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United States Department of Agriculture

Agricultural Research Service


item Oi, David
item Williams, David

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2002
Publication Date: N/A
Citation: N/A

Interpretive Summary: Imported fire ants are serious pests throughout the southern U.S. and in parts of California. These ants are an invasive species, originally from South America. There are two forms of imported fire ants: 1) a single- queen form which is territorial, and may have over 250,000 adult workers per colony; and, 2) a multiple-queen form which is not-territorial and can develop colony sizes of over 500,000 ants. Because of their large size an numerous queens, the multiple queen form is more difficult to control. In the U.S., there is a lack of natural enemies which may mediate the spread and impact of fire ants. In 1996, a pathogen of imported fire ants found in South America, called Thelohania solenopsae, was discovered in Florida. In laboratory studies with the single-queen form, T. solenopsae infected the ovaries of the queen, reduced her egg production, and caused premature death. However, the pathogen's impact on the multiple-queen form was unknown. Scientists from the Center for Medical, Agricultural, and Veterinary Entomology in Gainesville, FL determined that under laboratory conditions, multiple-queen colonies infected with T. solenopsae will stop reproducing within 46 weeks, which is approximately two times longer than single-queen colonies. In addition, under field conditions, only half of the queens per multiple-queen colony were found to be infected with the pathogen. Nevertheless, multiple-queen fire ant populations, which had a natural infection of T. solenopase, were reduced by as much as 65% over a two year period. However, it should be noted that this reduction fluctuated. These results suggest that this pathogen can reduce fire ant populations, but the presence of multiple queens can slow the impact.

Technical Abstract: A total of 57 of 122 queens (46.7%) from nine, field-collected, polygynous, red imported fire ant, Solenopsis invicta, colonies were infected with the entomopathogen Thelohania solenopsae. Infection rate of queens for each colony ranged from 25 to 75%. Laboratory colonies of polygyne S. invicta, with 3-12 queens, were inoculated and infected with T. solenopsae. Brood levels in all infected colonies declined to 0 after 26-52 weeks. Brood di not reappear in all colonies after 3-11 weeks, even though in 2 of the 8 infected colonies, 5 fertile queens were recovered. Thus, polygyne, S. invicta colonies infected with T. solenopsae, which were confined and isolated under laboratory conditions, did not recover. Field plots that contained polygynous S. invicta colonies, which were infected with T. solenopsae, were monitored over a 2-year period. Infection rates increased during the study and reached a maximum of 93%. Fire ant nest density and colony sizes fluctuated over time, with maximum reduction of 63% per plot. In general, reductions were attributed to smaller colony sizes.

Last Modified: 08/18/2017
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