Comparative Ecology of Bactrocera Cucumis (French) and Bactrocera Tryoni (Froggatt) (Diptera: Tephritidae) - Understanding the Life History Consequences of Host Selection and Oviposition Behaviour

Abstract

The host use patterns of tephritids (fruit flies) range from those that are highly specific (monophagous, oligophagous) to those that use a wide range of hosts from different plant families (polyphagous). Knowledge of the mechanisms responsible for such host use patterns is critical in understanding the insect-plant relationship and developing successful population management strategies. Several studies have revealed that the behavioural and environmental factors play a significant role in host use. A key theory thought to explain host use patterns of phytophagous insects has been optimality theory (preference-performance hypothesis), however, this has seldom been investigated in fruit flies. In this thesis I compared the host use pattern of the specialist (oligophagous) cucumber fruit fly, Bactrocera cucumis (French), and the generalist (polyphagous) Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae: Dacinae), both serious agricultural pests in Queensland, Australia. I primarily focussed on investigating the role of the preference-performance hypothesis in explaining the host use pattern of these two species at different plant taxonomic levels. I also examined the life history consequences of host use patterns by investigating the demographies of these two dacines on different host plants. The preference-performance hypothesis postulates that the host preference executed by an insect should be positively correlated to its offspring performance. While there were some correlations between preference and performance parameters for both B. cucumis and B. tryoni, these were dependent on the parameter used and were more relevant in explaining host use patterns in B. cucumis than B. tryoni. Both species exhibited significant host discrimination among the different hosts presented to them. More significantly, they preferentially used certain host plants even when hosts of purportedly equivalent status were presented to them simultaneously. The efficiency with which they used their hosts differed even when offered different varieties of a host species from their primary host family. These patterns, observed in both these dacine species, indicated that host use is more plausibly explained by behavioural and physiological factors associated with host use. Key factors affecting host acceptance behaviour and subsequent host use of dacine fruit flies include pre-alighting factors (e.g., host plant structure, fruit colour, shape and size) and post-alighting factors (e.g., pericarp toughness, fruit volatiles and chemical composition of fruit). I investigated the role of fruit size and pericarp toughness in explaining host use patterns. Pericarp toughness appeared to strongly affect host acceptance and subsequent host use in both dacine species. Although some fruit species were readily accepted for oviposition by receiving a high proportion of oviposition attempts from both fly species, very few of these oviposition attempts resulted in pericarp penetration (oviposition event). This behaviour was primarily attributed to the differences in pericarp toughness; fruits with a tougher pericarp provided greater resistance to ovipositor penetration, thus resulting in less use than those with a softer pericarp. In addition to the behavioural aspects of fruit fly-host plant relationships, life history parameters of tephritid species play a significant role in host use. Investigations of the demography of these species on different hosts revealed that the polyphagous B. tryoni has higher fecundity and net reproductive rate, and shorter generation time while the oligophagous B. cucumis has a lower fecundity and net reproductive rate with a longer generation time. While the demographic patterns of the B. cucumis appeared to conform to preference-performance hypothesis, B. tryoni showed some interesting departures from the predictions of the preference-performance hypothesis. For B. tryoni, it was evident that some hosts that may yield lower survival in terms of larval survival (e.g., plum) may still be demographically suitable because of the enhanced reproductive performance of adults emerging from these fruits. This highlights the significance of the quality of adult feeding resource in the demography of this species. Organizing demographic data and analysing them using population projection models also enabled me to identify critical life stages that influence the demography of these two species across different host fruits. This study found that for both the specialist B. cucumis and the generalist B. tryoni, population growth rate is highly sensitive at the adult reproductive stage, indicating that manipulating probability of survival at this life stage would be critical to manage the population of these pest species. This may explain the success of protein bait sprays as a technique to manage these pest species. Predators or pathogens that target the adult reproductive stage may also provide successful biological control of these pest fruit fly populations. In addition to survival of reproductive individuals, pupal survival was also important to the demography of these species, suggesting that biological control targeting pupal stage may also yield population suppression across different fruit production systems. In summary, my studies showed that, while the preference-performance hypothesis may be a useful heuristic tool in explaining comparative host use patterns in the Dacinae, it may be more valuable in understanding primary adaptations of different species that enable them to use different host plants. This understanding combined with the life-history consequences of host use would not only shed light on their ecology, but also prove valuable in designing suitable management strategies for pest dacines.