Protection from restrike provided by flystrike dressings

Abstract

Summary. An organophosphate-resistant strain of the Australian sheep blowfly, Lucilia cuprina, was used to initiate 4 artificial flystrikes onto each of 156 Merino sheep that carried 6 cm of wool growth. Strikes were allowed to develop for 2 days by which time large third instar larvae were present. At that time the strikes were shorn and 1 of 11 registered flystrike dressings was applied according to label directions. On 13 sheep, strikes were shorn but otherwise left untreated (controls). When inspected 1 day later only 4 out of 52 control (shorn only), and 1 out of 572 insecticide-treated strikes still contained larvae. One week after treatment and following exposure to 15 mm of artificial rainfall, the sheep were subjected to severe blowfly pressure in a fly-proof animal house. Egg masses were laid onto 25 healing wounds on 12 of the 13 control sheep. Restrikes developed from 17 of the 25 egg masses. Only sheep in 3 treatment groups had no eggs laid on the dressed lesions, however, compared with the controls, significantly fewer egg masses were laid on lesions on all but 1 treatment group. Although egg masses were found on 73 insecticide-treated lesions on 40 sheep, restrikes were recorded on only 2 lesions. Although most flystrike dressings have poor larvicidal efficacy, over 90% of strikes resolved by shearing alone. All of the registered products provided adequate protection from restrike either by repelling blowflies or by killing their eggs or the larvae that hatched from them before strikes established.