Sterilisation of the Screw‐worm Fly, Chrysomya Bezziana Villeneuve (Diptera: Calliphoridae), by Gamma Radiation

The effect of gamma radiation on the screw‐worm fly, Chrysomya bezziana, was studied to determine the optimum dose and stage for sterilisation. Laboratory cultured flies were exposed in a gamma irradiator with a cesium‐137 source. Treated flies were compared with untreated controls for adult emergence, survival, insemination and fertility. Puparia treated with 5 krad at 1–2 d before emergence were rendered infertile but not otherwise adversely affected. When flies were irradiated early in pupal development, adverse effects were apparent. When late puparia were irradiated with 1–8 krad, adult emergence, survival and rate of insemination were similar to controls. Complete sterility of males and females was achieved with 4 krad. There was no larval survival from eggs when both parents were treated with 1 or more krad but larval survival was recorded from eggs laid by untreated females mated with 1–2.5 krad treated males and from eggs laid by 1–1.5 krad treated females mated with untreated males. There was no recovery of fertility after sterilisation with 4 krad. Irradiation of males at 2–6 krad as late puparia did not impair mating competitiveness in laboratory tests. In a field evaluation of male competitiveness, the release of sterile males at a density of 144 males per km2 per week over a 78 km2 area for 7 weeks resulted in 25 per cent sterility in the native screw‐worm fly population after 4 weeks of releases.