Prevalence of Thelohania solenopsae (Microsporidia: Thelohaniidae) infection in monogyne and pologyne red imported fire ants (Hymenoptera: Formicidae)

Authors: Oi, David; Valles, Steven; Pereira, Roberto

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2003
Publication Date: 4/23/2004
Citation: Oi, D.H., Valles, S.M., Pereira, R.M. 2004. Prevalence of Thelohania solenopsae (Microsporidia: Thelohaniidae) infection in monogyne and pologyne red imported fire ants (Hymenoptera: Formicidae). Environmental Entomology. 33(2):340-345.

Interpretive Summary: Imported fire ants are an invasive pest that infests over 320 million acres in 14 states. In the U.S., there is a lack of natural enemies which can suppress populations and mediate the spread of fire ants. In 1996, a pathogen of imported fire ants in South America, called Thelohania solenopsae, was found in Florida by scientists from the Center for Medical, Agricultural, and Veterinary Entomology in Gainesville, FL (CMAVE). This pathogen debilitates fire ant queens and can cause reductions in fire ant populations. However, sustained infections seem to occur only in sites that contain colonies with several queens per colony. While colonies with multiple queens have larger populations and are probably harder to control, the predominant form of fire ants in the U.S. have colonies with only a single queen. To confirm these observations, we determined the natural prevalence of T. solenopsae in three pastures in Florida. For individual colonies at each study site, social form (i.e. single- or multiple-queen) and T. solenopsae infections were determined by examining the genotype of the ants and the pathogen using polymerase chain reaction (PCR) methods developed at CMAVE. Infections were only detected in multiple-queen colonies (100 of 164 total colonies infected). None of the 44 single-queen colonies from the three sites were infected with T. solenopsae. While infections were only detected in multiple-queen colonies in the field, CMAVE scientists were able to find infections in ants with the single-queen genotype. These ants came from four colonies that were reared from queens that recently mated and would normally attempt to establish a new colony. Thus, T. solenopsae infections can occur in ants with genotypes of either social form, but there are probably mechanisms that prevent the widespread detection of infected single-queen colonies. One important mechanism is that colonies with a single, infected queen die faster than infected multiple-queen colonies. Thus, there is a lower probability of finding infected single-queen colonies. These results will help scientists overcome barriers to the widespread establishment of this disease of fire ants. This pathogen has the potential to contribute greatly to the long term suppression of fire ants which will benefit everyone who is affected by this stinging, invasive ant.

Technical Abstract: We determined the prevalence of natural field infections of the fire ant pathogen Thelohania solenopsae Knell, Allen, and Hazard in the monogyne and polygyne social forms of the red imported fire ant, Solenopsis invicta Buren, in three pastures in Florida. Social form was determined by examining the genotype of ants at the Gp-9 locus. T. solenopsae infections were restricted to colonies of only the polygynous (multiple fertile queens) social form in study sites that had a mixture of social forms. Among the study sites, ratios of monogyne:polygyne colonies ranged from 3:55 to 28:22 and infections rates were 42 to 78% of the colonies, regardless of social form. While T. solenopsae was only detected in polygynous colonies in the field, T. solenopsae infections were found in ants with the monogyne genotype. Ants from four colonies reared from field collected, newly-mated queens that were naturally infected with T. solenopsae, were found to exhibit this genotype. T. solenopsae also was detected in individual, alate female reproductives possessing the monogyne genotype, which were collected from polygynous colonies. Polygynous colonies can contain individual ants which possess either the polygyne or monogyne genotype. Thus, T. solenopsae infections can occur in fire ants with genotypes of either social form. Because making genotypic determinations of S. invicta social forms may be impractical in the field, we compared visual and genotypic determinations of polygyny and monogyny. Visual determinations, based mainly on the relative preponderance of major workers, corresponded to 85% of the genotypic determinations.