THE IMPACT OF HOST-PLANT STRESS L. ____ ••••••- •• '.: .. ,., ... -......... -. ON THE PERFORMANCE OF Two INSECT BIOLOGICAL CONTROL AGENTS OF BROOM (CYT/SUS SCOPARIUS) A thesis submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy At Lincoln University By K.E. Galway Lincoln University 2005 11 . ... - _. - ~ .. -. " ... ~ DEDICATION " '. "--,"-'-'.--' :"'; ,',-'----'" This thesis is dedicated to my husband, Peter Jones. .-.!;.,:;-; iii ACKNOWLEDGMENTS I gratefully acknowledge the CRC for Australian Weed Management and Landcare Research New Zealand for jointly funding my scholarship. ':", I would like to express my appreciation to my supervisors, Pauline Syrett (LR) , Richard Duncan (LV), Rowan Emberson (LV), and Andy Sheppard (CSIRO/CRC). Their support, advice and friendship have been instrumental to the development of my career. I am also grateful for the comments and support from my adviser - Richard Groves (CSIRO/CRC). I would also like to thank: Peter Bellingham (LR) , Peter Williams (LR) , and John Hosking (NSW Ag) for feedback on my research proposal; Neal Smith (LV) for discussions on soil moisture approaches; Derek Moot (LV) for direction on plant physiology issues; Chris Frampton (LV) for statistical guidance; and Anne Austin (LR) and Christine Bezar (LR) for writing assistance. This research involved assistance from many people. I would especially like to thank Lindsay Smith (LR) for all his technical assistance and support, as well as the continual glasshouse assistance from Dave Purcell (LR). I greatly appreciated the help from my technical assistant Kirstin McLean (LU). I would also like to thank: Warick Carter (LR) and Stuart Oliver (LR) for additional glasshouse support; Helen Harman (LR) for information on the two insect agents; and all the people who assisted with field work - Sandy Hammond (LV), Rachael Smith (LV), Matthias Kloeppel (LR) , and Peter Hodge (CSIRO). : ;- -- ~ . - - . - ,-. '.- :"-" .:,-.-;-- Of course, my deepest thanks goes to my husband, Peter, who was involved in every aspect of this thesis. LR - Landcare Research, New Zealand LU - Lincoln University, New Zealand CSIROICRC - Commonwealth Scientific Industrial and Research Organisation I CRC for Australian Weed Management NSW Ag - NSW Agriculture, Australia iv ABSTRACT ..... .-_ .... ,'.-.,.... Abstract of a thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy THE IMPACT OF HOST-PLANT STRESS ON THE PERFORMANCE OF Two BIOLOGICAL CONTROL AGENTS OF BROOM (CITISUS SCOPARIUS) by K.E. Galway Weeds often grow over a wide range of environments, experiencing a number of environmental stresses with varying intensities. As a consequence these w~eds will often exhibit differences in morphology and physiology. It has been suggested that these differences in host-plant morphology· and physiology will affect insect herbivores. Determining how insect herbivores will be affected is important for weed management programs, so that environments where greatest establishment, population growth and impact on host-plants by weed biological control agents can be identified. The Plant Stress Hypothesis (PSH), Plant Vigour Hypothesis (PVH) , and Insect Performance Hypothesis (IPH) have been used to explain changes in insect herbivore performance, when host-plants are experiencing environmental stress. A review of more than 200 studies on insect performance across five insect feeding-guilds and 12 stress types was conducted. It was found that both insect feeding-guild and stress type were important determinants of predicting insect performance, which were not considered by these three hypotheses. -.:...... -~ '.;-.-,,'- --;- The performance of Leucoptera spartifoliella and Arytainilla spartiophila, two biological agents of Scotch broom (Cytisus scoparius) were assessed over a range of host-plant soil moistures and shading levels. It was found that overall performance of L. spartifoliella and A. spartiophila were greatest on broom growing in drier soil conditions. Populations of L. spartifoliella were greatest in sunnier environments under laboratory conditions, or in the field when populations were low. However, when populations in the field were medium or high, there were no differences v .... ~'. .. between shaded and non-shaded environments. The greatest impact to broom might - ............ occur in shaded environments, where broom is not growing optimally and is -combined with the added pressure of L. spartifoliella attack. As the laboratory experiments did not allow ovipositing females to select between hosts of varying quality, further laboratory experiments were conducted, giving ovipositing females the choice of host-plants. Relationships between oviposition preference and oviposition performance were similar in most cases for L. spartifoliella and A. spartiophila across a range of host-plant soil moisture and shade gradients. The results give some support to the hypothesis that preference and performance are linked in monophagous insects that have immobile juvenile stages. These positive preference-performance results also increase the strength of predictions made in the earlier laboratory experiments. In a survey on L. spartifoliella pupal populations in New Zealand and in Australia, pupae were found on all ages of broom surveyed, from plants as young as two to 23 years old. No evidence was found to suggest that L. spartifoliella pupal populations were influenced by the age of host-plants. However, populations increased as the amount of available feeding resource (one-year-old shoots) increased. The continual feeding damage inflicted by L. spartifoliella on plants of a young age may hasten plant senescence and death. This study showed that environmental stress significantly affects insect herbivore performance, and that both insect feeding-guild and stress type need to be considered when predicting insect performance. Broom management strategies can be enhanced as environments where insect establishment, population levels and impact may be greatest have been identified. Control of broom by these two biological agents will be greatest where broom is growing jn dry and shaded environments. Key words: Cytisus scoparius, Scotch broom, Leucoptera spartifoliella, Arytainilla spartiophila, Lyonetiidae, Psyllidae, insect-plant interactions, herbivory, insect performance, insect preference, biological control, weed management, plant stress vi hypothesis, plant vigour hypothesis, insect performance hypothesis, induced defence hypothesis, plant age hypothesis, moisture stress, light stress, plant age. CONTENTS PAGE Dedication 11 Acknowledgments iii Abstract iv Contents vi List of Tables x List of Figures xiii . Chapter 1: The impact of host-plant stress on biological control strategies for Scotch broom (Cytisus scoparius). 1.1 Introduction 1 1.2 Biological control of weeds 1 1.3 Environmental stress on weeds 3 1.4 Environmental stress on insects 4 1.5 Why study the Scotch broom system? 6 1.6 Scotch broom (Cytisus scoparius) 7 Broom as a weed Broom and its environment Biological control of broom 1.7 Predicting environments where biological control of broom will be most successful 13 1.8 Insect preference-performance relationships 13 1.9 The effects of host-plant age on insect herbivores 15 1.10 Thesis outline 16 1.11 References 17 Chapter 2: Insect performance and host-plant stress: a review from a biological ,'-'- . -'~ ~--~ control perspective. 2.1 Abstract 24 2.2 Introduction 25 2.3 Materials and methods 27 Selection of insect feeding-guilds and stress types Selection of studies Definition of a 'stressed host-plant' Analytical approach 2.4 Results 30 , '. ,~ .. - . 2.5 Discussion 32 2.6 References 41 vii 2.7 Appendix 44 2.8 Appendix references 50 V111 Chapter 3: The performance of two biological control agents of Scotch broom (Cytisus scoparius), Leucoptera spartifoliella (Lep: Lyonetiidae) and Arytainilla spartiophila (Hem: Psyllidae), on moisture stressed host-plants. 3.1 Abstract 60 3.2 Introduction 61 3.3 Materials and methods 63 Plant propagation Experimental design 1:- Minimum soil water vapour content (SWVC) treatments .. - ...... _, .. Insects Climate recordings Plant morphological measures Plant physiological measures Insect performance measures Analysis 3.4 Results 69 The effects of soil moisture on broom growth The effects of host-plant soil moisture on insect populations The effects of soil moisture and insects on broom growth and survival 3.5 Discussion 79 The effects of host-plant soil moisture stress on insect performance The effects of host-plant soil moisture stress on insect impact Implications for broom management programs 3.6 References 85 Chapter 4: The effects of host-plant shading on the performance of two insect biological control agents, Leucoptera spartifoliella (Lep: Lyonetiidae) and Arytainilla spartiophila (Hem: Psyllidae) of Scotch broom (Cytisus scoparius) 4.1 Abstract 89 4.2 Introduction 90 4.3 Materials and Methods Part a: Laboratory studies 92 Plant propagation Shade treatments and experimental design Layout of shade shelters Insects Climate recordings Plant and insect recordings Repeat of oviposition section Analysis Part b: Field studies 96 Selection of broom infestations Selection of plant age groups Survey timing and design Sampling procedure Analysis ix 4.4 Results 98 The effects of shading on broom growth .:-'.:.:-: "," .... - "," The effects of host-plant shading on insect performance The impact of insect herbivores in relation to host-plant shading L. spartifoliella