Predicting the spread of introduced species, such as natural enemies used in classical biological control programs, requires quantitative data on the rates of spread. Here, the pattern of spread of Pseudacteon tricuspis Borgmeier (Diptera: Phoridae), a parasitoid of the red imported fire ant (Solenopsis invicta Buren; Hymenoptera: Formicidae), was monitored at two release sites in Louisiana, USA. At both sites, P. tricuspis range expansion (measured as the mean radius of the range from four cardinal directions) was accelerating during the first four years post release. This suggests that population spread involved both a neighborhood diffusion and long-distance dispersal component. This is known as stratified or jump dispersal. Annual rates of spread were low in the first two years post release (possibly owing to an Allee effect), increased rapidly in years 3-4, and slowed down or leveled off by years 5-6. Annual spread rates reached a peak of 15-25 km/yr, with the northward spread being about 40% greater than the spread in the other cardinal directions. High rates of spread in the latter years and directional bias in the spread of P. tricuspis may have been driven by prevailing winds and two northward-moving hurricanes.
Keywords: Biological control; Long-distance dispersal; Range expansion; Spatial spread; Stratified dispersal