Causes and Consequences of Dispersal in the Mediterranean Fruit fly, Ceratitis capitata

The ability of insects to perform under challenging environmental conditions is paramount to their survival, population growth and evolutionary fitness. Understanding why some organisms persist in certain habitats but not others is the first step to comprehending present, past and future species distributions, of particular importance under future global climate change. Key traits that may assist a species to continue to perform under poor conditions and may potentially assist in surviving future variable and warming conditions include enhanced dispersal capabilities, a wide performance breadth and plastic responses. Repeated mark-release-recapture (MRR) experiments were conducted to measure the performance (dispersal) and plastic responses of the Mediterranean fruit fly, Ceratitis capitata (a prolific global invader). Dispersive and philopatric individuals were morphometrically assessed (including wing size and shape, body mass, abdomen mass, thorax mass and various ratios thereof) to identify phenotypic traits associated with enhanced dispersal. Thereafter, focussed laboratory experiments were undertaken to determine which aspects of flight performance are enhanced, or associated with, potential dispersal traits. Performance was then compared under various thermal limits (chill coma recovery, CCRT; heat knockdown time, HKDT; critical thermal minimum and maximum, CTmin and CTmax, respectively) to examine the influence of different thermal acclimation regimes and determine the responses of phenotypic plasticity in C. capitata. Subsequently, the costs and benefits of dispersal and its plasticity were measured under semi-field (greenhouse) and field conditions to determine how close laboratory predictions are to the real world. These experiments allowed the discovery of the phenotypic trait associated with dispersal (larger thorax mass: body mass). However, contrary to a widely-held expectation, it did not result in enhanced whole-animal flight performance, but was rather related to willingness to disperse (i.e. dispersal propensity). Furthermore, the
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integration of the three operational environments (laboratory, semi-field and field) illustrated that C. capitata's performance is influenced by thermal conditions and highlighted the best acclimation treatment (20°C acclimation, especially in warmer conditions) for enhanced performance. A challenge for invasion biology is the development of a predictive understanding of species invasion ability. Clarity on the species dispersal potential and the factors that influence it is an integral part of the problem. From this study, it is shown that dispersal is condition dependent (e.g. phenotypic traits and behaviour) as well as context dependent (e.g. thermal history and environmental temperature). This may benefit predictions of the future invasion risk of C. capitata and potentially improve current management strategies.