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

Abstract

Around the world, several pest tephritids are extending their ranges from warm tropical or Mediterranean climates into cooler temperate regions. The ability to tolerate climatic diversity is uncommon among insects, and understanding the population phenology drivers of such species across different parts of their range will be critical for their management. Here, we determined the role of temperature versus fruit availability on the population phenology of Queensland fruit fly, Bactrocera tryoni. Using a field site located at the subtropical/temperate interface, with host fruits continuously available, we monitored the development times and abundance of B. tryoni, a species which has invaded temperate Australia from the tropics. From fruit samples held at ambient and controlled conditions, the abundance of emerging flies was highly variable among collection dates, but the variance did not reflect the observed changes in temperature. For most samples, the survival rate of flies in a field site was lower than predicted by a day‐degree population model fitted with mean daily field temperatures. The development time of the immature stage in the field was prolonged, presumably due to cooler ambient conditions, but the fitted day‐degree population model consistently over‐predicted estimated development times. Our results indicate that at the subtropical/temperate interface, the decline in B. tryoni populations during winter is only partly driven by temperature and host availability. We classify B. tryoni as a climate generalist, which likely employs physiological as well as behavioural mechanisms to achieve broad climatic tolerance ranges.

A polyphagous, tropical insect herbivore shows strong seasonality in age-structure and longevity independent of temperature and host availability

“…At this point flies were visually reassessed (first assessment was done at time of capture) to confirm that only B. tryoni were held. The only other local fruit fly species easily confused with B. tryoni is B. neohumeralis (Hardy), and this species is rare at the study site 47 . Both species can be distinguished morphologically by the colour of humeral calli where B. tryoni has yellow and B. neohumeralis has brown humeral calli respectively 72 .…”

“…Notably, the “autumn to winter” breeding cessation is not temperature driven, as the lower temperatures are not cold enough to stop breeding 45 , 50 , 51 . Breeding at the site was also not halted due to lack of host fruit for larvae, as fruit are continuously available at that site 47 . The following discussion develops these points with respect to B. tryoni , but then broadens to examine the implications of the findings, and the methodology used, to understanding the demography of other tropical monsoonal insects.…”

“…The diversity of trees on the property meant that fruit suitable for B. tryoni breeding are available year-round. Because of the tourism aspect of the property, there is a minimal use of pesticides at the site 47 .…”

“…In 2018, the early autumn (16th Mar) and early spring (19th Sep) seasons were repeated. The timing of samples was chosen based on the known phenology of B. tryoni in subtropical Australia 43 , 44 , 47 which covered the end of winter period when population abundance is low just before the start of the spring population increase, the start of the spring increase, periods of high population abundance during summer and early autumn, and then during the late autumn decline. Note, the term “overwintering” in this paper is used for convenience to describe the period of low fly abundance which occurs from mid-autumn through to late winter.…”

“…However, paradoxically, this species also shows "overwintering" population declines in subtropical and tropical Australia in both endemic rainforest 42 and man-made habitats 43,44 , where lower threshold temperatures are not limiting 45,46 . A simple absence of suitable hosts during this population depression period in the tropics also does not explain the phenology of the fly, as a recent phenological study demonstrated little correlation of either temperature or host fruit availability on the population phenology of B. tryoni at an agricultural subtropical site 47 .…”