Response to laboratory selection with cyromazine and susceptibility to alternative insecticides in sheep blowfly larvae from the New South Wales Monaro.

To determine the significance of cyromazine resistance in blowfly larvae from a region where resistance has been previously confirmed and to measure susceptibility of the cyromazine-resistant blowflies to ivermectin and spinosad. Blowfly larvae from five properties located within 5 km of where cyromazine-resistant Lucilia cuprina were detected in 2011 were tested for resistance to cyromazine. The original cyromazine-resistant 'Nimmitabel' strain was reared for 13 generations on homogenised liver containing cyromazine at a concentration lethal to susceptible larvae. Larval development bioassays in which batches of neonate larvae were transferred onto homogenised bovine liver containing cyromazine or dicyclanil were used to determine whether the level of resistance had increased. The sensitivities of the 'Nimmitabel' strain to ivermectin and spinosad were also measured. Cyromazine-resistant larvae were identified in samples from four of the five properties near to the site of the original resistance detection. The 'Nimmitabel' strain responded to laboratory selection by becoming more resistant to cyromazine (8×) and to dicyclanil (3×). No larvae died after feeding on homogenate containing cyromazine at a concentration lethal to susceptible larvae. There was no cross-resistance to ivermectin or spinosad in the 'Nimmitabel' strain. The modest increases in resistance to cyromazine and dicyclanil following laboratory selection suggest that unless the situation worsens considerably, it is unlikely that this resistance will have a significant effect on flystrike protection provided by these insecticides if applied as directed on product labels. Moreover, cyromazine-resistant larvae remained susceptible to ivermectin and spinosad.