Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/14/2005
Publication Date: 12/24/2005
Citation: Fuxa, J.R., Sokolova, Y.Y., Milks, M.L., Richter, A.B., Williams, D.F., Oi, D.H. 2005. Prevalence, Spread, and Effects of the Microsporidium Thelohania solenopsae Released into Populations with Different Social Forms of the Red Imported Fire Ant (Hymenoptera: Formicidae). Environmental Entomology. 34(5):1139-1149. Interpretive Summary: The red imported fire ant is a stinging ant that infests over 320 million acres in 14 states and causes an estimated $6.5 billion annually in damage, control, and medical expenditures. 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 South American pathogen, called Thelohania solenopsae, that debilitates fire ant queens and can cause reductions in fire ant populations was found in Florida by scientists from the USDS-ARS Center for Medical, Agricultural, and Veterinary Entomology in Gainesville, FL (CMAVE). Sustained infections generally occur in areas where fire ant populations are comprised of colonies that contain several queens per colony. However, the predominant fire ant social form in the U.S. has only a single queen per colony. In a cooperative study among scientists at Louisiana State University and CMAVE, T. solenopsae infections in a mixed population of single and multiple-queen fire ant colonies indicated that the social form suffering higher prevalence of the disease decreased proportionally to the other form, possibly indicating a competitive disadvantage. The strongest evidence of disease impact on fire ant populations was a negative correlation between colony numbers versus percentage infection and a sporadic decrease in the number of foraging ants. These results characterized the dynamics of T. solenopsae infections relative to fire ant social form and will help scientists overcome barriers to the widespread establishment of this disease. This pathogen of fire ants has the potential to contribute to the long term suppression of fire ants which will benefit all who are affected by this invasive pest.
Technical Abstract: The microsporidium Thelohania solenopsae Knell, Allen, and Hazard was released into colonies of red imported fire ants, Solenopsis invicta Buren, at four field sites in Louisiana. Social form of the ant affected the establishment of the microsporidium; long-term epizootics developed at two sites with predominantly polygyne populations, whereas the pathogen produced a low disease prevalence before it disappeared or did not infect ants at all at two monogyne sites. This study is the first report of artificial infection of monogyne S. invicta field colonies. When the microsporidium became established in mixed monogyne/polygyne ant populations, prevalence rates peaked at >75% in both social forms. In mixed ant populations, the social form suffering higher prevalence of the disease decreased proportionally to the other form, possibly indicating a competitive disadvantage. Host population density or site characteristics may have influenced spread of the disease; the rate of spread was 0.9 m/mo at one epizootic site and reached 9.4 m/mo at the other. There was little seasonal variation in prevalence, which averaged 47% in February, 51% in April/May, and 57% in October/November at the two epizootic sites. The strongest evidence of microsporidian impact on S. invicta populations was a negative correlation between colony numbers versus percentage infection and a sporadic decrease in the number of foragers. There was some evidence of a decrease in the size and number of colonies at one epizootic site and a decrease in brood at the other. This sporadic weakening of the S. invicta populations did not lead to significant immigration of other ant species.