Luis A. Calcaterra

Social parasitism in fire ants (Solenopsis spp.): a potential mechanism for interspecies transfer of Wolbachia

One possible mechanism for interspecific transfer of Wolbachia is through the intimate contact between parasites and their hosts. We surveyed 10 species of fly parasitoids (Pseudacteon spp.) and one inquiline social parasite, Solenopsis daguerrei, for the presence and sequence identity (wsp gene) of Wolbachia. Two Wolbachia variants infecting S. daguerrei were identical to known variants infecting the two common ant host species, Solenopsis invicta and Solenopsis richteri, suggesting possible transfers of Wolbachia between this parasite and their hosts have occurred. Our data also revealed an unexpectedly high diversity of Wolbachia variants within S. daguerrei: up to eight variants were found within each individual, which, to our knowledge, is the highest reported number of Wolbachia variants infecting a single individual of any host species.

Ecological dominance of the red imported fire ant, Solenopsis invicta, in its native range

Despite the widespread impacts invasive species can have in introduced populations, little is known about competitive mechanisms and dominance hierarchies between invaders and similar taxa in their native range. This study examines interactions between the red imported fire ant, Solenopsis invicta, and other above-ground foraging ants in two habitats in northeastern Argentina. A combination of pitfall traps and baits was used to characterize the ant communities, their dominance relationships, and to evaluate the effect of phorid flies on the interactions. Twenty-eight ant species coexisted with S. invicta in a gallery forest gap, whereas only ten coexisted with S. invicta in a xerophytic forest grassland. S. invicta was the most numerically dominant species in the richest and complex habitat (gallery forest); however it performed better as discoverer and dominator in the simpler habitat. S. invicta was active during day and night. In spite of its poor capacity to discover resources, S. invicta showed the highest ecological dominance and the second-best behavioral dominance after Camponotus blandus. S. invicta won 78% of the interactions with other ants, mostly against its most frequent competitor, Pheidole cf. obscurithorax, dominating baits via mass recruitment and chemical aggression. P. cf. obscurithorax was the best food discoverer. S. invicta won 80% of the scarce interactions with Linepithema humile.Crematogaster quadriformis was one of the fastest foragers and the only ant that won an equal number of contests against S. invicta. The low presence of phorid flies affected the foraging rate of S. invicta, but not the outcome of interspecific interactions. This study revealed that the red imported fire ant ecologically dominated other terrestrial ants in its native range; however, other species were able to be numerically dominant or co-dominant in its presence.

Distribution and Abundance of Fire Ant Decapitating Flies (Diptera: Phoridae: Pseudacteon) in Three Regions of Southern South America

Abstract The distribution and abundance of fire ant decapitating flies (Diptera: Phoridae: Pseudacteon Coquillett) were studied in three regions of southern South America, primarily from September 2002 to September 2004. A total of 2,421 flies belonging to 14 Pseudacteon species were found at 51% of the 662 fire ant mounds examined at 125 collecting sites. Flies occurred in a variety of habitats at altitudes from sea level to 2,280 m. Pseudacteon obtusus Borgmeier (large form) was found at the highest altitude and at the most western longitude. Flies were active between 16 and 37°C, 20 and 90% RH, and 0 and 11.6 km/h wind speed. Pseudacteon curvatus Borgmeier showed the highest abundance and one of the broadest geographical distributions. Pseudacteon tricuspis Borgmeier, P. litoralis Borgmeier, the large form of P. obtusus, P. nudicornis Borgmeier, and P. nocens Borgmeier also were widely distributed. These species seem to be the most generalized within saevissima-group. Pseudacteon solenopsidis Schmitz was only collected attacking isolated workers. A new Pseudacteon species was discovered in northwestern Argentina. Seven fly species were reported for the first time on a new fire ant host in this region. Pseudacteon cultellatus Borgmeier was found for the first time on Solenopsis invicta Buren in Corrientes province in northeastern Argentina, where up to nine fly species have been found to cooccur. Males of P. tricuspis and P. obtusus were the only males normally attracted to disturbed fire ant colonies.

Ground ant diversity (Hymenoptera: Formicidae) in the Iberá Nature Reserve, the largest wetland of Argentina.

ABSTRACT The Iberá Nature Reserve in northeastern Argentina protects one of the largest freshwater wetlands and reservoirs of species in South America. However, key invertebrate groups such as the ants (Hymenoptera: Formicidae) remain almost unknown. The main objective of this work was to study the ground ant diversity in four main habitats of Iberá: grasslands, savannas, forests, and floating islands. Pitfall traps were used to characterize ground foraging ant assemblages, compare ant diversity among environments, and establish habitat associations. We also used bait traps, manual collections, and Winkler and Berlese extractors on the ground, vegetation, and litter strata to increase the species inventory. In total, 94 species in 30 genera were captured, representing the highest number of species reported from a survey of a protected area of Argentina. The richest genus was Pheidole Westwood with 23 species including three species reported for the first time in Argentina. The most common species was Solenopsis invicta Buren. Overall, the savanna was the richest and most diverse habitat with the highest number of exclusive species and functional groups. The grassland showed the highest number of rare species, but only half of the expected species were captured. The forest showed the lowest ant richness, diversity, and equitability, but one half of the species were exclusive to forest habitat. Generalized myrmicines were predominant and dominant in all habitats. Our findings indicate that habitat specialization could be an important factor determining the organization of ant assemblages in Iberá. The protection of each of these unique and threatened natural habitats of Argentina is needed to ensure the long-term preservation of their exclusive ant species.

New Survey for the Fire Ant Microsporidia Vairimorpha invictae and Thelohania solenopsae in Southern South America, with Observations on Their Field Persistence and Prevalence of Dual Infections

Abstract The exploration for the fire ant diseases Vairimorpha invictae Jouvenaz and Ellis (Microsporidia: Burenellidae) and Thelohania solenopsae Knell, Allen, and Hazard (Microsporidia: Thelohaniidae) was conducted from 2001 to 2005 in Argentina, Paraguay, Chile, Bolivia, and Brazil. A total of 2,064 colonies were sampled from 262 sites. Three sites with high prevalence of pathogens were monitored periodically for the persistence of the infections. V. invictae occurred at 12% of the sites and in 10% of the colonies. Except for one infected colony in Bolivia, its distribution was restricted to the eastern part of the region surveyed. The highest occurrence was in Santa Fe Province. T. solenopsae presented a much wider distribution. It occurred at 25% of the sites and in 13% of the colonies. The highest occurrence was in Buenos Aires Province. This is the first report of T. solenopsae in the northwest, in the west, in central Argentina, and in Bolivia, and infecting S. interrupta (Santschi). Simultaneous infections were found at 4% of the sites and in 2.2% of the colonies. The periodical examination revealed high infection levels in most occasions. The prevalence of T. solenopsae ranged from 10 to 90% of the colonies, V. invictae from 0 to 60%, and dual infections from 0 to 50%. Each microsporidium exhibited a characteristic enzootic/epizootic wave. Successive epizootic levels observed in both infections provide a more constant pressure against fire ant populations. These diseases are promising classical biological control agents of the imported fire ants in the United States.

Fire Ants (Solenopsis spp.) and Their Natural Enemies in Southern South America

We review the fire ant research conducted by the ARS-South American Biological Control Laboratory (SABCL) since 1987 to find a complex of natural enemies in southern South America and evaluate their specificity and suitability for field release as self-sustaining biological control agents. We also include those studies conducted by the ARS-Center for Medical, Agriculture, and Veterinary Entomology in the United States with the SABCL collaboration. Ecological and biological information is reported on local fire ants and their microsporidia, nematodes, viruses, phorid flies, eucharitid wasps, strepsiptera, and parasitic ants. Their biology, abundance, distribution, detrimental effect, field persistence, specificity, and phenology are discussed. We conclude that the objectives of the ARS program in South America are being achieved and that the pioneering studies have served to encourage further investigations in the United States and other countries and advanced the implementation of biological control programs to decrease imported fire ant densities and damage. Still, several promising organisms should be further investigated for eventual field release in the near future.

Survey of Solenopsis Fire Ants and Their Parasitoid Flies (Diptera: Phoridae: Pseudacteon) in Central Chile and Central Western Argentina

Abstract Twenty-two species of Pseudacteon flies (Diptera: Phoridae) are known to attack fire ants (Solenopsis spp.) in Argentina, Brazil, Bolivia, and Paraguay, but none are known in Chile. Surveys were conducted in central Chile and at similar latitudes in western Argentina to detect the presence of fire ants and parasitoid flies and to determine their relationship. Flies and fire ants were much more common and abundant in Argentina. In total, 100 colonies of four fire ant species were found at 63.6% of the sites surveyed. In contrast, only six colonies of one species, Solenopsis gayi (Spinola), were found at 4.7% of the sites surveyed in Chile. Our survey includes the first record of five parasitic fly species in central western Argentina and a new host, Solenopsis quinquecuspis Forel. The large form of Pseudacteon obtusus Borgmeier was found attacking S. gayi in Chile, which is the first record in that country, and the first record on this host species. The southern-most and western-most records were established for fire ant-decapitating flies. Mitochondrial DNA (mtDNA) indicates that S. gayi is genetically closer to the Argentine fire ants (saevissima-group) than the North American S. geminata-group. However, S. gayi venom alkaloid composition is similar to S. geminata-group, whereas cuticular hydrocarbon composition has characteristics of both groups. Analysis of mtDNA from the collected flies supports the monophyly of P. obtusus and suggests that the Chilean population is similar to populations in eastern Argentina. The presence of P. obtusus in Chile could be explained by immigrant parasitized fire ant species from Argentina or by an accidental introduction.

Activity Patterns and Parasitism Rates of Fire Ant-Decapitating Flies (Diptera: Phoridae: Pseudacteon spp.) in Their Native Argentina

Abstract This work describes the annual and daily activity patterns of two parasitoid fly communities of the fire ant Solenopsis invicta (Hymenoptera: Formicidae) in their native range in Argentina. Pseudacteon (Diptera: Phoridae) flies were censused monthly for 1 yr at two sites in northwestern Corrientes province. Additionally, parasitism rates of S. invicta colonies naturally attacked in the field by Pseudacteon flies are reported for the first time from its native habitat. In total, 4,528 flies (86.3% females) of eight Pseudacteon species were collected attacking ants in the field. Pseudacteon litoralis Borgmeier and Pseudacteon nocens Borgmeier represented 71–79% of female flies censused in both sites. Most species were active throughout the year, although abundances were variable over time and between sites. The highest occurrence peaks of flies were recorded in spring, whereas the lowest occurrence was in summer. Fly abundance was higher close to dusk, whereas species diversity was highest at midday. Relationships among species were established based on their activity patterns and genetic proximity. The presence and abundance of four species were explained by climatic variables, whereas two species may have inherited similar circadian rhythms from a common ancestor. Overall, the parasitism rate by the nine species recorded was very low (0.24%). The highest percentage of parasitized workers was found in spring (0.5%) and occurred within the most complex habitat (gallery forest). The highest parasitism rate per site and colony was also for this habitat in spring (1.16 and 2.81%, respectively). The highest rates of emergence were recorded for Pseudacteon nudicornis Borgmeier.

Pseudacteon calderensis, a New Fly Species (Diptera: Phoridae) Attacking the Fire Ant Solenopsis interrupta (Hymenoptera: Formicidae) in Northwestern Argentina

Abstract A new species of Pseudacteon phorid fly, Pseudacteon calderensis (Diptera: Phoridae), is described from females attacking worker ants of Solenopsis interrupta Santschi in Salta and Jujuy provinces, northwestern Argentina. P. calderensis differs from almost all other South American Pseudacteon species by the structure of the female ovipositor. This new species is morphologically similar to Pseudacteon borgmeieri Schmitz, but it differs slightly in the shape of the apex of the ovipositor and differs strongly because of the absence of large and stout ventral hairs under the ovipositor.

New Host for the Parasitic Ant Solenopsis Daguerrei (Hymenoptera: Formicidae) in Argentina

The parasitic ant Solenopsis daguerrei (Santschi) has been reported as an inquiline of fire ants in South America. It lacks a worker caste, so all adults are reproductive males and females. The parasite queens and occasionally some virgin females attach themselves to the host queens, and divert resources from them. The fire ant workers tend S. daguerrei in a manner similar to their own mother queens (Bruch 1930). The host workers also feed and maintain the brood of S. daguerrei. According to Silveira-Guido et al. (1973) this parasite inhibits the egg production of the fire ant mother queen, thus causing the ant colony to collapse and eventually die out. Recently it was reported that multiple-queen colonies of fire ants parasitized with S. daguerrei have fewer queens than non parasitized ones (Calcaterra et al. 1999). Because of these findings, this parasitic ant is a candidate for introduction for the biological control of imported fire ants in the United States and is under quarantine conditions at the USDA-ARS CMAVE, Gainesville, FL. However, before S. daguerrei can be released from quarantine, its host specificity must be determined. Surveys on fire ant natural enemies in South America revealed that S. daguerrei has been found in Argentina, Uruguay, and Brazil, in colonies of S. richteri Forel, S. invicta Buren, S. saevissima F. Smith, and S. macdonaghi Santschi (Silveira-Guido et al. 1973, Briano et. al. 1997, Pesquero et al. 1998, Calcaterra et al. 1999). Here we report a new host species for S. daguerrei. As part of the study of the specificity of S. daguerrei, a field host range survey was conducted in San Eladio (60 km W of Buenos Aires), Argentina, the only place where S. daguerrei has been found consistently since 1995. This area had the highest abundance (7% of fire ant colonies) of S. daguerrei recorded in South America to date (Briano et al. 1997). The surveys were conducted from December 1996 to May 1997 and from November 1997 to May 1998 and consisted in walking through the pastures to visually detect the ant colonies. When an ant nest was found, it was excavated, scattered on the ground, and thoroughly examined for S. daguerrei adults. Some colonies were excavated and put into 10-liter buckets for separation in the laboratory by flotation (Banks et al. 1981). The floated colonies were put in rearing trays (40 by 30 by 15 cm) and examined later. Alcohol samples were kept of most ant species found. Samples of 34 parasitized fire ant colonies were preserved in hexane to confirm their identification by gas chromatography analysis of cuticular hydrocarbons and venom alkaloids (Vander Meer & Lofgren 1988). Voucher samples were deposited at the USDA-ARS, SABCL collection and at the USDA-ARS, CMAVE, Gainesville, FL. We sampled a total of 4,316 ant colonies of 9 different species in 4 subfamilies, however 96% of them were fire ants. Other species examined were: Pheidole bergi Mayr, Acromyrmex lundi Gu6rin, A. ambiguus Mayr, Camponotus punctulatus Mayr, Neivamyrmex pertyi Shuckard, Linepithema humile Mayr, and Brachymyrmex sp. S. daguerrei was found exclusively in 161 colonies of fire ants (Table 1). Taxonomic studies revealed that 95% of the parasitized colonies corresponded to S. richteri. The