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Keywords:

  • ivermectin;
  • Lucilia cuprina;
  • spinosad

Abstract

Compared with the reference susceptible strain neonate larvae of composite field strains of the Australian sheep blowfly, Lucilia cuprina displaying either moderate diazinon and diflubenzuron resistance or, moderate diazinon and extreme diflubenzuron resistance, were equally cross-resistant to ivermectin and spinosad. Resistance factors were low (2–3×). As third instar larvae these strains exhibited cross-resistance to ivermectin by better tolerating exposure to higher concentrations, or longer exposure to the registered concentration (32 mg/L). Despite this, larval mortality in each strain exceeded 85% following exposure to ivermectin at the registered flystrike dressing concentration for as little as 30 s. As third instar larvae these strains and the reference strain, were equally susceptible to spinosad, but mortality was only around 40% even after immersion for 60 s in an aqueous solution at the registered rate (25 mg/L). Maximum mortality (99%) was observed in larvae immersed in 250 mg/L spinosad. Two hours after 30 s immersion in ivermectin (32 mg/L) 80% of third instar larvae were no longer moving. This figure rose to 99% by 6 h. In similar bioassays spinosad (25 mg/L) failed to elicit obvious symptoms within 10 h of treatment.