Department of Meteorology in association with Rothamsted Research The biometeorology of high-altitude insect layers Curtis Ron Wood A thesis submitted for the degree of Doctor of Philosophy March 2007 The biometeorology of high-altitude insect layers (ii) Declaration I confirm that this is my own work and the use of all material from other sources has been properly and fully acknowledged. Curtis Wood The biometeorology of high-altitude insect layers (iii) Abstract Flight at high altitude is part of a migration strategy that maximises insect population displacement. This thesis represents the first substantial analysis of insect migration and layering in Europe. Vertical-looking entomological radar has revealed specific characteristics of high-altitude flight: in particular layering (where a large proportion of the migrating insects are concentrated in a narrow altitude band). The meteorological mechanisms underpinning the formation of these layers are the focus of this thesis. Aerial netting samples and radar data revealed four distinct periods of high-altitude insect migration: dawn, daytime, dusk, and night-time. The most frequently observed nocturnal profiles during the summertime were layers. It is hypothesised that nocturnal layers initiate at a critical altitude (200–500 m above ground level) and time (20:00–22:00 hours UTC). Case study analysis, statistical analysis, and a Lagrangian trajectory model showed that nocturnal insect layers probably result from the insects’ response to meteorological conditions. Temperature was the variable most correlated with nocturnal insect layer presence and intensity because insects are poikilothermic, and temperatures experienced during high-altitude migration in temperate climates are expected to be marginal for many insects’ flight. Hierarchical effects were detected such that other variables—specifically wind speed—were only correlated with insect layer presence and intensity once temperatures were warm. The trajectory model developed comprised: ( i) insect flight characteristics; ( ii ) turbulent winds (which cause vertical spread of the layer); and ( iii ) mean wind speed, which normally leads to horizontal displacements of hundreds of kilometres in a single migratory flight. This thesis has revealed that there is considerable migratory activity over the UK in the summer months, and a range of fascinating phenomena can be observed (including layers). The UK has moved from one of the least studied to perhaps the best studied environments of aerial insect migration and layering in the world. The biometeorology of high-altitude insect layers (iv) Occam’s razor (lex parsimoniae) “Entia non sunt multiplicanda praeter necessitatem” ‘Entities should not be multiplied unnecessarily’ William of Occam (c. 1285–1350) The biometeorology of high-altitude insect layers (v) Acknowledgments Particular thanks must go to the following people and organisations. Janet Barlow for providing guidance in such subtle and tactful ways that I rarely knew that I was being directed; also for witty sidetracks. Jason Chapman and Ian Woiwod , of Rothamsted Research, for supervision of entomological material. Pete Clark for providing UM data and advice on modelling; plus tangential humour during meetings. Sue Gray and Paul Mason for academic guidance through the monitoring committee. Paul Mason, John Gloster, and Anthony Illingworth (chair) for examining me and the thesis in the PhD viva voce examination. Don Reynolds for guidance and humour, particularly during field work. Alan Smith for technical expertise, assistance, and lodgings. Dave Thomson and Andy Reynolds for comments on Lagrangian modelling. Suzanne Clark and Sue Welham for statistical advice. Richard Harrington and Kelvin Conrad for conducting a 2-year VIVA. Stephen Belcher and the boundary-layer meteorology group for constructive academic (and otherwise) discussions. The Meteorology Department for its elite qualities both academically and, most particularly, social aspects. Departmental PhD students for a friendly network of support and banter. Officemates for procrastinations when needed. Jon Shonk for sharing pedantry remarks on life, the universe, and everything. Duncan Ackerley , and other housemates, for supporting me. Rob Thompson, Ewan O’Conner , and Matt Sapiano for MATLAB help. Genstat , Matlab , UNIX , Google, and Amazon for making my life much easier. Microsoft for both making my life easier and testing my temper at the same time. The Concise Oxford Dictionary , which I have aimed to heed in the writing of this thesis. BBSRC & the Met Office for funding. RMetS for providing bursaries for conferences and fieldwork alike, for providing a strong community in the UK of atmospheric scientists, and for the opportunity to serve as chair of the Student Conference 2006. Kevin Waters and Hugo Ricketts for always being supportive friends. My family , parents in particular, for financial and emotional support. The biometeorology of high-altitude insect layers (vi) Contents 1: An introduction to the biometeorology of high-altitude insect layers ... 1 1·1 An introduction to the insects ................................................................................................1 1·2 The aerial insect fauna..............................................................................................................2 1·2·1 Introduction ........................................................................................................................................... 2 1·2·2 Migration................................................................................................................................................. 3 1·2·3 Migratory flight periodicity ............................................................................................................... 5 1·3 Environmental responses.........................................................................................................5 1·3·1 Effects of temperature on take-off and flight in migrants........................................................ 6 1·3·2 Effects of vision on migratory behaviour ...................................................................................... 7 1·3·3 Chemical cues........................................................................................................................................ 8 1·4 The atmospheric boundary layer ............................................................................................8 1·4·1 Introduction ........................................................................................................................................... 9 1·4·2 Diurnal cycle ........................................................................................................................................11 1·4·3 Static stability ......................................................................................................................................12 1·4·4 Vertical neutral profiles of meteorological variables ...............................................................14 1·4·5 Turbulent fluxes..................................................................................................................................16 1·4·6 Turbulence............................................................................................................................................18 1·4·7 The nocturnal boundary layer (NBL) .............................................................................................20 1·5 Migratory flight profiles ........................................................................................................ 24 1·5·1 The insect flight boundary layer (FBL) ..........................................................................................24 1·5·2 Vertical movements...........................................................................................................................25 1·5·3 Vertical distribution of insects in the atmosphere ...................................................................26 1·6 Layer concentrations.............................................................................................................. 28 1·6·1 Layer concentrations overview.......................................................................................................28 1·6·2 Daytime layering ................................................................................................................................29 1·6·3 Nocturnal layering .............................................................................................................................30 1·6·4 Summary of insect layering.............................................................................................................32 1·7 Motivation and thesis plan ................................................................................................... 33 The biometeorology of high-altitude insect layers (vii) 2: Monitoring of airborne insect populations ................................................35 2·1 Introduction.......................................................................................................................... 35 2·2 Methods ................................................................................................................................. 37 2·2·1 Ground trapping ..................................................................................................................................37