Table Of ContentINSECT CHEMORECEPTION
FUNDAMENTAL AND APPLIED
Insect Chemoreception
Fundamental and Applied
by
Michael F. Ryan
Department ofZoology, University College Dublin
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DEDICATED TO MY FAMILY
PREFACE
In this time of edited volumes when the list of individual contributors may reach
double figures, it is appropriate to question the usefulness of a volume, with such a
broad scope, by a single author. The answer is simple. For years he has believed that
the rather sharp distinction between fundamental and applied aspects of this
discipline, has ill-served the significance of each; and has diminished the incidence
of fruitful synergies. Yet the need for these was never greater, and this case may be
developed by a single author withexperience of each aspect.
The inclusion of a Chapter on Genetic Engineering may raise some doubts, but it
is enabled by the chosen title “Chemoreception”, as distinct from Chemoperception:
the latter implies detection of a chemical, followed by a behavioural response. But
the former broader category subsumes Chemoperception and allows for the
reception of a chemical toxin so potent as to prelude a behavioural or physiological
response, other than death. Accordingly, chemical toxins are a legitimate inclusion.
In which event, their delivery through a GM plant is as appropriate for study as their
application in a spray.
Following a Chapter devoted to an Introduction and Overview, Part I comprises
Chapters 2, 3, 4, 5, and 6 respectively, providing a fundamental framework. Part II
comprising Chapters 7, 8, 9, and 10 examines applied aspects. For internal
consistency, the sequence followed in Part I in regard to plant chemicals and
pheromones (Chapters 2 and 3 respectively), is also followed in Part II (Chapters 7
and 8 for plants and Chapter 9 for pheromones). However, as progress in genetic
engineering for insect pest control (Chapter 10) is less with insects than with plants,
Chapters 7, 8, and 10 may be read as a continuum on plants.
Chapter 10 brings to the fore the Third World dilemma of feeding, in the future,
vastly more people from less land and poorer-quality water. The problem is most
acute in sub-Saharan Africa where, from 1970 onward, food production grew at
about half the rate of population growth, entailing decreased per capita food
production. Undernourished people in this region are estimated to number some
300 million or half the world’s total. In seeking to find a balance between GM-based
and sustainable ecological solutions, this Chapter explicitly calls for a renewed
commitment from the West, especially from the young researcher. Shall a coalition
develop?
My most grateful thanks are due to the many who encouraged or helped,
especially President Art Cosgrove, University College Dublin (UCD), and my
colleagues at the Institute of Environment and Life, Department of Zoology,
Coimbra University, Portugal, where my presence was enabled by the PRAXIS XXI
programme of the EU, and by the Foundation for Science and Technology, Lisbon.
vii
viii
A very special debt of gratitude is owed to Professor Maria Susana de Almeida
Santos, Leader of the Nematology Group, whose kind hospitality sustained this
enterprise at difficult and decisive moments. She also, with her colleague Professor
Isabel Abrantes, critically read the manuscript leading to significant improvements
and the avoidance of countless solecisms.
The hand-drawn figures are by William Clarke, UCD except for Figure 33 by
Professor Mary Behan, and Figure 39 by Dr. Peter Daly, both originally of UCD.
Figures 18, 32, 35, 36, 37, 38, 72 and 74 are by Fernando Correia, Coimbra
University; chemical structures and all remaining diagrams are by Dr. Wen-Yuan
Chung, UCD. At Coimbra University, the species index was compiled by Sofia dos
S. da R. Costa, and the entire manuscript was prepared and formatted with
unforgettable insight, patience, and skill by M. Clara Vieira Santos.
MFR
September 2001
TABLE OF CONTENTS
Preface vii
1. INTRODUCTION AND OVERVIEW 1
FUNDAMENTAL ASPECTS 1
Historical 1
Insect phylogeny 2
Insect systems 2
Insect senses 4
Plant/insect relationships 7
APPLIED ASPECTS 8
Firstgeneration insecticides 9
Second generation insecticides 9
Organochlorines 9
Organophosphates 10
Carbamates 10
Pyrethroid insecticides 10
Avermectins 12
Chitin inhibitors 13
Third generation insecticides 13
Fourth generation insecticides 14
ECOLOGICAL CONSEQUENCES 15
Insect resistance 15
Mortality of beneficial and innocuous insects and wildlife 16
Target pest resurgence 16
Secondary pest outbreak 16
Mortality of wild life 16
Contamination of food 17
Cancer 18
The present position 19
Risk assessment 20
REFERENCES 23
PART I – FUNDAMENTAL ASPECTS
2. PLANT CHEMICALS 27
CHEMICAL DEFENCE BY NUTRIENTS 28
CHEMICALDEFENCE BY ALLELOCHEMICALS: STRUCTURAL CATEGORIES 29
Phenolics 29
Terpenoids 31
Monoterpenes 33
ix
x
Sesquiterpenes 35
Diterpenes 35
Triterpenes 36
Alkaloids 36
Sequestration 37
Effects on insects 38
Polyacetylenes and other compounds 39
ALLELOCHEMICALS: FUNCTIONAL CATEGORIES 40
Allomones 40
Jasmonic acid 43
Proteinase inhibitors 44
Kairomones 46
Synomones 47
Antimones 48
TRITROPHIC INTERACTIONS 48
Evolutionary aspects 49
Blend composition 51
Comparison of direct and indirect defence 51
MICROBIAL INTERACTIONS 52
Nitrogen-fixers 54
Endophytes 54
Phylloplane microflora 54
Plant pathogens 55
Insect mutualists 55
Insect pathogens 55
THE CONCEPT OF COEVOLUTION 56
Specific reciprocal coevolution 57
Diffusecoevolution 57
Escape and radiation coevolution 58
Diversifyingcoevolution 59
Objections 59
FATE IN INSECTS OF ALLELOCHEMICALS 61
Evacuation 61
Metabolism 61
Cytochrome P-450-dependent polysubstrate monooxygenases
(PSMOs) 62
Esterases and transferases 63
Reductases 64
Sequestration 65
Pheromone precursors 67
REFERENCES 69
xi
3. PHEROMONES 73
PHEROMONE-PRODUCING GLANDS 73
Coleoptera 73
Lepidoptera 75
CHEMICAL STRUCTURES AND NOMENCLATURE OF PHEROMONES 77
Pheromones of female Lepidoptera 79
Pheromones of male Lepidoptera, and of other Orders 79
SEX PHEROMONES 81
Female sex pheromones 82
Specificity 82
Male sex pheromones 83
Sex-pheromonebiosynthesis 84
The blend 86
Perception of the blend 86
Significance of the blend 87
Genetics of the blend 87
Hormonalregulation of sex-pheromoneproduction 88
Melanizationand reddish coloration hormone (MRCH) 89
Orientation to pheromone source 90
AGGREGATION PHEROMONES 92
Synergism 95
Chiral specificity 97
Aphid aggregation pheromones 97
OVIPOSITION PHEROMONES 97
SPACING (EPIDEICTIC) PHEROMONES 98
PHEROMONES OF SOCIAL INSECTS 99
Multifunctionality 100
Kin recognition 101
Overlapping defence and sex attractant functions 103
Alarmpheromones 104
Alarm pheromone glands 105
Dispersal and attack 105
Chemistry of the compounds 105
Function-shift in pheromone evolution 107
REFERENCES 109
4. THE CHEMORECEPTIVE ORGANS: STRUCTURAL ASPECTS 113
THE SENSORY NEURON 115
ASSOCIATED CELLS 117
SYSTEMATICS OF SENSILLA 119
xii
EVOLUTION OFSENSILLA 121
Collembola 121
Odonata 122
Lepidoptera 123
SENSILLA OF IMATURE INSECTS 125
Aporous mechanosensilla 125
Thermo-hygrosensilla 125
Chemosensilla 126
Gustatory chemosensilla 126
Olfactory chemosensilla 126
The sensory neuron 126
Associated cells 127
Morphogenesis and moulting 127
Sensillar distribution 128
Antenna 128
Maxillae 128
Labium 128
Clypeo-labrum 128
Mandibles 129
Embryological development of sensilla 129
SENSILLAR ACCESS 131
Dimensional movement 131
SIGNIFICANT ORGANELLES 132
EXTERNAL MORPHOMETRY 134
Morphological correlates 135
Internal morphometry 135
REFERENCES 138
5. ELECTROPHYSIOLOGY OF CHEMORECEPTION 140
THE NEURON 140
The action potential 140
Channel characteristics 141
Stimulus-response relationships 141
Central processing 142
THE INSECT ANTENNA 143
THE ELECTROANTENNOGRAM 144
Relationship of EAG to olfactory stimulation 145
Uses of the EAG 146
CIRCUITRY OF THE SENSILLUM 148
Ionic currents of the sensillum 150
