Dietary methoprene enhances sexual competitiveness of sterile male Queensland fruit flies in field cages

Queensland fruit flies Bactrocera tryoni (Froggatt) have a long adult maturation phase which, together with high mortality rates, can substantially reduce number of released flies that survive to mature and contribute to sterile insect technique (SIT) programmes. This constraint on SIT can potentially be addressed by incorporating methoprene, a juvenile hormone analogue, into an adult diet of sugar and yeast hydrolysate for 2 days after emergence. Methoprene treatments have been found to accelerate sexual development of male Queensland fruit fly, resulting in increased mating propensity of 5-7-day-old males in no-choice laboratory trials. Before considering deployment of methoprene as a pre-release treatment in SIT, it is necessary to demonstrate mating competitiveness and compatibility of methoprene-treated flies under field-like conditions. In the present study, we assessed whether methoprene treatment increases ability of sterile males (5 and 7 days old) to compete with mature (wild or laboratory) males for matings with mature (wild or laboratory) females in field cages. We also investigated mating compatibility to test for sexual isolation between sterile flies and mature (wild or laboratory) fertile flies. In mating competitiveness tests, methoprene-treated males of either age outperformed mature wild or laboratory males for matings with mature wild or laboratory females, respectively. Untreated 5- and 7-day-old males were less competitive than mature wild or laboratory mature males and hence had lower relative sterility indexes. Methoprene-treated males mated earlier in the evening and continued mating for longer than untreated sterile males and mature wild or laboratory males. In mating compatibility trials, methoprene-treated males mated equally with methoprene-treated females and mature females, whereas methoprene-treated females tended to mate more often with mature males than with methoprene-treated males. However, untreated flies of both sexes exhibited substantial sexual isolation. Pairings that comprised methoprene-treated males and mature females had shorter mating latency and longer copulations than other pairings. Unlike males, methoprene-treated females did not exhibit changes in mating latency or duration. Overall, the present study supports the use of pre-release dietary methoprene treatment in Queensland fruit fly SIT.