The value of grasslands for pest suppression: an assessment of grassland refuge quality and their integration into agricultural landscapes Rebecca Kate Nagy BSc. (Hons.) A thesis submitted for the degree of Doctor of Philosophy at The University of Queensland in 2020 School of Biological Sciences Abstract Grasslands provide valuable resources and refuges that support arthropod biodiversity in ever- changing agricultural landscapes. Potential exists to manipulate and manage grasslands to enhance the efficiency of natural biocontrol services provided to surrounding croplands, whilst discouraging the build-up of agricultural pests. In order to successfully achieve this, a deeper understanding of the ecological function of grassland habitats is needed. Grasslands are diverse habitats that differ significantly in their composition, function and management, despite consistently being bundled together in research. Exploring these differences and how they influence arthropod communities, and thus landscape pest suppression, remains a significant gap in biocontrol research. Current knowledge of the arthropod pests and natural enemies that utilise different grassland habitats is scarce, and even less is understood about the potential for different grasslands to be biological control service providers, making progress in this area challenging. Three distinct grassland types were distinguished in the mixed farming regions of south-east Queensland, Australia. Native grasslands comprised a mix of predominantly native grass species, Mixed Improved grasslands were a sown mix of both introduced and native grass species, and Pure grasslands comprised a single dominant grass species. To compare the above-ground arthropod communities that inhabit these grasslands, field sampling was carried out over a two-year period. Arthropods were collected from grassy fields by sweep netting, while vegetation assessments monitored grassland composition, floristic diversity, quality and management practices. Only 6% of all arthropods collected were agriculturally important pests, while 25% were beneficial predators and parasitoids. Arthropods were almost 50% more abundant in Native grasslands than in the other types, although Pure grasslands supported the largest communities of natural enemies. Pest and other herbivorous insects favoured grasslands with higher proportions of weeds and other undesirable vegetation (i.e. Native and Mixed), and were less abundant in grasslands with high proportions of desirable introduced grass species (i.e. Pure). Natural enemies used all grasslands as a refuge habitat when adjacent crops were absent, while pest and other herbivorous arthropods did not. This study highlights the importance of grasslands as valuable arthropod refuges, particularly for beneficial natural enemies, and draws attention to the differences between different grassland types that can influence these communities. The observation that grasslands harbour significant communities of predatory and parasitic arthropods raises the question of whether these natural enemies are providing biological control benefits to the greater landscape. An experiment using sentinel prey was conducted to test the hypothesis that grasslands do provide pest suppressive services to surrounding crops, although ii services provided by different grassland types will differ. A detailed pilot study evaluated prey types and methods, before live Helicoverpa armigera eggs, dead H. armigera larvae and artificial plasticine larvae were used to assess predation in two different grassland types (Native and Mixed Improved) and their adjacent grain sorghum crops in south-east Queensland. Predation rates were significantly higher in grasslands, coinciding with higher natural enemy abundance, and decreased with distance into the cropped fields. This suggests that the natural enemies supported by grassland habitats are providing valuable biological control services not only within the grasslands themselves, but at minimum to the edges of crops in the surrounding landscape. Significant differences in predation rates were observed between the two grassland types, again highlighting the fact that not all grasslands are the same, and that the pest suppressive services they provide differ. Field experiments have their limitations, so to further explore grassland characteristics and their influence on landscape pest suppression, a dynamic spatially explicit simulation model was developed. This model compared the suppression of aphid pests by lady beetle predators in crop- dominated landscapes comprising grasslands that varied in type, size and configuration. Native grasslands provided the most efficient aphid suppression despite supporting the highest density of aphids at the beginning of the simulation. This was a result of their larger lady beetle communities, suggesting that encouraging predators to build-up in non-crop habitats, ready for early colonisation of surrounding crops, is a key strategy for boosting landscape biocontrol. Additionally, grassland area and configuration were important factors predicting efficient pest suppression. Landscapes with both 5 and 10% grassland had the potential to control pests, although their success was dependent on configuration, with linear grasslands (e.g. field margins) providing more effective pest suppression than block elements (e.g. pastures). These results suggest that with careful planning and management, even landscapes with limited grassland can significantly benefit from biocontrol services. This thesis addresses several important research gaps and presents some of the first research evaluating differences in grassland type by means of both empirical and modelling studies. While all grasslands in this study supported important natural enemy communities that provide valuable biological control services to the greater agricultural landscape, significant differences were observed between different grassland types. Additional research is needed to explore these differences further, in order to develop our understanding of different grassland habitats so they can be successfully exploited for their biocontrol potential. Preliminary practical methods for managing grassland habitats to boost biocontrol services, without also increasing pest pressure, are proposed, and it is recommended that grassland habitats should be considered as an important part of pest management decision making. iii Declaration by author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis. I have clearly stated the contribution of others to my thesis as a whole, including statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, financial support and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my higher degree by research candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award. I acknowledge that an electronic copy of my thesis must be lodged with the University Library and, subject to the policy and procedures of The University of Queensland, the thesis be made available for research and study in accordance with the Copyright Act 1968 unless a period of embargo has been approved by the Dean of the Graduate School. I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis and have sought permission from co-authors for any jointly authored works included in the thesis. iv Publications included in this thesis Nagy, Rebecca K., Bell, Lindsay W., Schellhorn, Nancy A., Zalucki, Myron P. (2020). The role of grasslands in pest suppressive landscapes: how green are my pastures? Austral Entomology, 59, 227- 237 – incorporated as Chapter 1. Nagy, Rebecca K., Schellhorn, Nancy A., Zalucki, Myron P. (2020). Fresh, frozen or fake: a comparison of predation rates measured by various types of sentinel prey. Journal of Applied Entomology, 144, 407-416 – incorporated as Chapter 3. Submitted manuscripts included in this thesis No publications included. Other publications during candidature Conference abstracts Nagy, Rebecca K., Bell, Lindsay W., Schellhorn, Nancy A., Zalucki, Myron P. The value of pastures for pest suppression: valuable predator habitat or pest refuge? Conference abstract: 49th Australian Entomological Society AGM and Scientific Conference, Alice Springs, NT, 23-26 September 2018. v Contributions by others to the thesis Prof Myron P. Zalucki (UQ), Associate Prof Michael Furlong (UQ), Dr Nancy A. Schellhorn (CSIRO) and Dr Lindsay W. Bell (CSIRO) were advisors of this thesis. Nancy Schellhorn and Lindsay Bell were responsible for conception of the thesis project, and along with Myron Zalucki made significant contributions through conception of project design, monitoring research progress, providing guidance for data analysis and editing draft manuscripts. Rebecca Nagy,