Larval dispersal as related to density in wild and laboratory strains of Delia (=Hylemya) antiqua Meigen.

Using confined larval populations of Hylemya antiqua the effects of increasing larval density on larval dispersal, pupation and mortality were assessed. The experiments were performed initially using a laboratory strain (35 generations rearing in the laboratory). It was shown that dispersal, pupation and mortality were significantly different for the two strains with the wild strain dispersing at a much lower density level than the laboratory strain and with a significantly shorter mean dispersal time. The experiments were repeated four generations later to monitor the effects of laboratory colonization on the wild strain. This time dispersal, pupation and mortality were not significantly different for the two strains; the same was true for larval dispersal time. It was concluded that laboratory colonization had significantly altered larval dispersal behaviour in the wild strain. Multiple comparisons between all four strains revealed that larval dispersal in the original wild strain was significantly different from that in the other three strains. The same pattern emerged following multiple comparisons of the mean larval dispersal times. Therefore, the two laboratory strains and the wild strain reared for a total of six generations in the laboratory exhibited identical responses to increasing larval density as measured by dispersal, pupation, mortality and dispersal time. It was further demonstrated that several self-regulatory mechanisms mitigating against the adverse effects of increasing larval density were present i.e. the magnitude of the dispersal process, the time at which dispersal occurred and regulation of pupal size. It was shown that these processes did not always operate concurrently and often exhibited some interactions. The results were discussed in the framework of laboratory adaptation of the wild strain and the relevance of these behavioural changes when such individuals would be released into a field population during genetic control procedures.