Dietary methoprene and caffeine as pre-release supplements for Queensland fruit fly Bactrocera tryoni (Froggatt) sterile insect technique

Sterile insect technique (SIT) is a sustainable pest management technique that involves releasing millions of sterile insects to suppress reproduction of pest populations. SIT has been employed to manage outbreaks of Queensland fruit fly (Bactrocera tryoni Froggatt, 'Q-fly'), the most difficult and costly challenge to market access for most Australian fruit growers. Due to a long adult maturation phase, immature Q-flies released in SIT can take a week or longer after release to attain sexual maturity. This delay from release until maturation can result in large proportion of sterile flies dying before becoming sexually active, and this can substantially constrain the efficacy of SIT. Treatments that promote early sexual maturity, as well as mating performance and longevity, provide valuable means of increasing abundance of sexually mature sterile males. Building on this understanding, my PhD investigated the viability of the juvenile hormone analogue methoprene and caffeine as pre-release treatments for Q-fly SIT. In laboratory trials, incorporation of a mosquito larvicide Nomoz (containing 40% s-methoprene) in a diet of sugar and yeast hydrolysate was found to substantially increase mating propensity of young male Q-flies; this effect was comparable to the effects obtained using analytical standard methoprene, providing an economic and practical method of methoprene application. The effect of methoprene on sexual activity of young male Q-flies is not only a behavioral response, but rather methoprene treatments resulted in accelerated maturation of reproductive organs. The effect of methoprene on mating propensity of young male Q-flies was confirmed in field cage studies that simulate field conditions; young treated sterile males (5-7 days old) sexually outcompeted untreated mature males. The effects of methoprene treatment on mating extended to post-copulatory success; the additional matings obtained by young treated males exhibited typical efficacy at inducing sexual inhibition in their mates. Contrasting potent effects on males, no effects of methoprene treatment on mating propensity or development were evident for females either in the laboratory or in field cages. The effects of methoprene treatment were not limited to sexual development physiologically but had a broad effect on physiology that present risks for operational use. Methoprene supplementation resulted in greatly increased activity levels in young flies, and reduced ability to tolerate challenges of nutritional and desiccation stress. In addition, potential of pre-release caffeine as a novel means to accelerate sexual maturation in Q-fly was demonstrated in one of my PhD projects. Dietary supplementation of newly emerged males with analytical caffeine significantly enhanced mating propensity and promoted development of ejaculatory apodeme and testis in male Q-fly. Overall, my thesis demonstrates substantial potential of dietary methoprene and caffeine treatment to enhance the performance of Q-fly SIT through (1) increasing the proportion of released males that mature and contribute to the program and (2) by biasing operational sex ratio of released flies in favour of sterile males due to sex differences in response to methoprene application -- summary.