The Mating System and Courtship Behaviour of the Queensland Fruit Fly, Bactrocera Tryoni (Froggatt) (Diptera: Tephritidae)
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The Mating System and Courtship Behaviour of the Queensland Fruit Fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) W M Thilini Darshika Ekanayake B.Sc. (Hons) (Zoology) Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy 2017 Earth, Environmental and Biological Sciences Science and Engineering Faculty Queensland University of Technology Keywords Bactrocera neohumeralis, Bactrocera tryoni, courtship, cuticular compounds, land- marking, leks, mating system, mating behaviour, Queensland fruit fly, Tephritidae i Abstract The Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), is one of Australia’s most destructive horticultural pests, attacking a wide range of fruit commodities including drupes, pomes and berries; causing between $28.5 million and $100 million loss per annum. Understanding the mating system of B. tryoni remains a key component to resolving its behavioural ecology from both pure and applied perspectives. Despite much of B. tryoni’s mating system having been investigated previously, critical elements of its sexual behaviour remain either completely unknown or in need of further research, such as: how individuals locate mates within the landscape; whether this species leks as is often assumed; the sequence of its courtship behaviour; if mate choice occurs and varies with flies of different physiological state; and whether close range cues (esp., cuticular compounds) may be involved in mate recognition and mate choice. To address this, I carried out large and fine scale experiments to evaluate: 1) the role of plant architectural features in determining mating site; 2) if the presence or absence of host fruit influences mating site selection and whether B. tryoni behaviours conforms to ‘classical lek’ expectations or has a resourced based mating system; 3) what are the fine-scale behaviours involved in courtship and mate recognition; 4) how variation in male quality (age, size, & sexual experience) may drive differential mating success and whether there is evidence of female choice depending on these factors; and 5) whether cuticular chemical compounds (esp. hydrocarbons) play a role in mate recognition. The first two objectives are addressed in Chapter 2, for which behavioural observations were made on virgin males and females released in large field cages in ii comparative studies using tall vs short artificial trees (to test for land-marking behaviour), and fruit vs no fruit-bearing artificial trees (to test for host-fruit resource use in mating). To address the third objective, Chapter 3 focussed on close-range courtship behavioural observations of B. tryoni males and females using Noldus analytical software to construct an ethogram of the courtship sequence, comparing male and female behaviours for successfully vs unsuccessfully mated individuals in separate replicates. In Chapter 4 the potential effects of male physiological attributes (i.e., young/old, large/small and virgin/mated) were tested in single-tree field cage studies, with further fine-scale courtship analysis to identify possible mechanisms explaining why males of one type were more successful at securing a mate (as demonstrated in Chapter 3). The final research component, Chapter 5, addressed the fifth objective using gas chromatography mass spectrometry (GCMS) studies to identify cuticular compounds, specifically cuticular hydrocarbons, which may be implicated in mate recognition and also mate choice. Results of this thesis demonstrate B. tryoni as a tephritid species that does not preferentially mate on plants containing fruit vs those without (i.e., is a non- resource based system), and that significantly more mating aggregations form on tall over short trees (i.e., displays land-marking behaviour). Male/female timing of aggregation is approximately the same, demonstrating no evidence of preliminary male occupancy of mating site before females, as predicted under a classical lek hypothesis. Within this selected mating site, both B. tryoni males and females perform sequences of behaviours before they initiate copulations. Among these behaviours, a male directing wing (i.e., extending both wings perpendicular to the thorax and abdomen) was the most significantly different behaviour between successful and unsuccessful copulations. This implies the presence of visual iii signalling in B. tryoni males which they may use to indicate their presence or readiness to mate. Significantly greater mating success was demonstrated by young males over old males, and by large males over small males; there was no effect of male sexual experience with respect to mating success, but no specific fine-scale behavioural differences were observed between successful vs unsuccessful males. Subsequent GCMS analyses of male and female cuticular compounds revealed evidence of sexual dimorphism in the relative quantities of fourteen cuticular compounds; however, intra-sexual differences were not observed between males that mated vs those that did not. In conclusion, B. tryoni has a non-resource based, aggregated mating system for which they likely use land-marking when selecting a mating site in the field. Within this mating arena, the most important male behaviour that leads to a successful copulation is whether a male initiates mounting; no other behavioural differences were observed between males and females for successful vs unsuccessful males. Differences in male physiological attributes affect mating success; however, no evidence was found that, this is driven by female choice and is instead driven by intrinsic male qualities that may render them more capable of repeated attempts at copulation or fending off other males. Analysis of cuticular compounds revealed differences between males and females, implying a potential mate recognition role; however, absence of differences among males suggests that, these compounds may not be involved in individual mate acceptance or rejection. Further resolution of the mating system of B. tryoni reinforces the notion that considerable differences may exist among tephritid fruit fly species, despite them often being considered in the literature as having highly similar mating systems. The outcome of this research contributes with information useful for pest management decisions for adopting iv approaches that rely on an intimate knowledge of the mating system, such as the Sterile Insect Technique (SIT). v Table of Contents Keywords ...................................................................................................................... i Abstract ........................................................................................................................ ii Table of Contents ........................................................................................................ vi List of Figures ............................................................................................................. xi List of Tables ............................................................................................................. xvi List of Appendices .................................................................................................... xix List of supplementary material ................................................................................... xx Statement of Original Authorship ............................................................................. xxi Acknowledgements .................................................................................................. xxii Chapter 1: General Introduction ........................................................................ 1 1.1 General introduction ........................................................................................... 2 1.2 Tephritid fruit flies .............................................................................................. 4 1.3 Sexual reproduction and mating systems of tephritids ....................................... 5 1.3.1 Tephritid “leks” as a mating system .......................................................... 6 1.3.2 Tephritid “swarms” as a mating system .................................................... 7 1.4 Mating site selection ........................................................................................... 8 1.4.1 Extrinsic mate location .............................................................................. 8 1.4.1.1 Environmental factors as extrinsic mate location mechanisms .............. 8 1.4.1.2 Resources for mating site selection ......................................................... 9 1.4.2 Intrinsic mate location mechanisms ........................................................ 11 1.4.2.1 Pheromones ........................................................................................... 11 1.4.2.2 Auditory signals .................................................................................... 12 1.4.2.3 Visual signals ........................................................................................ 13 1.5 Male-male competition in tephritids ................................................................. 14 1.6 Tephritid courtship and courtship behaviours .................................................. 15 1.6.1 Courtship ................................................................................................. 15 vi 1.6.2 Individual courtship signals .................................................................... 19 1.6.2.1 Pheromones ..........................................................................................