the vast majority of queens perish before they can establish nests.
Once established, a queen at peak productive capacity can lay half her own weight in eggs daily and may live several years, until sperm depletion (Tschinkel, 2006). Before development of her first brood, the queen does not feed and must rely on stored food in the digestive tract and breakdown of flight muscles for nutrition (Taber, 2000). The queen loses a substantial amount of weight during care for the first brood. Eggs hatch into larvae, which pass through four instars;last stage larvae become pupae, which transition into adults. Workers, whether minor, media or major, change behavioural roles with age, first acting as nurses for queen and brood, then reserves (nurse + food reception from foragers) and, finally foragers. A new colony can start producing winged reproductives within 6-8 months, with production of several thousand individuals per year. It takes approximately 2 years for a colony to reach full maturity.
Physiology and Phenology
S. richteri is an adaptable species in a variety of ways, which contributes to its success. It is primarily a creature of disturbed habitats, both natural and manmade, in both its adopted and native countries (Tschinkel, 2006). It can aggressively exploit such areas, which is even more evident in S. invicta, allowing it to colonize and exclude other species that are potential competitors. The good fortunes of imported fire ants are closely tied to human activities, especially since the arrival of Europeans in the New World and the accompanying huge areas of ecological disturbance that resulted (Tschinkel, 2006). Fire ants are perhaps best viewed as pioneer species, evolved to exploit relatively rare and short-lived habitat patches derived from disturbance. They evolved high reproductive output as a response to dealing with such rare and unpredictable optimum habitat;effective dispersal mechanisms were also required to exploit habitats unpredictable in space. Additional adaptations to such habitats include rapid colony growth and early reproduction over a long season. Thus, fire ants have successfully exploited the highly disturbed landscape of the southeastern USA (Tschinkel, 2006). S. richteri produces a glycerol-type antifreeze which enables it to withstand colder temperatures than S. invicta, increasing its potential to move into habitats outside the range of S. invicta. Hybrid vigor associated with the S. richteri / S. invicta hybrid may also increase ability to withstand low temperatures (Callcott et al., 2000). S. richteri can readily adjust to varying environmental conditions, within limits, for example, moving brood around in mounds or underground to areas of optimum temperature and humidity. Its generalist feeding habits are an obvious adaptive advantage, allowing it to exploit habitats more efficiently.
Nutrition
S. richteri’s colony populations, foraging behaviours, diets and feeding behaviours are similar to those of S. invicta, which has been studied much more intensively (Taber, 2000;Tschinkel, 2006). Ants communicate through vision (sight), vibration (sound), touch and chemicals (pheromones), including a queen pheromone that attracts workers and a trail pheromone associated with the worker ant stinger. Upon locating food resources, a pheromone trail is produced which directs other worker ants to the site. Fire ants are omnivorous, consuming primarily other arthropods and honeydew produced by aphids and related insects (primarily Order Hemiptera, Suborder Sternorrhynca), but also seeds and other plant parts like developing or ripening fruit, and dead plant and animal tissues (Vinson, 1997). Living prey may be subdued by stinging. Foraging ants may bring solid or liquid food back to the colony;however, only certain larvae can process solid foods. Workers store liquid food in their crops, from where it can be regurgitated for nest mates (trophallaxis) (Glancey et al., 1981). Optimum ambient foraging temperatures range between 70 and 85 ° F (Rhoades and Davis, 1967).
Associations S. richteri symbionts have been studied in both South America and the southern USA. Caterpillars of the metalmark butterfly (Hamearis epulus signatus) spend much of their lives inside S. richteri mounds in South America, leaving the nest at night to feed on a leguminous host plant. When caterpillars are in the ant nest, workers feed on their bodily secretions. Other associates found in nests in South America included millipedes, short-winged mold beetles, seed bugs, lace bugs, wingless phorid flies, and rove beetles (Taber, 2000). One darkling beetle species native to South America, where it inhabits nests, is also found in the southern USA, although it has not actually been found in fire ant nests there (Taber 2000,).
Latitude/Altitude Ranges
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Latitude North (°N)|Latitude South (°S)|Altitude Lower (m)|Altitude Upper (m)
36
42
0
0
Air Temperature
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Parameter
Lower limit
Upper limit
Mean annual temperature (ºC)
15.2
20.4
Mean minimum temperature of coldest month (ºC)
0.8
9.3
Rainfall
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Parameter|Lower limit|Upper limit|Description
Mean annual rainfall|1011|1680|mm;lower/upper limits
Natural enemies
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Natural enemy|Type|Life stages|Specificity|References|Biological control in|Biological control on
Beauveria bassiana| Pathogen
All Stages| not specific
Burenella dimorpha| Pathogen
Caenocholax fenyesi
Adults| not specific
Kneallhazia solenopsae| Pathogen
All Stages| to genus
Argentina
Neivamyrmex opacithorax| Predator
Pachydiplax longipennis| Predator
Pseudacteon
Adults| to genus
Argentine, Brazil, Uruguay, USA (introduced)
Pseudacteon tricuspis| Parasite
Pyemotes tritici| Predator
All Stages| not specific
Solenopsis daguerrei| Parasite
Adults
Solenopsis molesta| Predator
Steinernema| Pathogen
Larvae/Pupae| not specific
Stichotrema wigodzinsky| Parasite
Vairimorpha invictae| Pathogen
All Stages| to genus
Notes on Natural Enemies
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There is a great deal of information available on imported fire ant natural enemies, including S. richteri, due primarily to the potential for biological control in areas where fire ants are invasive. The socially parasitic fire ant, Solenopsis daguerrei, was first discovered in S. richteri mounds in South America (Santschi, 1930). Colony parasitization rates in a given area can be as high as 31% and mound densities in affected areas are lower than densities where the parasite is not found (Calcaterra etal., 1999). Phorid flies in the genus Pseudacteon and several related genera produce larvae that decapitate worker ants and pupate inside their empty heads (Porter, 1998). Each species of fly parasitizes a characteristic size range of ants (Morrison et al., 1997;Morrison and Gilbert, 1999). Species that attack fire ants appear to be specific to fire ants (Porter, 1998). In addition to mortality, phorids appear to affect fire ant worker behaviour in important ways. Once flies are recognized, most ant workers seek cover, others curl into a stereotypical c-shaped defensive posture, and yet others freeze their posture (Porter, 1998). These behaviours generally result in reduced foraging rates;the presence of a single fly can stop or greatly inhibit the foraging of hundreds of workers within 2-3 minutes (Feener and Brown, 1992;Orr et al., 1995;Porter et al., 1995). In Argentina, the presence of six phorid species that attack S. richteri reduced the number of ants at food resources in the field, as well as foraging activity in general (Folgarait and Gilbert, 1999).

Hosts


Neither S.richteri nor S. invicta are considered major pests of crops although S. invicta is documented to feed on several crops, at times causing minor damage. S. invicta is well known to feed, and S. richteri workers probably feed, on honeydew produced by certain sternorrhyncan hemiptera (e.g., aphids, scale insects, mealybugs, etc.). Since the ants may protect these insects, their numbers may increase on some horticultural crops, especially if their natural enemies are reduced by fire ants.

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