The incidence of sheep strike by Lucilia sericata on sheep farms in Britain: a simulation model.
Using a combination of a temperature-dependent simulation of the population dynamics of L. sericata and a logistic model of sheep susceptibility, based on patterns of faecal soiling, fleece length and fleece humidity, a deterministic simulation model was constructed to predict the incidence of sheep strike by L. sericata on lambs and ewes in Britain. Strikes were assigned to individual sheep in the flock according to a negative binomial distribution. The model showed that the majority of the first strikes of the season occurred equally on ewes and lambs, when ewe fleeces were long and before lamb susceptibility increased through faecal soiling. With each subsequent generation of gravid blowflies, however, a greater proportion of strikes occurred on lambs, while the number of strikes observed on ewes remained relatively consistent throughout the season. The increase in strike of lambs was due to the seasonal increase in lamb faecal soiling, associated with rising helminth infections and lamb fleece growth. The model showed that at the beginning of the season the incidence of strike is limited by the low number of flies present; towards the end of season, however, the blowfly population had grown large enough for the number of strikes to be limited by the number of susceptible ewes and lambs. The model showed that the seasonal incidence of strike is highly sensitive to the interactions between temperature, rainfall and wool length. Strike was most prevalent under warm, wet conditions. However, there was a critical period during spring, where the short fleeces of lambs and ewes, in the latter case due to shearing, resulted in the desiccation of egg batches. If this coincided with dry conditions, the high levels of mortality resulted in suppression of the fly population and a reduction in the subsequent incidence of strike.