Survival advantage of cyromazine-resistant sheep blowfly larvae on dicyclanil- and cyromazine-treated Merinos

Objective: To determine whether a cyromazine-resistant strain of Lucilia cuprina was able to establish strikes sooner than a susceptible strain on cyromazine- or dicyclanil-treated sheep.

Methods: Groups of 7 sheep were treated with cyromazine or dicyclanil in accordance with label directions. Beginning 5 weeks after treatment, 5 sheep from each group were challenged by implantation of neonate larvae belonging to the cyromazine-resistant strain 'Nimmitabel-selected' and the susceptible blowfly strain 'Field 2011' according to standard larval implant technique. The implant sites were alternated between the shoulder, mid-back and rump within the treatment zone. Similarly, implants of the resistant and susceptible strain larvae were alternated at each challenge such that the strain implanted on the left side of the sheep at one challenge was implanted on the right side at the next. Challenges were conducted at 3-weekly intervals until the susceptible larvae formed strikes on at least 2 of the 5 sheep in a treatment group or until 29 weeks after treatment.

Results: Sheep treated with cyromazine or dicyclanil were protected from flystrike by the cyromazine-susceptible strain for periods consistent with, or longer than, the registered product label claims. The cyromazine-resistant strain created strikes several weeks sooner after treatment than did the susceptible strain. Accordingly, the protection periods provided by cyromazine and dicyclanil against the resistant strain were reduced from 14 and 18-24 weeks to <8 weeks and <11 weeks, respectively.

Conclusion: Resistance, even in the pure-breeding resistant strain, was not so severe as to cause treatment failure with cyromazine or dicyclanil, but was sufficient to reduce the protection period provided. It is recommended that producers adopt management practices that minimise the development of resistance to these and other compounds.