Sensitivity analysis of a stochastic model for the sheep blowfly Lucilia sericata

Using empirically derived relationships between temperature and development rate, a stochastic simulation model was used to predict the seasonal pattern of abundance of the sheep blowfly Lucilia sericata. Sensitivity analysis suggests that, at the levels of mortality observed on sheep farms in Britain, L. sericata abundance may not be strongly influenced by the number of oviposition sites available. Hence, control techniques aimed at reducing the number of sites in the field may have little impact on the blowfly population and the resulting incidence of sheep strike. The model shows that variations in adult mortality have a much greater effect on the blowfly population than variations in pre-adult mortality, both in terms of the number of flies present in each generation and the probability of extinction of the population. With levels of pre-adult mortality of 50% per generation, adult mortality levels in excess of 30% per day would be needed to prevent the growth of a population of L. sericata. In contrast, at the lowest level of adult mortality examined, pre-adult mortality levels of 90% per generation were unable to prevent the population from increasing. The simulations suggest that unless consistently high levels of larval mortality can be maintained by a control programme, suppression of a L. sericata population is unlikely to be achieved. In contrast, relatively small increases in adult mortality may have a significant impact on L. sericata abundance, and attempts to suppress L. sericata populations by targeting the adult population may be an effective future approach.