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.