Population dynamics of the sheep blowfly Lucilia sericata: seasonal patterns and implications for control

Abstract

Summary The infestation of sheep with blowfly larvae (sheep strike) is a significant animal welfare and economic problem in many regions of the world. Improved control requires greater knowledge of the population dynamics of the primary agent of strike in temperate areas, the blowfly Lucilia sericata (Diptera: Calliphoridae). The abundance of L. sericata was recorded on two farms in south‐west England between 1990 and 2000, and the time‐series analysed to describe and explain the nature of population change within and between years. To examine the catch time‐series for periodicity, the data were detrended, demeaned and tapered, and plotted against a day‐degree time base. Spectral analysis of periodograms showed that the clearest signal present was the low‐frequency seasonal cycle. The only other significant signal in both series had a periodicity corresponding to the day‐degree requirements for the entire life cycle of this insect species. This suggests that, within each season, the abundance pattern is composed of a series of semi‐discrete generation peaks. Significant density‐dependence was detected in the seasonal change in fly abundance and this was shown to be approximately compensatory in action. The results suggest that populations of L. sericata show relatively stable and predictable dynamics, with populations passing through three or four relatively discrete generations each year prior to diapause and limited by strong, apparently compensatory, density‐dependence each season. The results have important implications for the control of this insect parasite. In general, earlier and more intense farmer intervention, to reduce sheep susceptibility and treat struck animals during the blowfly season, would result in a lower L. sericata population and reduced strike incidence. However, at the start of the season, when fly abundance may be lower than the number of susceptible hosts, direct fly control, in addition to treatment, may be a more effective strategy.