Juan A. Briano

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.

Parasitoid-host matching between the little decapitating fly Pseudacteon curvatus from Las Flores, Argentina and the black fire ant Solenopsis richteri

Matching biotypes of potential biocontrol agents to target host populations can greatly improve the effectiveness of control. This study was designed to determine if the fly Pseudacteon curvatus Borgmeier from Las Flores, Buenos Aires Province, Argentina prefers its natural host, the black fire ant, Solenopsis richteri Forel. We found that P. curvatus strongly preferred S. richteri from Argentina, S. richteri from the United States, and hybrid (S. richteri S. invicta) fire ants from the United States when each was tested against S. invicta from the United States. The time to pupation of developing parasitoids was 10% and 21% longer in hybrid and red fire ants than in black fire ants. Parasitism rates, however, were not significantly different among these ant hosts in no-choice parasitism tests.

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.

Protozoan and Fungal Diseases in Solenopsis richteri and S. quinquecuspis (Hymenoptera: formicidae) in Buenos Aires Province, Argentina

The diversity and abundance of protozoa and fungi infecting colonies of the fire ants Solenopsis richteri Forel and S. quinquecuspis Forel were surveyed in Buenos Aires province, Argentina. A total of 185 roadside sites was selected, and 1,836 colonies were sampled and examined under phase-contrast microscopy. Pathogens were found at 32% of the sites and in 10% of the colonies. The microsporidium Thelohania solenopsae Knell, Allen & Hazard was the most common microorganism; it was present at 25% of the sites and 8% of the colonies. In some sites within the surveyed region, the percentage of infected colonies with T. solenopsae ranged from 40 to 80%. Other pathogens present were the microsporidium Vairimorpha invictae Jouvenaz & Ellis and the fungus Myrmecomyces annellisae Jouvenaz & Kimbrough. A field site was selected for future ecological studies.

Field host range of the fire ant pathogens Thelohania solenopsae (Microsporida: Thelohaniidae) and Vairimorpha invictae (Microsporida: Burenellidae) in South America

We studied the field host specificity of the microsporidia Thelohania solenopsae and Vairimorpha invictae and their prevalence in the imported fire ants, Solenopsis invicta and S. richteri. Terrestrial ants were sampled by using bait traps and/or nest sampling at preselected sites in Argentina and Brazil. The sampling included the genera Solenopsis, Pheidole, Camponotus, Crematogaster, Linepithema, Brachymyrmex, Paratrechina, Dorymyrmex, and Wasmannia. The samples were examined under a phase-contrast microscope for the presence of microsporidian infections. The bait trap sampling revealed that: (1) T. solenopsae infected only S. richteri, S. invicta, and Solenopsis sp. at 6–67% of the sites and in 1.5–29% of the traps; (2) V. invictae infected only S. invicta at 6% of the sites and in 3% of the samples. The nest sampling revealed that: (1) T. solenopsae infected S. invicta, S. richteri, and S. macdonaghi, at 41–67% of the sites and in 11–58% of the colonies; (2) V. invictae infected the same species at 15–50% of the sites and in 2–26% of the colonies. We detected T. solenopsae and V. invictae in equal percentages of S. invicta sites (41%); however, the percentage of colonies infected with V. invictae was 20% and with T. solenopsae only 11%. At S. richteri sites, in contrast, T. solenopsae occurred at 46% of the sites and 15% of the colonies and V. invictae occurred at only 15% of the sites and 2% of the colonies. In S. macdonaghi, T. solenopsae was detected at 67% of the sites and 58% of the colonies, and V. invictae was detected at 50% of the sites and 26% of the colonies. This is the first report of V. invictae infecting S. macdonaghi. The proportion of S. richteri and S. invicta infected with T. solenopsae was similar. In contrast, the proportion of S. invicta infected with V. invictae was higher than S. richteri. We conclude that the microsporidia, T. solenopsae and V. invictae, show a very high specificity for Solenopsis ants in the field. It appears that T. solenopsae infects S. invicta and S. richteri equally but V. invictae may be more adapted to infect S. invicta. Published by Elsevier Science (USA).

Long-Term Studies of the Red Imported Fire Ant, Solenopsis invicta, Infected with the Microsporidia Vairimorpha invictae and Thelohania solenopsae in Argentina

Abstract A study was conducted on populations of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), infected with the microsporidia Vairimorpha invictae Jouvenaz and Ellis (Microsporidia: Burenellidae) and Thelohania solenopsae Knell, Allen, and Hazard (Microsporidia: Thelohaniidae). Fire ant populations and microsporidia prevalence were monitored three to five times per year for 3–4 yr in eight field plots in northern Argentina. The mean population index per plot showed an overall reduction of 69%. The percentage of infection with V. invictae and T. solenopsae showed fluctuations that ranged from 29.2 to 1.4% and 13.6 to 2.6%, respectively. The highest infection rates were observed at the beginning of the study. A total of 394 colonies were sampled during the study: 325 (82.5%) were healthy and 69 (17.5%) were infected with microsporidia. The proportion of infected colonies with brood was 81% (56/69), similar to the proportion of healthy colonies with brood (78%; 255/325). The proportion of infected and healthy colonies in the population index categories was significantly different. Of the infected colonies with brood, 49.3% were medium and 1.4% were large in size. In contrast, healthy colonies were generally larger, with 29.7 and 10.4% being medium and large, respectively. The general environmental conditions in the area of the plots were appropriate for fire ant population growth; consequently, they do not explain the overall reduction in the populations. These results, combined with additional evidence reported previously, suggest that infection with V. invictae and T. solenopsae has a deleterious effect on native populations of S. invicta.

Host specificity of a biotype of the fire ant decapitating fly Pseudacteon curvatus (Diptera: Phoridae) from northern Argentina

Abstract We tested the host specificity of Pseudacteon curvatus Borgmeier from Formosa, Argentina, on North American colonies of the red imported fire ant, Solenopsis invicta Buren, and the native fire ants Solenopsis geminata (F.) and Solenopsis xyloni McCook. No-choice tests showed that P. curvatus hovered over and attacked all three species of fire ants. The number of pupae successfully completing development to adult flies in the trials was 5.03 ± 1.55 (mean ± SE) per female fly in S. invicta, 0.66 ± 0.24 per female fly in S. xyloni, and 0 per female fly in S. geminata. Paired preference tests showed that P. curvatus preferred to hover over S. invicta instead of S. xyloni 77 ± 3% (mean ± SE) of the time and preferred S. invicta over S. geminata 87 ± 4% of the time. The oviposition attempts of active female P. curvatus were 2.8 times higher on S. invicta than on S. xyloni and 16 times higher on S. invicta than on S. geminata. These results demonstrate that this new biotype of P. curvatus is more host specific to North American red imported fire ants than a previous biotype collected from black imported fire ants.

Natural Occurrence and Laboratory Studies of the Fire Ant Pathogen Vairimorpha invictae (Microsporida: Burenellidae) in Argentina

Abstract We surveyed 154 sites in north-central Argentina and sampled 2,528 fire ant colonies for the presence and intracolonial prevalence of the microsporidium, Vairimorpha invictae Jouvenaz & Ellis, in the red imported fire ant, Solenopsis invicta Buren. The concentrations of meiospores and binucleate spores were quantified in workers and sexuals; and the occurrence and intracolonial prevalence of dual infections with Thelohania solenopsae Knell, Allen & Hazard were studied. To study the effect of V. invictae in infected colonies of S. invicta, we compared the proportion of infected living workers to the proportion of infected dead workers, and compared the survival of uninfected and infected workers. V. invictae occurred at 13% of the sites and 2.3% of the colonies. At times, the disease reached epizootic levels in certain areas. We found vegetative stages in 4.8–52.3% of eggs, larvae, pupae, and queens, meiospores in 4–56.3% of pupae and mature stages, and binucleate spores in 9.5–63% of all life stages, except eggs. Evidence for transovarial transmission is provided. The percentage of sexual males infected was significantly higher than that of sexual females (44.9 versus 15.9%, respectively). Dual infections (V. invictae + T. solenopsae) occurred in 0.24% of the colonies. V. invictae was present in 9.3% of living workers and in 56.7% of dead workers. Mortality rates of workers from Vairimorpha-infected colonies were higher than those of workers from uninfected colonies. Survival times of infected workers were 18.8–31.7% less than those of uninfected workers. The studies reported here contribute to the evaluation of V. invictae for use as a classical biological control agent against the red imported fire ant in the United States.

The Large Decapitating Fly Pseudacteon litoralis (Diptera: Phoridae): Successfully Established on Fire Ant Populations in Alabama

ABSTRACT The large fire ant decapitating fly, Pseudacteon litoralis Borgmeier, from northeastern Argentina was successfully released as a self-sustaining biocontrol agent of imported fire ants in south central Alabama in 2005. Five years later, this fly is firmly established at the original release site and has expanded outward at least 18 km. Nevertheless, populations remain very low considering P. litoralis is one of the most abundant fire ant decapitating flies in large areas of its range in South America. The reasons for low densities and why we were only able to establish this fly at 1 of 9 release sites in 4 states (2003–2006) are unknown, but problems with host-matching, release procedures, weather conditions, and competition with previously released decapitating flies are discussed as possible factors.

SIMULTANEOUS DETECTION OF VAIRIMORPHA INVICTAE (MICROSPORIDIA: BURENELLIDAE) AND THELOHANIA SOLENOPSAE (MICROSPORIDIA: THELOHANIIDAE) IN FIRE ANTS BY PCR

A PCR-based method capable of detecting Thelohania solenopsae and/or Vairimorpha invictae infection in the red imported fire ant, Solenopsis invicta, was developed. Multiplex PCR allows simultaneous detection of both species of microsporidia in a single reaction.