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

Abstract

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°C to À4°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.

Fly foregut and transmission of microbes

“…The fly has now expanded its range into temperate Australia and, this has created a major survival problem for the fly. Unlike most other tephritid species, it has now been shown that the adults overwinter in a reproductive diapause, and this may help explain its expansion ability into temperate climates (Clarke et al, 2019). Thaochan et al (2010) reported that one group of bacteria, Firmicutes, was found mainly within the adult crop.…”

Temperature effects on “overwintering” phenology of a polyphagous, tropical fruit fly (Tephritidae) at the subtropical/temperate interface

“…Reproduction ceases during winter, with already sexual mature females resorbing eggs (Fletcher, ) and newly emerged flies remaining sexually immature (Fletcher, ). Overwintering of the fly does not occur in the pupal form (Bateman & Sonleitner, ; Clarke, Merkel, Hulthen, & Schwarzmueller, ; O'Loughlin et al, ).…”

“…Secondly, we asked how well the observed data align with predictions based on published day‐degree data (Yonow et al, ), specifically testing: (c) if development times in the field during the winter match predictions for temperature‐dependent development times; and (d) if the number of flies emerging match predictions based on temperature‐dependent mortality rates. The overall aim of the study was to determine the role of temperature and breeding hosts in the previously reported decline in B. tryoni populations in the subtropical/temperate interface during winter (Clarke et al, ; Fletcher, ; Muthuthantri et al, ). Specifically, we sought to determine whether the decline correlates with decreased adult activity and a cessation of egg laying (as occurs in temperate areas), whether egg laying and juvenile development continue through winter but are simply delayed in a predictable fashion based on day‐degree accumulation (as could be predicted for a tropical insect in marginal temperature conditions) or whether there was evidence for a combination of these drivers.…”

“…Overwintering of the fly does not occur in the pupal form (Bateman & Sonleitner, 1967;Clarke, Merkel, Hulthen, & Schwarzmueller, 2018;O'Loughlin et al, 1984).…”

“…Secondly, we asked how well the observed data align with predictions based on published day-degree data (Yonow et al, 2004), specifically testing: (c) if development times in the field during the winter match predictions for temperature-dependent development times; and (d) if the number of flies emerging match predictions based on temperature-dependent mortality rates. The overall aim of the study was to determine the role of temperature and breeding hosts in the previously reported decline in B. tryoni populations in the subtropical/temperate interface during winter (Clarke et al, 2018;Fletcher, 1974;Muthuthantri et al, 2010).…”