Interactions between insect pests, grasses and their endophytes under resource limitation : A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Lincoln University

Abstract

Environmental resource limitations, such as drought and reduced availability of phosphorus (P) pose significant challenges to pasture systems, impacting plant health and productivity. Despite advances in developing resilient pasture species and systems, little is known about how such environmental changes influence the complex relationship between Epichloë endophytes, host cool-season grasses, and pasture insect pests. This study focuses on how environmental resource constraints, specifically drought and P availability, affect pasture grasses, with attention to the modulating role of fungal Epichloë endophytes under these conditions. In a glasshouse pot trial, the effect of drought on root aphid populations and plant performance was measured using endophyte-infected perennial ryegrass (AR37, NZCT) (Chapter 3). Endophyte infection reduced root aphid numbers. In general, aphid populations were higher in drought-stressed than in well-watered plants. Under drought conditions, compared to endophyte-free plants, root aphid populations increased 4-fold and 8-fold for NZCT andAR37, respectively. In endophyte-free plants root aphids reduced shoot dry weight by 16% in drought-exposed and 26% in well-watered plants while reducing root biomass by 49%. These results suggest that root aphids are likely able to exploit the higher availability of amino acids in the plant sap of drought-exposed plants. This study provides evidence that climate change-mediated impacts of root aphids could reduce production in perennial ryegrass-dominant pastures. In a no-choice bioassay, African black beetle (ABB) were fed semi-synthetic diets containing freeze-dried foliage material from drought-exposed and well-watered perennial ryegrass containing no endophyte (Nil) or with endophyte strains AR37 or NZCT (Chapter 4). Diet consumption, beetle weight change, fungal alkaloid concentrations, and phytohormones were measured. Except for AR37, ABB consumed more diets containing drought-exposed rather than well-watered material, with a 61% increase in Nil and a 50% increase in diets containing NZCT. This study discusses the implications of these results on ABB damage in New Zealand pastures in the context of increasing drought. Areas that do not currently have high populations of ABB such as Manawatū-Whanganui, Wairarapa and Marlborough are predicted to become increasingly vulnerable to damage. This research also investigated the interplay between P, plant growth, and grass grub (Costelytra giveni) fitness in Epichloë endophyte-infected perennial ryegrass (Epichloë sp. LpTG-3 strain AR37) and meadow fescue infected with E. uncinata (strain MaxR) (Chapter 5). In a glasshouse trial, plants were grown in P-enriched soil with varying Olsen P levels (9, 18, 28, and 78 mg L-1), and grass grubs were introduced. Their survival, weight gain, and plant performance were measured. Results highlighted a decrease in the survival of grass grub on plants infected with MaxR endophyte, but not with AR37. Increasing Olsen P levels correlated with diminished grass grub performance, revealing a nuanced relationship between soil fertility and pest dynamics. The study underscores the pivotal role of selected Epichloë endophyte-grass associations in mitigating grass grub damage across varying phosphorus levels. This study highlights the potential to integrate P applications for sustainable pest control against grass grub. This study examined the indirect impact of various P fertiliser regimes on cool-season grasses, which serve as food sources for porina larvae (Wiseana copularis). Endophyte-infected (Epichloë sp. LpTG-3 strain AR37) perennial ryegrass and meadow fescue infected with E. uncinata (MaxR), were grown in P-enriched soil with varying Olsen P levels (9, 18, 28, and 78 mg L-1) (Chapter 6). Freeze-dried foliage was added to semi-synthetic diets and fed to porina larvae in a no-choice assay. Measurements included porina survival, weight gain, fungal alkaloid concentration, fungal mass, and plant nutrient levels. Endophyte infection of AR37 and MaxR and AR37 significantly reduced porina diet consumption, larval weight gain and survival irrespective of soil Olsen P levels. The concentration of loline alkaloids in herbage infected with MaxR increased with rising soil Olsen P levels, whereas the concentration of epoxyjanthitrem in AR37 remained unchanged despite variations in Olsen P levels. The fungal mass remained unchanged in response to variations in Olsen P levels. These results suggest that the improved performance of porina feeding on endophyte-free plants is largely driven by P-induced changes in food quality. This study discusses the implications of porina damage in New Zealand pastures in the context of decreasing P availability. This study also assessed the effects of repeated P-fertilizer applications on pasture invertebrate pest fauna over three years in two long-term trials in New Zealand: Ballantrae (0 and 375 kg P ha-1 annually) in the hill country in the lower North Island and Winchmore (0, 175, 188, 250, and 375 kg P ha-1 annually) in the flat land of the Canterbury Plains of the South Island (Chapter 7). The aim was to determine how P applications influenced the abundance and order richness of pasture insect pests and their relationship with pasture production and botanical composition. While order richness did not change significantly, this study observed increased total invertebrate pest abundance in flat land pastures. Notably, grass grub abundance decreased with higher P input, whereas clover root weevil (Sitona obsoletus) increased, positively correlating with clover content. This study highlights the complex interactions between P-fertiliser application and pasture invertebrates and plant communities, suggesting that strategic P management can enhance productivity while naturally reducing pest populations like grass grub. The key novel contributions of this research are that drought and P availability significantly influence the dynamics between cool-season grasses, fungal Epichloë endophytes and pasture insect pests; drought imposes stress on grasses, altering their chemical composition and attracting herbivorous pests such as root aphids and ABB, while P fertilisation enhances plant quality by improving palatability and nutrient availability for some insects, although its availability can also negatively affect others such as grass grub. These findings highlight the importance of understanding the adaptive mechanisms of endophytes in maintaining pasture health under environmental resource limitations. As climate change increases the frequency and intensity of droughts, there is an escalating need for precision nutrient management. Ultimately, recognising and harnessing these complex interactions, especially regarding integrating endophytes into these systems, will be essential for developing effective pest management strategies that promote the long-term resilience and sustainability of agricultural systems in an increasingly variable environment.