Divergent east-west lineages in an Australian fruit fly, (Bactrocera jarvisi), associated with the Carpentaria Basin divide.

Bactrocera jarvisi is an endemic Australian fruit fly species (Diptera: Tephritidae). It occurs commonly across tropical and subtropical coastal Australia, from far-northern Western Australia, across the 'Top End' of the Northern Territory, and then down the Queensland east coast. Across this range, its distribution crosses several well documented biogeographic barriers. In order to better understand factors leading to the divergence of Australian fruit fly lineages, we carried out a population genetic study of B. jarvisi from across its range using genome-wide SNP analysis, utilising adult specimens gained from trapping and fruit rearing. Populations from the Northern Territory (NT) and Western Australia were genetically similar to each other, but divergent from the genetically uniform east-coast (= Queensland, QLD) population. Phylogenetic analysis demonstrated that the NT population derived from the QLD population. We infer a role for the Carpentaria Basin as a biogeographic barrier restricting east-west gene flow. The QLD populations were largely panmictic and recognised east-coast biogeographic barriers play no part in north-south population structuring. While the NT and QLD populations were genetically distinct, there was evidence for the historically recent translocation of flies from each region to the other. Flies reared from different host fruits collected in the same location showed no genetic divergence. While a role for the Carpentaria Basin as a barrier to gene flow for Australian fruit flies agrees with existing work on the related B. tryoni, the reason(s) for population panmixia for B. jarvisi (and B. tryoni) over the entire Queensland east coast, a linear north-south distance of >2000km, remains unknown.

Prion Propagation is Dependent on Key Amino Acids in Charge Cluster 2 within the Prion Protein.

To dissect the N-terminal residues within the cellular prion protein (PrPC) that are critical for efficient prion propagation, we generated a library of point, double, or triple alanine replacements within residues 23-111 of PrP, stably expressed them in cells silenced for endogenous mouse PrPC and challenged the reconstituted cells with four common but biologically diverse mouse prion strains. Amino acids (aa) 105-111 of Charge Cluster 2 (CC2), which is disordered in PrPC, were found to be required for propagation of all four prion strains; other residues had no effect or exhibited strain-specific effects. Replacements in CC2, including aa105-111, dominantly inhibited prion propagation in the presence of endogenous wild type PrPC whilst other changes were not inhibitory. Single alanine replacements within aa105-111 identified leucine 108 and valine 111 or the cluster of lysine 105, threonine 106 and asparagine 107 as critical for prion propagation. These residues mediate specific ordering of unstructured CC2 into ß-sheets in the infectious prion fibrils from Rocky Mountain Laboratory (RML) and ME7 mouse prion strains.

Competition: A Missing Component of Fruit Fly (Diptera: Tephritidae) Risk Assessment and Planning.

Tephritid fruit flies are internationally significant pests of horticulture. Because they are also highly invasive and of major quarantine concern, significant effort is placed in developing full or partial pest risk assessments (PRAs) for fruit flies, while large investments can be made for their control. Competition between fruit fly species, driven by the need to access and utilise fruit for larval development, has long been recognised by researchers as a fundamental component of fruit fly biology, but is entirely absent from the fruit fly PRA literature and appears not be considered in major initiative planning. First presenting a summary of the research data which documents fruit fly competition, this paper then identifies four major effects of fruit fly competition that could impact a PRA or large-scale initiative: (i) numerical reduction of an existing fruit fly pest species following competitive displacement by an invasive fruit fly; (ii) displacement of a less competitive fruit fly pest species in space, time or host; (iii) ecological resistance to fruit fly invasion in regions already with competitively dominant fruit fly species; and (iv) lesser-pest fruit fly resurgence following control of a competitively superior species. From these four major topics, six more detailed issues are identified, with each of these illustrated by hypothetical, but realistic biosecurity scenarios from Australia/New Zealand and Europe. The scenarios identify that the effects of fruit fly competition might both positively or negatively affect the predicted impacts of an invasive fruit fly or targeted fruit fly control initiative. Competition as a modifier of fruit fly risk needs to be recognised by policy makers and incorporated into fruit fly PRAs and major investment initiatives.

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.

Fruit Fly Larval Survival in Picked and Unpicked Tomato Fruit of Differing Ripeness and Associated Gene Expression Patterns.

The larvae of frugivorous tephritid fruit flies feed within fruit and are global pests of horticulture. With the reduced use of pesticides, alternative control methods are needed, of which fruit resistance is one. In the current study, we explicitly tested for phenotypic evidence of induced fruit defences by running concurrent larval survival experiments with fruit on or off the plant, assuming that defence induction would be stopped or reduced by fruit picking. This was accompanied by RT-qPCR analysis of fruit defence and insect detoxification gene expression. Our fruit treatments were picking status (unpicked vs. picked) and ripening stage (colour break vs. fully ripe), our fruit fly was the polyphagous Bactrocera tryoni, and larval survival was assessed through destructive fruit sampling at 48 and 120 h, respectively. The gene expression study targeted larval and fruit tissue samples collected at 48 h and 120 h from picked and unpicked colour-break fruit. At 120 h in colour-break fruit, larval survival was significantly higher in the picked versus unpicked fruit. The gene expression patterns in larval and plant tissue were not affected by picking status, but many putative plant defence and insect detoxification genes were upregulated across the treatments. The larval survival results strongly infer an induced defence mechanism in colour-break tomato fruit that is stronger/faster in unpicked fruits; however, the gene expression patterns failed to provide the same clear-cut treatment effect. The lack of conformity between these results could be related to expression changes in unsampled candidate genes, or due to critical changes in gene expression that occurred during the unsampled periods.

Gene selection for studying frugivore-plant interactions: a review and an example using Queensland fruit fly in tomato.

Fruit production is negatively affected by a wide range of frugivorous insects, among them tephritid fruit flies are one of the most important. As a replacement for pesticide-based controls, enhancing natural fruit resistance through biotechnology approaches is a poorly researched but promising alternative. The use of quantitative reverse transcription PCR (RT-qPCR) is an approach to studying gene expression which has been widely used in studying plant resistance to pathogens and non-frugivorous insect herbivores, and offers a starting point for fruit fly studies. In this paper, we develop a gene selection pipe-line for known induced-defense genes in tomato fruit, Solanum lycopersicum, and putative detoxification genes in Queensland fruit fly, Bactrocera tryoni, as a basis for future RT-qPCR research. The pipeline started with a literature review on plant/herbivore and plant/pathogen molecular interactions. With respect to the fly, this was then followed by the identification of gene families known to be associated with insect resistance to toxins, and then individual genes through reference to annotated B. tryoni transcriptomes and gene identity matching with related species. In contrast for tomato, a much better studied species, individual defense genes could be identified directly through literature research. For B. tryoni, gene selection was then further refined through gene expression studies. Ultimately 28 putative detoxification genes from cytochrome P450 (P450), carboxylesterase (CarE), glutathione S-transferases (GST), and ATP binding cassette transporters (ABC) gene families were identified for B. tryoni, and 15 induced defense genes from receptor-like kinase (RLK), D-mannose/L-galactose, mitogen-activated protein kinase (MAPK), lipoxygenase (LOX), gamma-aminobutyric acid (GABA) pathways and polyphenol oxidase (PPO), proteinase inhibitors (PI) and resistance (R) gene families were identified from tomato fruit. The developed gene selection process for B. tryoni can be applied to other herbivorous and frugivorous insect pests so long as the minimum necessary genomic information, an annotated transcriptome, is available.

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).

Aversive responses of Queensland fruit flies towards larval-infested fruits are modified by fruit quality and prior experience.

For frugivorous fruit flies, the decision whether to accept or reject a host fruit for oviposition is influenced by a variety of fruit quality factors. Additionally, ovipositing flies may be influenced by the presence of eggs or larvae already within the host fruit. Species of the genus Bactrocera have been shown to avoid ovipositing into larval-infested fruits. However, the observed oviposition aversion in Bactrocera is variable, with some studies showing that deterrence to infested fruits may not always occur, but what may influence such variation is unknown. Using the polyphagous fruit fly Bactrocera tryoni (Froggatt), we tested if the quality of host fruit for offspring survival was a factor in influencing a female fly's decision whether to oviposit or not into larval-infested fruits. In both small cages and field cages, ovipositing B. tryoni did not discriminate between infested and non-infested high-quality fruits. However, when given a choice between poor-quality infested and non-infested fruits, significantly more flies selected and oviposited in non-infested fruits. For example, B. tryoni did not discriminate between infested and non-infested guava (a fruit in which there is high offspring survival), but more flies selected and oviposited on non-infested than on infested green apples (a fruit in which there is poor offspring survival). Small cage experiments also showed that prior oviposition experience on a larval-infested host negated the previously observed aversive response for that particular infested host fruit. The results are discussed in the light of a long recognised, but often ignored fact that herbivore host choice is about the sum of both the positive and negative cues received from the host.

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.

Male-lure type, lure dosage, and fly age at feeding all influence male mating success in Jarvis' fruit fly.

Males of certain Dacini fruit flies are strongly attracted to, and feed upon, plant secondary compounds such as methyl eugenol, raspberry ketone and zingerone. The consumed lure is generally found to induce physiological and behavioural changes that enhance the mating performance of lure-fed males. Male Bactrocera jarvisi respond strongly to zingerone from a young age, but only weakly respond to raspberry ketone. We hypothesized that this selective lure-response would be reflected in the physiological importance of the lure to the fly. We found that zingerone feeding by young males resulted in significantly greater mating success in competitive mating trials with lure-deprived flies, but the mating advantage was lost in older males. Lure dosage had a significant effect on the duration of the mating advantage, for example when fed 20 µg of zingerone, the advantage lasted only 1 day post-feeding, but when fed of 50 µg zingerone the advantage lasted 7 days. Raspberry ketone feeding did not confer any mating advantage to males except at one dosage (50 µg) for 1 day after feeding. When given a choice, B. jarvisi females preferred to mate with zingerone-fed versus to raspberry ketone-fed males. This study revealed lure, dosage and age of fly at time of lure administration are all important factors for maximising lure-enhanced fruit fly mating performance. These findings contribute to a better theoretical understanding of the evolution of fruit fly-lure interactions and may help improve fruit fly pest management via the Sterile Insect Technique through semiochemical-mediated enhancement of sterile male mating performance.

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.

The "sequential cues hypothesis": a conceptual model to explain host location and ranking by polyphagous herbivores.

Successfully locating a host plant is crucial for an insect herbivore to feed and/or oviposit. However, locating a host within a complex environment that may contain an array of different plant species is a difficult task. This is particularly the case for polyphagous herbivores, which must locate a host within environments that may simultaneously contain multiple suitable and unsuitable hosts. Here we review the mechanisms of host selection used by polyphagous herbivores, as well as exploring how prior experience may modify a generalist's response to host cues. We show that recent research demonstrates that polyphagous herbivores have the capacity to detect both common cues from multiple host species, as well as specific cues from individual host species. This creates a paradox in that generalists invariably rank hosts when given a choice, a finding at odds with the "neural limitations" hypothesis that says generalist insect herbivores should not have the neural capacity to identify cues specific to every possible host. To explain this paradox we propose a model, akin to parasitoid host location, that postulates that generalist herbivores use different cues sequentially in host location. We propose that initially common host cues, associated with all potential hosts, are used to place the herbivore within the host habitat and that, in the absence of any other host cues, these cues are sufficient in themselves to lead to host location. As such they are true "generalist" cues. However, once within the host habitat, we propose that the presence of a smaller group of cues may lead to further host searching and the location of preferred hosts: these are "specialist" cues. This model explains the current conflict in the literature where generalists can respond to both common and specific host-plant cues, while also exhibiting specialist and generalist host use behavior under different conditions.

Development of internal COI primers to improve and extend barcoding of fruit flies (Diptera: Tephritidae: Dacini).

Accurate species-level identifications underpin many aspects of basic and applied biology; however, identifications can be hampered by a lack of discriminating morphological characters, taxonomic expertise or time. Molecular approaches, such as DNA "barcoding" of the cytochrome c oxidase (COI) gene, are argued to overcome these issues. However, nuclear encoding of mitochondrial genes (numts) and poor amplification success of suboptimally preserved specimens can lead to erroneous identifications. One insect group for which these molecular and morphological problems are significant are the dacine fruit flies (Diptera: Tephritidae: Dacini). We addressed these issues associated with COI barcoding in the dacines by first assessing several "universal" COI primers against public mitochondrial genome and numt sequences for dacine taxa. We then modified a set of four primers that more closely matched true dacine COI sequence and amplified two overlapping portions of the COI barcode region. Our new primers were tested alongside universal primers on a selection of dacine species, including both fresh preserved and decades-old dry specimens. Additionally, Bactrocera tryoni mitochondrial and nuclear genomes were compared to identify putative numts. Four numt clades were identified, three of which were amplified using existing universal primers. In contrast, our new primers preferentially amplified the "true" mitochondrial COI barcode in all dacine species tested. The new primers also successfully amplified partial barcodes from dry specimens for which full length barcodes were unobtainable. Thus we recommend these new primers be incorporated into the suites of primers used by diagnosticians and quarantine labs for the accurate identification of dacine species.

Changes in Energy Metabolism Trigger Pupal Diapause Transition of Bactrocera minax After 20-Hydroxyecdysone Application.

Correct timing of diapause entry and exit is critical for a species' survival. While many aspects of insect diapause are well-studied, the mechanisms underlying diapause termination remain largely unknown. The Chinese citrus fly, Bactrocera minax, is a univoltine insect with an obligatory pupal diapause. The application of 20-hydroxyecdysone (20E) is known to terminate diapause in B. minax, and we used this approach, along with isobaric tags for relative and absolute quantitation technology, to determine the proteins associated with diapause termination in this fly. Among 2,258 identified proteins, 1,169 proteins significantly differed at 1, 2, and 5 days post-injection of 20E, compared with the solvent-injected control group. Functional annotation revealed that the majority of differentially expressed proteins were enriched in the core energy metabolism of amino acids, proteins, lipids, and carbohydrates as well as in signal transduction pathways including PPAR signaling, Calcium signaling, Glucagon signaling, VEGF signaling, Ras signaling, cGMP-PKG signaling, and cAMP signaling. A combined transcriptomic and proteomic analysis suggested the involvement of energy metabolism in the response of diapause transition. RNA interference experiments disclosed that a 20E injection triggers diapause termination probably through non-genomic actions, rather than nuclear receptor mediated genomic actions. Our results provide extensive proteomic resources for insect diapause transition and offer a potential for pest control by incapacitating the regulation of diapause termination either by breaking diapause prematurely or by delaying diapause termination to render diapausing individuals at a high risk of mortality.

From laboratory to the field: consistent effects of experience on host location by the fruit fly parasitoid Diachasmimorpha kraussii (Hymenoptera: Braconidae).

Associative learning is well documented in Hymenopteran parasitoids, where it is thought to be an adaptive mechanism for increasing successful host location in complex environments. Based on this learning capacity, it has been suggested that providing prerelease training to parasitoids reared for inundative release may lead to a subsequent increase in their efficacy as biological control agents. Using the fruit fly parasitoid Diachasmimorpha krausii we tested this hypothesis in a series of associative learning experiments which involved the parasitoid, two host fruits (tomatoes and nectarine), and one host fly (Bactrocera tryoni). In sequential Y-tube olfactometer studies, large field-cage studies, and then open field studies, naïve wasps showed a consistent preference for nectarines over tomatoes. The preference for nectarines was retained, but not significantly increased, for wasps which had prior training exposure to nectarines. However, and again consistently at all three spatial scales, prior experience on tomatoes led to significantly increased attraction to this fruit by tomato-trained wasps, including those liberated freely in the environment. These results, showing consistency of learning at multiple spatial scales, gives confidence to the many laboratory-based learning studies which are extrapolated to the field without testing. The experiment also provides direct experimental support for the proposed practice of enhancing the quality of inundatively released parasitoids through associative learning.

Rh6 gene modulates the visual mechanism of host utilization in fruit fly Bactrocera minax.

BACKGROUND: Vision plays a critical role in host location and oviposition behavior for herbivorous insects. However, the molecular mechanisms underlying visual regulation in host recognition and oviposition site selection in insects remains unknown. The aim of this study was to explore the key visual genes that are linked to the host plant location of the fruit fly, Bactrocera minax. RESULTS: Using a host specialist fruit fly, B. minax, which lays eggs only into immature green citrus fruit, we undertook behavioral, transcriptomic, and RNAi research to identify the molecular basis for host fruit color recognition. In laboratory and field assays we found that adults prefer green over other colors, and this preference is significantly increased in sexually mature over immature flies. Furthermore, we identified that the Rh6 gene, responsible for green spectral sensitivity, has elevated expression in mature flies over immature flies. RNAi suppression of Rh6 eliminated the preference for green, resulting in a significant decrease in the number of eggs laid by B. minax in green unripe citrus. CONCLUSION: These results show that the Rh6 gene modulates the visual mechanism of host utilization in B. minax, providing a genetic basis for visual host location in a non-model insect herbivore. © 2018 Society of Chemical Industry.

Population structure of a global agricultural invasive pest, Bactrocera dorsalis (Diptera: Tephritidae).

Bactrocera dorsalis, the Oriental fruit fly, is one of the world's most destructive agricultural insect pests and a major impediment to international fresh commodity trade. The genetic structuring of the species across its entire geographic range has never been undertaken, because under a former taxonomy B. dorsalis was divided into four distinct taxonomic entities, each with their own, largely non-overlapping, distributions. Based on the extensive sampling of six a priori groups from 63 locations, genetic and geometric morphometric datasets were generated to detect macrogeographic population structure, and to determine prior and current invasion pathways of this species. Weak population structure and high genetic diversity were detected among Asian populations. Invasive populations in Africa and Hawaii are inferred to be the result of separate, single invasions from South Asia, while South Asia is also the likely source of other Asian populations. The current northward invasion of B. dorsalis into Central China is the result of multiple, repeated dispersal events, most likely related to fruit trade. Results are discussed in the context of global quarantine, trade, and management of this pest. The recent expansion of the fly into temperate China, with very few associated genetic changes, clearly demonstrates the threat posed by this pest to ecologically similar areas in Europe and North America.

The impact of nutritional quality and gut bacteria on the fitness of Bactrocera minax (Diptera: Tephritidae).

To examine how nutritional quality and resident gut bacteria interplay in improving the fitness of an oligophagous fruit fly, Bactrocera minax, artificial sucrose diets and full diets (sucrose, tryptone and yeast extract) were fed to flies with and without antibiotic supplementation. Furthermore, Klebsiella oxytoca and Citrobacter freundii were supplemented to sucrose-only diets. Flies were maintained in the laboratory and the fitness parameters, male and female longevity, number of copulations and female fecundity, were recorded. Full diet without bacterial depletion significantly increased fecundity and copulation. In the absence of gut bacteria, flies fed with full diets had significantly decreased mean fecundity and copulation rate. Flies that were fed with sucrose diet had a very low copulation rate and produced no eggs. Diet type and the presence of bacteria did not have any effect on the average longevity of male and female flies. Bacterial supplementation in sucrose diets did not improve any of the measured parameters. The results demonstrate that gut bacteria interact with diet to influence mating and reproduction in B. minax. Symbiotic bacteria significantly and positively impact reproduction in B. minax; however, their impact can only be fully realized when the flies are fed with a nutritionally complete diet.

The responsiveness of Bactrocera jarvisi (Diptera: Tephritidae) to two naturally occurring phenylbutaonids, zingerone and raspberry ketone.

The males of different species of Bactrocera and Zeugodacus fruit flies are commonly attracted to plant-derived phenylpropanoids (e.g. methyl eugenol (ME)) or phenylbutanoids (e.g. raspberry ketone (RK)) but almost never to both. However, one particular plant-derived phenylbutanoid, zingerone (ZN), which possesses an intermediate chemical structure between ME and RK, weakly attracts both ME- and RK-responding fruit fly species. Bactrocera jarvisi, an Australian fruit fly species, is weakly attracted to cue lure (an analogue of RK) but strongly attracted to ZN. Here, we investigated the minimum olfactory threshold and optimum sensitivity of B. jarvisi males to ZN and RK as a function of dose, time and sexual maturation. Our results show that B. jarvisi males had a marked preferential response to ZN, with a much lower olfactory threshold and faster response time to ZN than RK. Probit analysis demonstrated that ZN was at least >1600× more potent than RK as a male attractant to B. jarvisi. Although fruit fly male attraction to the phytochemicals is generally associated with sexual maturity, in B. jarvisi immature males were also attracted to ZN. Our results suggest that B. jarvisi males have a fine-tuned olfactory response to ZN, which appears to play a central role in the chemical ecology of this species.