Abstract
	Acknowledgements
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Chapter 1: General Introduction

• 1.1 Introduction
• 1.2 Increased host-plant quality: a universal cause of insect outbreaks?
• 1.2.1 Overview
• 1.2.2 Environmental effects on host-plant quality: the plant stress hypothesis
• 1.2.3 Direct inputs of nitrogen to the host-plant: fertilisation and pollution
• 1.2.4 Temporal variability in host-plant quality
• 1.3 An opposing view: "the world really is green"
• 1.3.1 Overview
• 1.3.2 Climatic Release
• 1.3.3 Natural enemies and "enemy-free space"
• 1.4 Nutritional Compensation
• 1.5 Host Races
• 1.6 Effects of CO₂ enrichment on host-plant quality and insect herbivory
• 1.7 Thesis Structure

Chapter 2: Description of moorland O. brumata outbreaks

• 2.1 Introduction
• 2.2 Study species
• 2.2.1 Operophtera brumata host range and life history
• 2.2.2 Calluna vulgaris and associated moorland plant species
• 2.3 Insect outbreaks on Calluna vulgaris
• 2.3.1 Description and history of moorland O. brumata outbreaks in Scotland
• 2.3.2 Study sites
• 2.3.3 Mechanisms of moorland decline: effects of defoliation on Calluna

Chapter 3: Effects of alternative host availability and winter weather on moorland O. brumata populations

• 3.1 Introduction
• 3.1.1 Overview
• 3.1.2 Host quality, insect abundance and development
• 3.1.3 Population variability and climate
• 3.2 Methods
• 3.2.1 Patterns of host exploitation by newly-emerged larvae
• 3.2.2 Host exploitation and development of late-instar larvae
• 3.2.3 Larval mortality and development on Calluna and Vaccinium
• 3.2.4 Growth rate, frass production and pupal weight on Calluna and Vaccinium
• 3.2.5 Population densities of adult O. brumata
• 3.2.6 Population densities of larval O. brumata
• 3.3 Results
• 3.3.1 Host exploitation by newly-emerged larvae
• 3.3.2 Late-season host exploitation and development
• 3.3.3 Mortality and development on Calluna and Vaccinium
• 3.3.4 Growth rate, frass production and pupal weight
• 3.3.5 Population densities at Hunthill and Orkney
• 3.4 Discussion
• 3.4.1 Patterns of host use in the field
• 3.4.2 Nutritional qualities of Calluna and Vaccinium
• 3.4.3 Population densities: a "climatic release" hypothesis
• 3.5 Summary

Chapter 4: Life history, metabolism, host specialisation and parasitism in O. brumata populations

• 4.1 Introduction
• 4.1.1 Overview
• 4.1.2 Life history variability in O. brumata
• 4.1.3 Metabolic adaptation to local climate
• 4.1.4 Host specialisation
• 4.1.5 Enemy-free space
• 4.2 Methods
• 4.2.1 Collection and rearing of larvae; determination of parasitism levels
• 4.2.2 Pupal weights, adult emergence dates and adult morphology
• 4.2.3 Reciprocal transfer experiment to detect host specialisation
• 4.3 Results
• 4.3.1 Pupal weights and viabilities
• 4.3.2 Adult emergence dates
• 4.3.3 Adult size, pupal weight loss and inferred pupal respiration
• 4.3.4 Reciprocal transfer experiment
• 4.3.5 Parasitism levels
• 4.4 Discussion
• 4.4.1 Life history variability
• 4.4.2 Metabolism
• 4.4.3 Host specialisation
• 4.4.4 Enemy-free space
• 4.4.5 Wing lengths
• 4.4.6 Possible artefacts of rearing conditions
• 4.5 Summary

Chapter 5: The importance of hatch-budburst synchrony for O. brumata on C. vulgaris

• 5.1 Introduction
• 5.1.1 Overview
• 5.1.2 Hatch-budburst synchrony, and insect life history adaptation
• 5.2 Methods
• 5.2.1 Hatching date in situ in a moorland outbreak
• 5.2.2 Hatching dates of moorland and oak woodland populations
• 5.2.3 Manipulation of hatch dates in moorland and woodland populations
• 5.2.4 Chemical analysis
• 5.2.5 Data analysis
• 5.3 Results
• 5.3.1 Hatching dates in situ
• 5.3.2 Hatching dates of O. brumata populations under constant conditions
• 5.3.3 Manipulation of O. brumata hatch dates
• 5.3.4 Calluna vulgaris tissue chemistry
• 5.4 Discussion
• 5.4.1 Unimportance of hatch-budburst synchrony
• 5.4.2 Differential sensitivity to host quality on different hosts: a model
• 5.4.3 Differences in survival and hatch date between biotypes
• 5.5 Summary

Chapter 6: An experiment to test for compensatory feeding by O. brumata on C. vulgaris

• 6.1 Introduction
• 6.1.1 Overview
• 6.1.2 Nutritional compensation
• 6.2 Methods
• 6.2.1 Growth of C. vulgaris seedlings
• 6.2.2 O. brumata feeding trials
• 6.2.3 Calculations and data analysis
• 6.3 Results
• 6.3.1 Seedling responses
• 6.3.2 Larval responses
• 6.3.3 Testing for compensation: responses of consumption rates and growth rate
• 6.4 Discussion
• 6.4.1 Evidence for compensatory feeding
• 6.4.2 Implications for O. brumata performance
• 6.5 Summary

Chapter 7: Experimental CO2 and nitrogen enrichment of the C.vulgaris-O. brumata system, and a test for nitrogen-enrichment in the field

• 7.1 Introduction
• 7.1.1 Overview
• 7.1.2 Carbon dioxide enrichment, nutrient limitation and phenology
• 7.1.3 Nitrogen enrichment of Calluna
• 7.2 Methods
• 7.2.1 Experimental CO₂ and nitrogen enrichment of C. vulgaris, and the effects on O. brumata performance: a feeding trial
• 7.2.2 Sampling and chemical analysis of Calluna from the field
• 7.3 Results
• 7.3.1 Performance of open-top chambers
• 7.3.2 Responses of C. vulgaris to experimental CO₂ and nitrogen enrichment
• 7.3.3 Responses of O. brumata to the experimental treatments of C. vulgaris
• 7.3.4 Nitrogen concentration of field-sampled C. vulgaris
• 7.4 Discussion
• 7.4.1 Chamber effects
• 7.4.2 Lack of response to CO₂ enrichment
• 7.4.3 Responses to experimental nitrogen enrichment
• 7.4.4 Response of O. brumata survival to C. vulgaris shoot length
• 7.4.5 Nitrogen concentration of C. vulgaris in the field
• 7.4.6 Implications for outbreak potential
• 7.5 Summary

Chapter 8: General Discussion

• 8.1 "Bottom-up" and "top-down" constraints on O. brumata populations
• 8.1.1 Overview
• 8.1.2 "Bottom-up" effect of host quality
• 8.1.3 "Top-down" effect of parasitism, and its interaction with altitude
• 8.1.4 Direct effects of climatic variability on O. brumata populations
• 8.1.5 A schematic model of high-altitude moorland O. brumata outbreaks
• 8.2 Effects of environmental change on the O. brumata-C. vulgaris system

	References
	Appendices

• Appendix I: Calculation of larval growth rates
  
• I.1 Overview
• I.2 The mean relative growth rate, and some problems with its use
• I.3 Growth rate calculations in this thesis
• Appendix II: Comparison of nitrogen assay techniques
• II.1 Overview
• II.2 Methods
• II.3 Results
• II.4 Discussion

Outbreaks of winter moth, Operophtera brumata L. (Lepidoptera: Geometridae), have recently become common on moorlands in Scotland. This thesis describes the ecology of moorland O. brumata populations, and seeks to determine whether increased host-plant nutritional quality is a probable cause of outbreaks.

The principal moorland host used by outbreak O. brumata populations is common heather, Calluna vulgaris (L.) Hull (Ericaceae), which is shown here to be of low nutritional quality compared to Vaccinium myrtillus (L.) (Ericaceae), an alternative moorland host.

Operophtera brumata growth rate and pupal weight on C. vulgaris are shown to be increased by experimental nitrogen-enrichment of this host. Nevertheless, this study finds no evidence that outbreaks are caused by nitrogen-enrichment of C. vulgaris in the field. There is no evidence that moorland O. brumata populations are specialised to feeding on C. vulgaris, and no evidence that synchrony between larval emergence and C. vulgaris budburst is an important determinant of larval survival and development. There is some evidence that O. brumata exhibits compensatory feeding, a mechanism which may be important for successful development on C. vulgaris.

The nutritional quality of C. vulgaris is not significantly affected by growth in carbon dioxide-enriched atmospheres, and there is therefore no evidence that moorland O. brumata populations will be affected by this aspect of environmental change.

A comparative study reveals differences between moorland and non-moorland O. brumata populations in adult emergence dates, pupal respiration rates and hatching dates under constant conditions. However, these differences are most readily explained as physiological adaptations to local climate, and are unlikely to be caused by nutritional specialisation of O. brumata populations to different host species.

Evidence is presented that winter weather conditions strongly affect O. brumata abundance in high-altitude moorland outbreak sites. Further evidence is presented that O. brumata escapes from parasitism at such high-altitude sites.

The relative importance of "top-down" factors, such as natural enemies, and "bottom-up" effects, such as host-plant quality, in determining O. brumata abundance is discussed. It is argued that, in high-altitude moorland sites, the interaction between winter weather conditions and the "top-down" effect of natural enemies is a more important determinant of outbreak potential than the "bottom-up" effect of host quality. The effects of future environmental changes on this system are considered.

Time and forgetfulness prevent me from acknowledging everyone who has contributed to this work. Nevertheless, I shall list those whose help has been indispensable, and hope to be forgiven the inevitable omissions:

First and foremost, I thank my supervisors, Sue Hartley and Sarah Woodin, for being constant sources of support, encouragement and common sense, and for tolerating my grumpy, erratic and wilfully unconventional working style.

I am also extremely grateful to David Elston, whose statistical advice and enthusiasm were inspirational.

For financial assistance, I gratefully acknowledge the support of the Aberdeen Research Consortium, the Scottish Landowners' Federation and the Leverhulme Trust.

For discussion and advice at various stages of the work, I thank Allan Watt, Simon Fraser, Mark Young, Prof. Charles Gimingham, Lars Högbom, Carmen Gordon, Nick Picozzi, Karen Haysom, Loeske Kruuk, Richard Smith, Paul Metcalfe, and Peter Reynolds.

For practical assistance, I thank the owners, factors and keepers of Hunthill, Auchnafree, Invercauld, Fasque and Balmoral Estates. I am particularly grateful to the staff at Hunthill for "rescuing" the I.T.E. Land Rover.

I thank Eric Meek and Liz McTeague for their help on Orkney, Mark Shaw for parasitoid identification, Lionel Cole for supplying winter moth eggs from Wytham Woods, and the staff of Andy Midwood's lab at the Macaulay Land Use Research Institute for the use of their analytical facilities.

I gratefully acknowledge everyone who assisted me with the more laborious and physically demanding parts of the work, especially Ken Nelson, Sue Auld, Joanne Harvey, Maria Luisa Graziani, Stuart Munro and Seth Racey.

I am grateful to the staff of the Cruickshank Botanic Gardens, especially Bob Rutherford, for tolerating the presence of my frequently weed-infested experiments.

I thank also Betty Johnson, Jim McRobb, Jim McGregor, Kathryn Lamza and Nora Morrison for their kindness and help.

I am grateful to Norman Little for the production of seemingly endless quantities of white-on-black slides; I am particularly grateful both to Norman and to Mike and Stef Swaine for helping to preserve my sanity in various Aberdonian public houses at critical moments throughout the course of this enterprise.

I am extremely grateful to the Astell family of Mary Culter, without whose tremendous kindness and hospitality I would not have been able to undertake much of the work presented here.

For the original inspiration and encouragement, I gratefully acknowledge my debts to David Hanke and to the late John Treherne.

I thank all the friends who have offered me food, drink, hospitality, advice and encouragement. They include Deborah Pearce, Sarah Barton, Sophie Barnes, Justin Baron, Mo Docherty, Lorna Brown, Ruth Dunn, Jenny Lowe, Adam Langsdale and Elspeth McKechnie.

In particular, I gladly acknowledge those close friends who have done most to "keep the show on the road" over the past four years: Ann Davies for bodhran and soup, Clare Woolgrove for concertina and clogs, and Dr. Kevin T. Hicks (pending) for the guitar, the Laphroaig 15, and copious moral and immoral support.

It remains for me to thank my parents, and the rest of my family, for their constant support and tolerance of the inevitable neglect that accompanies work such as this.

Finally I thank Agata, for being bothered to wait for me to finish all this nonsense.

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The work described in this thesis is entirely my own, and has not been accepted in any previous application for a degree, with the exception of the following:

All data shown in Figures 3-5, 3-6 & 3-7 were obtained by Loeske Kruuk, as part of an M.Sc. Ecology project at Aberdeen University. The Mann-Whitney and Kruskal-Wallis tests associated with Figures 3-5 and 3-6 were also performed by L. Kruuk. These results were previously presented in:
Kruuk, L.E.B. (1993) Winter moth (Operophtera brumata) outbreaks on heather moorland: patterns of host plant utilisation. M.Sc. (Ecology) thesis, University of Aberdeen.

All other data and statistics in this chapter were obtained/performed by me, and all text is entirely my own.

David Elston of Biometrics and Statistics Scotland gave advice on the use of GLIM to fit generalised linear models to data in these chapters. However, all analyses were ultimately performed by me.

Assays of total nitrogen concentration of plant material by the acid digestion method were performed by W.K. Hicks.

All quotations in this thesis are distinguished by quotation marks, and all sources of information are acknowledged.

Signed................................................................................ James Kerslake

Confirmed................................................................................ Dr. Sarah Woodin

July 1996

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