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

A study was conducted on populations of the red imported Þre 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 Þve times per year for 3Ð4 yr in eight Þeld 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 showedßuctuations that ranged from29.2 to 1.4%and13.6 to 2.6%, respectively.Thehighest infection rateswereobservedat thebeginningof the study.A total of 394colonieswere sampledduring the study: 325 (82.5%)were healthy and 69 (17.5%)were infectedwithmicrosporidia. 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 signiÞcantly 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 Þre ant population growth; consequently, they do not explain the overall reduction in the populations. These results, combinedwith additional evidence reported previously, suggest that infection with V. invictae and T. solenopsae has a deleterious effect on native populations of S. invicta.