38th Annual Meeting of the Society for Invertebrate Pathology

Epizootiology of the microsporidium Thelohania solenopsae was investigated in red imported fire ants, Solenopsis invicta. The microsporidium was detected at 16% of 165 sites and in 10% of 1309 colonies surveyed throughout Louisiana. Its random distribution was not affected by type of habitat. It infected 2.4% of monogyne (single-queen) and 53.3% of polygyne (multiple-queen) colonies. Its occurrence was positively correlated with number of colonies and with four soil parameters. Microsporidium-infected colonies in the survey were less likely to have brood than healthy colonies. A natural T. solenopsae epizootic was monitored in a mixed monogyne/polygyne ant population. The 89-100%-infected polygyne ants disappeared, possibly because they were at a competitive disadvantage to 15-26%-infected monogyne ants. The monogyne form did not sustain the pathogen after polygyne ants disappeared. Long-term epizootics developed when the microsporidium was released in two predominantly polygyne populations but not at two monogyne sites. In mixed host populations, prevalence peaked at >75% in both social forms; the form suffering higher prevalence decreased proportionally to the other. Prevalence averaged 47-57% and did not vary seasonally. The microsporidian rate of spread was 0.8-9.4 m/month. T. solenopsae weakened ant populations only sporadically, through decreases in numbers of foragers, colony numbers, colony size, or brood.