Thelohania solenopsae (Microsporidia: Thelohaniidae) infection in reproductives of red imported fire ants (Hymenoptera: Formicidae) and its implication for intercolony transmission

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 the transfer of brood among colonies during intercolony competition 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, 47 field-collected, newly mated queens that were reared in the laboratory established T. solenopsae-infected colonies that contained egg, larvae, pupae, and adults. Because T solenopsae is transovarially transmitted, this indicated that infected founding queens generated infected colonies. A life span of less than or equal to52 wk was documented for 81% of the infected queens and 59% for uninfected queens. To determine whether imported fire ant colonies can become infected with T solenopsae via colony raiding, seven pairs of S. invicta colonies consisting of large, uninfected and small, infected colonies were given access to each other in the laboratory. T solenopsae infection was detected in four of seven of the large colonies. In the four large, infected colonies, brood levels declined an average of 64% after 22 wk in contrast to a 116% increase in the controls. Thus, there was evidence that incipient, T solenopsae-infected colonies could provide a source of inocula for the horizontal transmission of T solenopsae through the transfer of brood during colony raiding.