Niche differentiation within the Wiseana (porina) species complex: pasture pests of New Zealand : A thesis submitted in partial fulfilment of the requirements of the Degree of Doctor of Philosophy at Lincoln University

Abstract

The most important sector of New Zealand’s primary industries is pastoral agriculture, but the production and quality of the pasture itself is threatened by porina moths. Porina is a complex of species belonging to the genus Wiseana (Lepidoptera: Hepialidae) which is endemic to New Zealand. The larvae are major pests of exotic pastures, particularly in the central and southern North Island and most of the South Island of New Zealand. Despite this impact, no such damage of any New Zealand native plants has been ascribed to these insects and, to date, no studies of porina feeding on native plants have been done to understand this discrepancy. However, to develop more stategic approaches to improve management of porina in pastures, it is critical to understand why porina have moved into exotic pastures and become so successful, relative to what the populations must have been in their native habitats. Here, fundemental information is developed to unravel key aspects that may be linked to porina’s success. In particular, to confirm that porina do indeed develop and survive better on exotic compared to native host plants, determine what the identities are of the species that have moved into pastures and if there are differences in their seasonal flight patterns. Behavioural studies are needed to observe larval feeding and survival on exotic and native host plants under controlled conditions, and to determine if any differences may be associated with the relative nutrient content of those hosts. Such information may then enable hypotheses to be developed as to which New Zealand native plants are, or were, likely hosts for these species, as well as to what might have been the evolutionary mechanism to move from those hosts to exotic pasture plants, i.e. host range expansion or host switch. Finally, if egg-larval development data can be developed, can this be used to build preliminary forecasting models that might enable the best timing of application of control measures in the field be determined. Results of the experimental work to address these knowledge gaps showed with a pilot study at the outset that larval development and survival of field-collected porina species on native and exotic plants showed mortality from 13% to 75% and larval survival time from 96 to 177 days (Chapter 2). Subsequently light trap catches showed W. cervinata and W. copularis to be the two main species infesting pastures in Canterbury, with W. cervinata flying from late spring to early summer and W. copularis flying from early summer to late summer (Chapter 3). A simple method was developed that improved laboratory rearing and survival of porina larvae from eggs by 90%, and revealed that eggs of W. cervinata at 15 and 22℃ hatched earlier compared to W. copularis and W. umbraculata (Chapter 4). Larvae of W. cervinata, W. copularis and W. umbraculata reared from eggs further confirmed the differential success of their growth and development, being significantly better on exotic (Trifolium repens and Lolium perenne × Lolium multiflorum) than native (Aciphylla squarrosa, Festuca actae, Chionochloa rubra, Poa cita and Phormium tenax) plants and with implications for species fitness, population dynamics and management (Chapter 5). The relative nutritional status of these plants was measured and over 100 metabolites were detected in each plant species (Chapter 6). Of these metabolites, which were classified into 11 groups (amino acids, cyclitols, fatty acids, organic acids, other N-compounds, sugars, sugar acids, sugar alcohols, phytosterols, miscellaneous and unknowns), N and silica were shown to be higher in the exotic host species, but C, CN ratios and fibre content were higher in the native host plants. Observations on the foraging behaviour of Wiseana larvae over a three day period showed that W. copularis were more active grazers, creating multiple burrows as they forage with 63% of larvae emerging to feed, compared to W. cervinata creating a single burrow and 50% of larvae emerging to feed (Chapter 7). However the locomotory ability of the pupal stage to move up and down the burrow showed that W. cervinata descended slightly faster than W. copularis in response to stimuli, suggesting it may have a better ability to avoid predators (Chapter 8). Together, the prediction of peak flight times, and the time when early instar larvae are vulnerable at the soil surface can help researchers and farmers to assess porina population dynamics more effectively for the application of management decisions. Lastly, data on porina development, seasonal flight and weather were used to design a simple prototype model for predicting the best timing for the application of pesticides. Overall, the approaches and results obtained in this thesis have provided essential biological evidence to help understand that a mixture of host range expansion and host switching mechanisms may have contributed to the success of Wiseana spp. in New Zealand pasture. It has also helped to identify components of their phenology that can be used in improved management stategies. The conclusions offer new research directions for the investigation of the Wiseana invasion processes in general.