Submitted to: Environmental Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/1/2003
Publication Date: 12/15/2003
Citation: Oi, D.H., Williams, D.F. 2003. Thelohania solenopsae (Microsporidia: Thelohaniidae) Infection In Reproductives of Red Imported Fire Ants (Hymenoptera: Formicidae) and Its Implication for Intercolony Transmission. Environmental Entomology. v. 32(5). p. 1171-1176. Interpretive Summary: Imported fire ants are an invasive pest that has spread throughout the southern U.S. and into California. 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 discovered in Florida by scientists from the Center for Medical, Agricultural, and Veterinary Entomology in Gainesville, FL (CMAVE). The pathogen infects fire ant queens, causing lower egg-laying rates and premature death which can result in the eventual death of entire fire ant colonies. The pathogen can be artificially transmitted by introducing live, infected, immature ants, or brood, into an uninfected colony. However, in order to more efficiently utilize this pathogen as a biological control agent of fire ants, the natural mechanism of transmission between colonies needed to be determined. CMAVE scientists found that brood raiding, the stealing of brood among colonies, may be a natural mechanism of the inter-colony transmission of the disease. It was shown that infected, virgin male and female reproductive fire ants could initiate mating flights, resulting in newly-mated queens that were infected. These infected queens were capable of only founding weak, incipient colonies that contained infected brood. The majority (85%) of the infected queens from these colonies lived less than 6 months in contrast to a healthy queen which can live over 5 years. Because these queens are able to produce infected brood, they provide a potential source of inoculums that could be spread by brood raiding. This was demonstrated by the infection of 67% of large colonies that stole brood from small, infected colonies. These results help to explain how the fire ant pathogen, T. solenopsae, can spread and be self-sustaining. These are important factors of the long term suppression of fire ants which will benefit everyone who is affected by this stinging, invasive ant.
Technical Abstract: The natural mechanism of intercolony transmission of Thelohania solenopsae, a pathogen of red imported fire ants, Solenopsis invicta, is unknown. However, T. solenopsae can be transmitted by introducing infected brood into an uninfected colony. We hypothesized that brood raiding, the stealing of brood among colonies, may be a mechanism for the horizontal transmission of T. solenopsae. Male and female reproductive caste alates, collected during the initiation of mating flights from infected colonies, had T. solenopsae infection rates of 93 and 75%, respectively. In addition, 31 field-collected, newly-mated queens that were reared in the laboratory, established T. solenopsae infected colonies that contained all life stages (egg, larvae, pupae, adults). Because T. solenopsae is transovarially transmitted, this indicated that infected founding queens generated infected colonies. The lifespan of 85% of these queens was less than 26 weeks compared to over 3 years observed for uninfected queens. To determine if imported fire ant colonies can become infected with T. solenopsae via brood raiding, 7 pairs of S. invicta colonies consisting of a large uninfected and a small, infected colony were given access to each other in the laboratory. T. solenopsae infection was detected in 4 of 7 of the large colonies. In the 4 large, infected colonies, brood levels declined an average of 64% after 22 weeks in contrast to a 116% increase in the controls. Thus, it was evident that incipient, T. solenopsae infected colonies could provide a source of inocula for the horizontal transmission of T. solenopsae by brood raiding among red imported fire ant colonies.