Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) overwintering: an overview

The Dacini fruit flies are tropical and subtropical insects for which a winter diapause is largely unknown. However, some pest dacines have extended their geographic ranges into temperate areas where winter temperatures may be limiting. During the 1900s, Bactrocera tryoni (Froggatt) extended its Australian endemic tropical range to include temperate southern Australia. Starting nearly 100 years ago, several research teams have spent considerable effort understanding how 'tropical' B. tryoni survives temperate winters: this review summarises that research. Across three study areas (Sydney and surrounding districts, New South Wales; Melbourne and surrounding districts, Victoria; Stanthorpe, south-eastern Queensland), there is highly consistent data showing that the fly overwinters almost exclusively in the adult form. Eggs, larvae and pupae generated in late autumn or early winter either fail to develop due to lack of sufficient day-degree accumulation or if immature development is successful and adults emerge, then those adults do not become sexually mature until the spring. Females which are already sexually mature when entering winter resorb some or all of their eggs. During winter, adults rest in sheltered locations and will forage for food on warm days. In field cages, mortality of adults is approximately 8% per week during winter, and so significant adult mortality occurs during winter. Based on laboratory studies, adult survival at cool to cold (15 degrees C to -4 degrees C) temperatures is increased if the fly or earlier developmental stages (pre-pupal maggots and fully developed pupae) undergo an acclimation process, rather than being subjected to a sudden drop in temperature. Early climate change modelling suggests that a decrease in the number of extreme cold events (i.e. hard frosts) is more likely to lead to improved fly survival in temperate areas than the overall mean temperature increase. Gaps in knowledge include quantification of what makes a preferred overwintering site; how climate change over the last 30 years (when the last detailed experiments were done) have modified overwintering phenologies; and where on the Australian east coast does temperature become limiting such that year-round breeding ceases.