Female-biased density-dependent dispersal of a tephritid fly in a fragmented habitat and its implications for population regulation

The aim of this study was to quantify the rate of dispersal as a response t o density in the specialist tephritid fly Paroxyna plantaginis (the main se ed predator on its patchily distributed host plant, Tripolium vulgare, Aste raceae). Marked flies were released at three different fly densities in art ificial host patches. The individual histories of recaptures were recorded as well as migration between patches and invasion by unmarked flies. The lo ss of marked flies relative to initial density was analysed using maximum l ikelihood estimation. Females generally had the highest loss rate. When com paring a density-independent model with a density-dependent model of the lo ss rate, the density-dependent model won four times out of six for the fema les but not a single time for the males. A stronger immigration rate of fem ales relative to males supported the suggested female-biased dispersal. Thi s indicates a sit-and-wait strategy for the territorial males and a pre-emp tive competition strategy for egg-laying substrates for the females. These results may be of general importance for non-frugivorous tephritid systems with unpredictable and almost ephemeral accessibility to host plants and wi th a dynamics characterised by a high turnover rate and high attack levels. The study presents a method for measuring the propensity of individuals to leave an area as a response to local density. It is further an example of the consequences individual behavioural responses may have on the populatio n dynamics of a patchy population.