In vitro larvicidal efficacy of flystrike dressings against the Australian sheep blowfly

Abstract

The abilities of 14 registered flystrike products to prevent development of Australian sheep blowfly (Lucilia cuprina) larvae were measured by laboratory bioassay. Larvae were exposed to the products for periods ranging from 5 to 180 s. Most products were quite effective against susceptible-strain larvae but less effective in field strains due to organophosphate resistance. Some products killed fewer than 20% of field larvae, even after 180 s exposure. However, field strain and susceptible-strain L. cuprina were equally likely to succumb after thorough exposure to a powder formulation of diazinon or to products containing ivermectin or cyromazine. Most products had no obvious effects on larvae until 2–8 h after treatment. Exceptions were non-aqueous, ready to use formulations such as Kleendok with diazinon, Defiance S and Coopers Flystrike Powder. In the cases of Kleendok with diazinon and Defiance S it is likely that formulation components other than the active ingredient were responsible for the fast action. Of the aqueous organophosphate formulation products Supona, containing chlorfenvinphos, was the fastest acting. Greater proportions of susceptible larvae responded over time but among field larvae that responded initially, there were some that recovered within 24 h. Increasing concern over insecticide residues in wool following jetting (topical treatment) of whole sheep, has placed greater reliance on dressing products to control flystrike. With so few effective products available there is a ready market for fast-acting, efficacious products.