Prevalence and impact of the microsporidium Thelohania solenopsae (Microsporidia) on wild populations of red imported fire ants, Solenopsis invicta, in Louisiana.

We surveyed 165 sites to determine the ecological factors that might influence the distribution and prevalence of Thelohania solenopsae, and its effect on the demography of the red imported fire ant (Solenopsis invicta) in Louisiana. The microsporidium was found in 9.9% of colonies and at 16% of sites. Its distribution was clumped within the state with the majority of infected colonies and sites occurring in two infection patches. The proportion of polygyne colonies was a strong (positive) predictor of the proportion of infected colonies at a site. Infected monogyne colonies, however, still accounted for nearly 20% of infected colonies, a much higher proportion than anticipated. Several other factors, including the numbers of colonies at a site, precipitation, proximity to commercial waterways and ports, and type of habitat were also retained in the multiple logistic regression model describing T. solenopsae prevalence. The microsporidium appears to adversely affect the occurrence of brood, and possibly the size of S. invicta colonies and the mass of workers. It, however, was not included in the multiple regression model of the number of colonies or the density of ants at a site. Although our findings do not imply causation, they have identified several variables that might influence the epizootiology of T. solenopsae. Future work should concentrate on experimentally manipulating these variables to confirm these relationships.

Threshold distances and depths of nucleopolyhedrovirus in soil for transport to cotton plants by wind and rain.

Two aspects of abiotic transport of nucleopolyhedrovirus from soil to cotton plants were examined in greenhouse experiments: the distance from the plants and depth in soil from which the virus could be transported under controlled conditions of soil type and moisture, wind, and precipitation. Transport distance and depth were tested separately under relatively conducive (precipitation/sandy soil and wind/clay soil) and non-conducive (precipitation/clay soil and wind/sandy soil) conditions, as determined in previous research. The amount of virus transported by precipitation generally decreased as distance from the plant increased, but in wind the amounts of virus transported were best described by polynomial models, with transport efficiency usually peaking at a distance of 60 cm. Depending on plant height and tissue, the farthest distances that virus was transported ranged from 30 to 60 cm by precipitation from clay soil, 60-75 cm in precipitation/sand, 60-80 cm in wind/clay, and 60-80 cm in wind/sand. In the depth experiments, transport by precipitation and wind generally decreased as the depth of virus in soil increased. The greatest depth from which NPV was transported ranged from 0 to 0.5 cm by precipitation from clay soil, 0.5-1.0 cm in precipitation/sand, 1.0-2.0 cm in wind/clay, and 0.5-1.0 cm in wind/sand. All of the experimental parameters (distance or depth, soil type, plant height, plant tissue) and all two-way interactions significantly (P<0.05) affected transport in all four experiments, except for the "soilxplant tissue" interaction in the depth/wind experiment. In all of the experiments, transport was significantly greater (P<0.05) to lower than to upper portions of plants and to leaves than to buds and squares. Transport was significantly greater from sandy soil than from clay in precipitation, and it was greater from clay than from sandy soil in wind. The results will contribute to NPV epizootiology, microbial control, and risk assessment.

Interaction of an entomopathogen with an insect social form: an epizootic of Thelohania solenopsae (Microsporidia) in a population of the red imported fire ant, Solenopsis invicta.

This is the first report of Thelohania solenopsae infections in monogyne (single-queen) Solenopsis invicta colonies in the field. In a 0.2-ha plot near Baton Rouge, Louisiana, inter-colony prevalence was 63% infection in June, 1999, when the population was 100% monogyne. In February, 2000, 21% of 33 monogyne and 90% of 10 polygyne colonies were infected. By May, 2001, the polygyne colonies had disappeared and only one of 34 monogyne colonies was infected, the final detection of T. solenopsae in the plot. Colony size did not differ significantly among the four types (monogyne versus polygynexinfected versus uninfected).