The seasonal phenology of Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) in Queensland

The Fallacy of Year-Round Breeding in Polyphagous Tropical Fruit Flies (Diptera: Tephritidae): Evidence for a Seasonal Reproductive Arrestment in Bactrocera Species

The genus Bactrocera (Diptera: Tephritidae) is endemic to the monsoonal rainforests of South-east Asia and the western Pacific where the larvae breed in ripe, fleshy fruits. While most Bactrocera remain rainforest restricted, species such as Bactrocera dorsalis, Bactrocera zonata and Bactrocera tryoni are internationally significant pests of horticulture, being both highly invasive and highly polyphagous. Almost universally in the literature it is assumed that Bactrocera breed continuously if temperature and hosts are not limiting. However, despite that, these flies show distinct seasonality. If discussed, seasonality is generally attributed to the fruiting of a particular breeding host (almost invariably mango or guava), but the question appears not to have been asked why flies do not breed at other times of the year despite other hosts being available. Focusing initially on B. tryoni, for which more literature is available, we demonstrate that the seasonality exhibited by that species is closely correlated with the seasons of its endemic rainforest environment as recognised by traditional Aboriginal owners. Evidence suggests the presence of a seasonal reproductive arrest which helps the fly survive the first two-thirds of the dry season, when ripe fruits are scarce, followed by a rapid increase in breeding at the end of the dry season as humidity and the availability of ripe fruit increases. This seasonal phenology continues to be expressed in human-modified landscapes and, while suppressed, it also partially expresses in long-term cultures. We subsequently demonstrate that B. dorsalis, across both its endemic and invasive ranges, shows a very similar seasonality although reversed in the northern hemisphere. While high variability in the timing of B. dorsalis population peaks is exhibited across sites, a four-month period when flies are rare in traps (Dec-Mar) is highly consistent, as is the fact that nearly all sites only have one, generally very sharp, population peak per year. While literature to support or deny a reproductive arrest in B. dorsalis is not available, available data is clear that continuous breeding does not occur in this species and that there are seasonal differences in reproductive investment. Throughout the paper we reinforce the point that our argument for a complex reproductive physiology in Bactrocera is based on inductive reasoning and requires specific, hypothesis-testing experiments to confirm or deny, but we do believe there is ample evidence to prioritise such research. If it is found that species in the genus undergo a true reproductive diapause then there are very significant implications for within-field management, market access, and biosecurity risk planning which are discussed. Arguably the most important of these is that insects in diapause have greater stress resistance and cold tolerance, which could explain how tropical Bactrocera species have managed to successfully invade cool temperate regions.

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

Bactrocera tryoni is a polyphagous fruit fly that is predicated to have continuous breeding in tropical and subtropical Australia as temperature and hosts are not limiting. Nevertheless, in both rainforest and tropical agricultural systems, the fly shows a distinct seasonal phenology pattern with an autumn decline and a spring emergence. Temperature based population models have limited predictive capacity for this species and so the driver(s) for the observed phenology patterns are unknown. Using a demographic approach, we studied the age-structure of B. tryoni populations in subtropical Australia in an agricultural system, with a focus on times of the year when marked changes in population abundance occur. We found that the age-structure of the population varied with season: summer and autumn populations were composed of mixed-age flies, while late-winter and early-spring populations were composed of old to very old individuals. When held at a constant temperature, the longevity of adult reference cohorts (obtained from field infested fruits) also showed strong seasonality; the adults of spring and early autumn populations were short-lived, while late autumn and late winter adults were long-lived. While still expressing in modified landscapes, the data strongly suggests that B. tryoni has an endogenous mechanism which would have allowed it to cope with changes in the breeding resources available in its endemic monsoonal rainforest habitat, when fruits would have been abundant in the late spring and summer (wet season), and rare or absent during late autumn and winter (dry season).

Age-related changes in the reproductive potential of the Queensland fruit fly

Despite the known negative impacts of aging on the reproductive potential of many insects, Bactrocera tryoni populations show a rapid increase in abundance from early to late spring when the population is composed of predominantly old individuals. While some aspects of how male and female reproductive potential are influenced by age for this species are known, no study investigates lifelong reproductive potential of either sex. We conducted a whole-of-life study in the laboratory to assess the effect of age and mating-partner age on reproductive potential of B. tryoni. The fertility of 70 individual females was directly measured by the number of eggs laid and hatched; while 70 individual males' fertility was assessed indirectly by measuring the hatch rate of eggs laid by a female partner. Half of the males and females had access to a same-age virgin mating partner, while the other half received a prime-age virgin partner (17-19 days old): in both groups mating partners were replaced weekly. Results showed that independent of the age of male mating partner, increasing age significantly reduced the fecundity and fertility of female B. tryoni after a peak at approximately 20 days of age. However, females mated with prime-age males showed higher egg hatch rates during early life than did females mated with a same-age mating partner. As indirectly measured through their partner's egg hatch rate, the fertility of B. tryoni males was also affected by the age of the male and their mating-partner's age. Males mated consistently with a prime-age partner showed an increasing trend in the egg hatch rate of their partner: indirect evidence of increasing fertility in males with increasing age. No such affect was seen when males were mated with a same-age female, possible because of the age-related changes in female fecundity and fertility. While fecundity is greatly reduced in old females, the whole-of-life data shows that the very old flies present in the field at the end of winter are physiologically capable of starting the new season's F₁ generation. Beyond getting it begun, old females are unlikely to further contribute to the new season's population as their fecundity does not increase even if mated with a prime-age, new generation male. In contrast, old males, if they have subsequent access to new generation females, have the capacity to help contribute to the rapid spring population growth which is observed in the field.

Response of Male Queensland Fruit Fly (Diptera: Tephritidae) to Host Fruit Odors

The surveillance and management of Dacini fruit fly pests are commonly split by fly gender: male trapping focuses on the dacine 'male-lures', whereas female trapping focuses on lures based on host-fruit volatiles. Although the males of several Dacini species have been reported to be attracted to host fruit volatiles, the option of using host-fruit traps for males has, to date, been ignored. Males of the cue-lure responsive fruit fly Bactrocera tryoni (Froggatt) have been recorded as responding to host-fruit volatile blends, but it is not known how frequently this happens, if it is age-dependent, or the strength of the response relative to cue-lure throughout the year. Here, we conducted an olfactometer experiment to test the lifetime (weeks 1-15) response of B. tryoni males to the odor of tomato, a known host of this fly, and compare catches of wild males to tomato-based traps and cue-lure traps in the field. Bactrocera tryoni males started to respond to tomato odor as they sexually matured (2 to 3 wk olds) and thereafter showed consistent olfactory response until advanced age (15 wk). In the field, wild males were captured by tomato-based traps throughout the year at a level not significantly different from cue-lure traps. The reason for the consistent B. tryoni male response to host fruit odor at this stage is not known, but it certainly occurs at a level greater than can be continued to be ignored for both basic and applied research.

Effects of advanced age on olfactory response of male and female Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae)

Olfaction is an essential sensory modality of insects which is known to vary with age. In short-lived insects odour response generally declines rapidly with increasing age, but how increasing age affects the olfactory response of long-lived insects is less known and there may be different life-time patterns of olfactory response. Here, we examine the effect of age on olfactory response and exploratory activity of a long-lived tephritid fruit fly, Bactrocera tryoni from sexual maturity (3 weeks) to advanced age (15 weeks). Males were tested against a male-specific attractant, cue-lure, which is associated with courtship and sexual selection in this species; while females were tested against guava-juice, a highly attractive oviposition host fruit odour. Trials were done in the laboratory using a Y-tube olfactometer at three weekly intervals. The probability of olfactory response of both males and females to tested odours declined with age. Males retained a constant attraction to cue-lure until 12 weeks of age, but then showed a significant drop in olfactory response at 15 weeks. However, females showed the highest attraction to guava-juice odour until six weeks of age and declined gradually thereafter. The change on odour response over time can be associated with an age-related change in initial locomotor activity for females as there was no change, over the life of the experiment, in selective female orientation to the odour source once flies started exploring within the olfactometer. However, for 15 week-old males, there was a simultaneous drop in both locomotor activity and selective olfactory orientation. The consistent attraction of male to cue-lure might be related to life-long reproductive activities of males, as males are thought to mate continuously during life. On the other hand, females' highest attraction to guava-juice odour in early life followed by a gradual decline might be linked with their oviposition rate which peaks in early life.

Impacts of climate change on high priority fruit fly species in Australia

Tephritid fruit flies are among the most destructive horticultural pests posing risks to Australia’s multi-billion-dollar horticulture industry. Currently, there are 11 pest fruit fly species of economic concern in Australia. Of these, nine are native to this continent (Bactrocera aquilonis, B. bryoniae, B. halfordiae, B. jarvisi, B. kraussi, B. musae, B. neohumeralis, B. tryoni and Zeugodacus cucumis), while B. frauenfeldi and Ceratitis capitata are introduced. To varying degrees these species are costly to Australia’s horticulture through in-farm management, monitoring to demonstrate pest freedom, quarantine and trade restrictions, and crop losses. Here, we used a common species distribution model, Maxent, to assess climate suitability for these 11 species under baseline (1960–1990) and future climate scenarios for Australia. Projections indicate that the Wet Tropics is likely to be vulnerable to all 11 species until at least 2070, with the east coast of Australia also likely to remain vulnerable to multiple species. While the Cape York Peninsula and Northern Territory are projected to have suitable climate for numerous species, extrapolation to novel climates in these areas decreases confidence in model projections. The climate suitability of major horticulture areas currently in eastern Queensland, southern-central New South Wales and southern Victoria to these pests may increase as climate changes. By highlighting areas at risk of pest range expansion in the future our study may guide Australia’s horticulture industry in developing effective monitoring and management strategies.

Potential impacts of climate change on habitat suitability for the Queensland fruit fly

Anthropogenic climate change is a major factor driving shifts in the distributions of pests and invasive species. The Queensland fruit fly, Bactrocera tryoni Froggatt (Qfly), is the most economically damaging insect pest of Australia's horticultural industry, and its management is a key priority for plant protection and biosecurity. Identifying the extent to which climate change may alter the distribution of suitable habitat for Qfly is important for the development and continuation of effective monitoring programs, phytosanitary measures, and management strategies. We used Maxent, a species distribution model, to map suitable habitat for Qfly under current climate, and six climate scenarios for 2030, 2050 and 2070. Our results highlight that south-western Australia, northern regions of the Northern Territory, eastern Queensland, and much of south-eastern Australia are currently suitable for Qfly. This includes southern Victoria and eastern Tasmania, which are currently free of breeding populations. There is substantial agreement across future climate scenarios that most areas currently suitable will remain so until at least 2070. Our projections provide an initial estimate of the potential exposure of Australia's horticultural industry to Qfly as climate changes, highlighting the need for long-term vigilance across southern Australia to prevent further range expansion of this species.

Population dynamics and management of diamondback moth (Plutella xylostella) in China: the relative contributions of climate, natural enemies and cropping patterns

Diamondback moth or DBM is the major pest of Brassica vegetable production worldwide. Control has relied on insecticides, and DBM resistance to these compounds has evolved rapidly. We review and summarize data on DBM population dynamics across a large latitudinal gradient from southwest to northeast China: DBM is, on average, more common in southern locations than in northern locations. The species' phenology is consistent: in southern and central locations there is a decline during hot summer months, while in the north, the species can only exist in the summer following migrations from the south. A cohort-based discrete-time model, driven by daily maximum and minimum temperatures and rainfall, which was built using the DYMEX modelling software, captures the age-structured population dynamics of DBM at representative locations, with year round cropping and threshold-based insecticide applications. The scale of the simulated pest problem varies with cropping practices. Local production breaks and strict post-harvest crop hygiene are associated with lower DBM populations. Biological control appears to improve the management of DBM. Of the management strategies explored, non-threshold based applications of insecticides with reduced spray efficacy (due to poor application or resistance) appear the least effective. The model simulates the phenology and abundance patterns in the population dynamics across the climatic gradient in China reasonably well. With planned improvements, and backed by a system of field sampling and weather inputs, it should serve well as a platform for a local pest forecast system, spanning the range of DBM in China, and perhaps elsewhere.

Population dynamics and "outbreaks" of diamondback moth (Lepidoptera: Plutellidae) in Guangdong province, China: climate or failure of management?

Diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), became the major pest of Brassica vegetable production in Guangdong, a province in southeastern China, in the late 1980s and has continued to challenge growers, particularly during the spring and autumn. Control has relied on insecticides and, as has happened in other parts of the world, resistance to these has evolved and subsequent field control failures have occurred. We review and summarize the history of diamondback moth management in Guangdong. We show that the geographic distribution of the pest in China is well described by a simple climate niche model. Our model predicts the seasonal phenology and some of the variation in abundance among years in Guangdong. Discrepancies may reflect migration and insecticide use at a landscape level. The scale of the pest problem experienced varies with management practices. Local production breaks, and strict post harvest hygiene are associated with lower pest pressure on large-scale production units. As more and more insecticides become ineffective the need to implement an insecticide resistance management strategy, as well as basic integrated pest management practices, will become more pressing. The potential use and development of a better forecasting system for diamondback moth that will assist these developments is outlined.