Strategic early treatment for control of sheep flystrike: potential economic benefits examined using a weather-driven model of flystrike risk

The application of preventive treatment against flystrike early in spring (before strike occurs) may reduce the reproductive success of the sheep blowfly, thereby maintaining a low risk of flystrike throughout the fly season. This hypothesis has been examined using a weather-driven model of flystrike (incorporating a reduced risk due to early treatment), based on previous experimental studies. The model indicates that in the Gunning area (with a high flystrike risk), this reduction in seasonal risk would reduce total costs related to flystrike, reduce the overall use of preventive chemical treatment and reduce the number of sheep struck. However, in a lower risk area (Flinders Island), the value of early treatment would depend on the date of shearing and the local risk of flystrike. Under some conditions, no preventive treatment would be necessary in most years because of a regional low risk of flystrike, so there would be no economic benefit from always using early treatment. Early treatment is least likely to be cost-effective when shearing is required soon after the optimal date for early treatment. Further experimental studies are recommended to fully examine whether the previously reported reduction in flystrike resulting from early treatment is consistent over a wider range of climatic conditions.