Toxins and blowfly population dynamics.

This thesis studies the effects of toxins upon larvae of the blowfly Lucilia sericata. A field study of fly populations infesting carcasses showed aggregations in space and time of a number of fly species, although L, sericata was not common. The presence of cadmium or deltamethrin in the larval diet was shown to have deleterious effects upon the larvae. Development was slowed down and the resultant adults were smaller. When the diet contained cadmium, adults had a lower fecundity than those arising from larvae fed upon the control diet. These effects became more pronounced as larval population density was increased. Models were constructed that simulated the population dynamics of L. sericata under two conditions. In each population, the larval diet was limited to 20 g/day whilst in one the diet was contaminated with 50 mg Cd/kg diet. These models allowed the underlying dynamics and their driving forces to be identified. The control model predicted sustained population cycles with a period of 67 days, approximately twice the generation time calculated from cohort life-tables. The cadmium model predicted that these cycles would be dampened and the mass of individual pupae increased relative to those from the control simulation model. These theoretical results, which apparently contradict the predictions made by scope for growth theory, are consistent with results from a long-term population study and were due to the interaction of cadmium with the effects of population density.