When natives go wild... Why do some insect species become invasive in their native range?

Abstract

Since the inception of invasion ecology as a modern field of research, there have been numerous attempts to establish why and how some species become invasive. To date, research emphasis has been almost entirely on exotic species introduced into new areas. As a result, most of the established theories relative to invasion success are not applicable for species that become invasive in their native range. The study of such species can, therefore, offer new directions for the search for mechanisms and the characterisation of traits associated with invasion success. This thesis aimed at investigate some alternative hypotheses using the New Zealand native species Costelytra zealandica (White) and C. brunneum (Broun) (Scarabaeidae: Melolonthinae)as an invasive and non-invasive congeneric species model.For many years, the larvae of C. zealandica and C. brunneum have been considered cryptic. To progress the research in this thesis, two novel molecular methods, based on the use of frass and larval exuviae as non-invasive sources of DNA, were developed to differentiate them. In addition, a detailed comparative taxonomic assessment between C. zealandica and C. brunneum revealed that three morphological characters allowed accurate identification of third instar larva. For live larvae, especially in field conditions, the morphology of the septula of the raster is usually sufficient to differentiate the two species. Several comparative studies that determined the feeding preferences and fitness performance of the model species when fed with native and exotic host plants were performed. The results of these experiments supported the existence of strong intra-specific variations in the diet of C. zealandica, suggesting the occurrence of evolutionary processes (i.e. host-shift or host range expansion) and the existence of a pre-existing ability to use ‘new’ hosts in this invasive species. The processes of host-shift and host range expansion were subsequently investigated in C. zealandica, using tussock and white clover as example of native and exotic host plants, respectively. The comparison of fitness response of several populations of this species to various feeding treatments, comprising an artificial host-shift, revealed that C. zealandica populations occurring in exotic pastures have experienced an ecological host-shift rather than just a hostrange expansion. Furthermore, the results of this study suggested the existence of distinct hostraces in this species. Additionally, further experiments were performed to determine whether or not C. zealandica became a successful invader through a pre-existing capacity to tolerate the detrimental effects of its new host plant’s defence chemicals. The comparison of the fitness response of C. zealandica and C. brunneum to host defences, artificially triggered and enhanced by the phytohormone jasmonic acid, suggested the existence of a pre-existing ability in the invasive species to tolerate and benefit from the defence chemicals of its exotic host plant. Finally, the environmental tolerance of C. zealandica and C. brunneum was compared through their survival and growth responses to different temperature regimes, and an attempt was made to relate these fitness responses to the phosphoglucose-6-isomerase (PGI) enzyme system. The invasive species C. zealandica was found to be more tolerant to challenging temperatures than its congener. However, no relationship was observed between the PGI enzyme system and the individual fitness performance of C. zealandica or C. brunneum under various temperature regimes. This thesis presents new insights into the mechanisms that may have led C. zealandica to reach the status of invader in its native range. The combined results presented in this thesis highlights, in particular, the importance that phenotypic plasticity might have played in the invasion success of C. zealandica. The conclusions raised here offer new research directions for the investigation of the invasion process in phytophagous insects in general.