POPULATION ECOLOGY OF CICADELLA VIRIDIS (L.) AND BIONOMICS OF GRAPHOCEPHALA COCCINEA (FORSTER) (HONOPTERA CICADELLIDAE). By Tay Eong Beok A thesis submitted for the Degree of Doctor of Philosophy, University of London. Imperial College Field Station, April, 1972. Silwood Park, Sunninghill, nr. Ascot, Berkshire. ABSTRACT The biology and population ecology of Cicadella viridis (L.) were studied in conjunction with the bionomics of Graphocephala coccinea (Forst.) in Silwood Park, Berkshire, Southern England. Both species are univoltine and overwinter in the egg stage. Study of undercooling point indicated hat eggs of both species are extremely cold-hardy. Most eggs of both species'could hatch without having to pass through a period at low temperature, but the proportion capable of doing this is greater in C. viridis than in G. coccinea. Larvae of both species hatched during the second half of May, and adults emerged in July. Oviposition in C. viridis commenced towards the end of August, but was slightly later and during the second week of September in G. coccinea® It was found through rearing experiments that although C. viridj laid its eggs in Juncus effusu.s L., the larvae required Holcus Mollis L. for growth and development. The number of C. viridis eggs produced each season was estimated by sampling the egg-bearing Juncus stems. The larvae and adult numbers were estimated by sampling at regular intervals with a Dietrick vacuum suction sampler, as well as by direct counting. During the egg stage, mortality is caused mainly by fungal attack, parasite and predators being the other main contributory factors. Predation by spiders and Nahidae ii appeared to be the main cause of mortality iii larvae and in the adult stage. Adults were also attacked by a fungus EntomoEhLhora sp. but its effect during 1969 to 1971 was slight. Population budgets for 1969, 1970 and 1971 are presented. A brief study of the flight activity and dispersal of C. viridis and G. coccinea was made. Their movements are essentially trivial and largely confined to the immediate vicinity of their habitats, but adequate power for dispersal is shown in occasional individuals caught in traps far away from their breeding grounds. iii TABLE OF CONTENTS ABSTRACT i. TABLE OF CONTENTS iii GENERAL INTRODUCTION 1 SECTION I. Cicadella viridis(L.) I. DESCRIPTION OF THE STUDY AREA 3 II. BIOLOGY OF C. viridis 8 1.... Methods of rearing 8 1.1 Eggs 8 1.2 Larvae and adults 8 • 2. Life history 10 3. Food plants. 13 4. Description of the egg 30 5. Oviposition 31 5.1 Host plants and the number of eggs per egg site 31 5.2 Distribution of oviposition site and its relationship to the diameter of Juncus stem 37 5.3 Effect of direction on oviposition 43 6. Egg development 45 6.1 Humidity and egg development 45 6.2 Effect of low temperature on egg survival 45 6.2a Determination of undercooling of eggs of C. viridis 45 6.2b Incubation at sub-zero temp- erature 57 6.3 Diapause in the eggs of C. viridis 60 6.3a Introduction 60 iv 6.3b Experiment 1. Hatching of eggs that have not been exposed to low temperatures 62 6.3c Experiment 2. Hatching of eggs after a period at low temperatures 65 6.3d Experiment 3. The effect of lengths of exposure to low temperature on hatching 66 6.3e Discussion 68 6.4 The effect of temperature on the rate of development of the eggs 74 7. Hatching in the field 77 8. Duration of larval instars 83 8.1 Rearing in the outdoor insectary 83 8.2 Duration of instars under different constant temperatures 85 9. Description of larvae and adults 85 9.1 The larval stages 85 9.2 Sex ratio 92 9.3 The adults 93 9.4 Relationship between measurements of succeccive instars 94 10. Internal reproductive organs and sexual maturation 98 10.1 Male 98 10.2 Female 100 11. Weight changes in adults and its relation -ship to oviposition cycle in the females 102 12. Longevity of adult C. viridis 105 13. Fecundity 106 14. Some observations on the biology of egg parasite Anagrus atomus(L.) (Mymaridae:Hymenoptera) 109 III. POPULATION OF C. viridis 115 1. Method of sampling eggs 115 1.1 Estimates of oviposition sites 115 1.2 Method of estimating eggs .116 2. Methods of sampling larvae and adults 117 2.1 Methods tried 118 2.2 Methods chosen for population estimate of larvae and adults 119 2.3 Efficiency of the Dietrick Machine 123 2.4 Comparison between direct count method and the Dietrick Machine for the estimation of adults 123 3. Estimated number of eggs in the field 125 4. Number of larvae and adults 126 5. Pattern of distribution of larvae and adults C. viridis in the field 128 6. Estimation of recruits and mortality in the larval and adult stages 140 6.1 Methods of estimating recruits entering a stage 140 6.2 Estimates of larvae and adults 142 6.3 Population budget of C. viridis 147 7. Mortality factors 153 7.1 Mortality in the egg stage 153 7.1a Fungal infection 154 7.1b Egg parasite 156 7.1c Egg predation 158 7.1d Sterility of egg 158 7.1e Mortality caused by other factors 159 7.1f Relative importance of various mortality factors 160 7.2 Mortality in the larvae and adults 162 7.2a Parasitism 162 vi 7.2b Fungal infection 166 7.2c Predation 167 (i) Laboratory feeding tests 167 (ii) The precipitin test 170 (iii) Populations of spiders 179 (iv) Populations of Nabidae 187 7.3 Analysis of mortality in the egg, larval and adult stages 188 IV. DISPERSAL AND FLIGHT ACTIVITY 195 1. Dispersal 195 1.1 Mark and recapture method 196 1.2' Water traps 196 1.3 Sticky traps 197 2. Flight ability of C. viridis 202 3. General discussion 204 SECTION II. BIONOMICS OF Graphocephala coccinea(Forst) 206 1. Introduction 206 2. Methods of rearing 208 2.1 Eggs 208 2.2 Larvae and adults 208 3. Life history of G. coccinea 209 4. Egg stage 212 4.1 Description of the egg 212 4.2 Determination of the undercooling point of the eggs 213 4.3 The effect of temperature on the rate of development of the eggs 218 5. Larval stages and the adult 221 5.1 Duration of the larval instars 221 5.2 Description of the larval stages 221 5.3 The adults 225 vii 5.4 The relation between measurements of successive instars 226 5.5 Internal_reproductive organs 229 5.5a Male 229 5.5b Female 229 5.6 Weight changes in the adults and fecundity of the females 231 6. Mortality of the eggs, larvae and adults G. coccinea 235 6.1 Egg 235 6.2 Mortality in larvae and adults 237 7. Flight and dispersal 238 7.1 Flight ability of G. coccinea 238 7.2 Periodicity of flight 242 7.3 Dispersal 243 7.4 Discussion 245 GENERAL DISCUSSION 249 SUMMARY 265 ACKNOWLEDGEMENTS 275 REFERENCES 277 1 GENERAL INTRODUCTION The subfamily Cicadellinae is represented in Britain by two species, Cicadella viridis (L.) and Graphocenhala coccinea (Forst.). In Europe and East Asia, C. viridis is known to be a pest of young fruit trees, and has often been recorded from agricultural crops, but it is of no economic importance in Britain. The British strain of C. viridis also differs from the European one in voltinism. G. coccinea is a relatively recent9introduced species from North America. In Britain this insect is generally associated with 'bud blast' disease of Rhododendron and is therefore a pest of minor importance. The thesis is mainly concerned with a population study of C. viridis, but its biology is also studied in order to get a better understanding of the species. Because of the time factor, only bionomics of G. coccineawesQ., studied. The thesis is divided into two sections, the first section dealing with C. viridis. The undercooling point and diapause of the eggs, the ovipositithn of the adults, and the food plants of immature stages were investigated. -As the insect has not previously been studied in any detail in Britain, an account of its life history and description. of the immature and adult stages is also given. The population study includes a comparison of current methods of estimating the numbers of recruits entering a stage in a population with widely overlapping 2 stages. Population budgets for the years 1969, 1970 and 1971 are presented. Investigations were carried out to acertain the cause of mortality occurring in all stages. Predators were studied by means of serological techniques and laboratory feeding trials. Varley and Gradwell's (1960) graphical key factor analysis was employed to identify the principal mortality in the egg stage and the stage where mortality was most likely to have a regulating effect on the population as a whole. Might activity and dispersal were studied, but not in great details. In Section II, the bionomics of G. coccinea are described, and compared with those of C. viridis. .A. brief study was also made of flight and dispersal of this insect. 3 SECTION I Cicadella viridis(L.) I DESCRIPTION OF THE STUDY AREA Imperial College Field Station, Silwood Park is o ' situated in southern England at 51 24 N and 0 39 W. It consists of some 230 acres of parkland, including marsh, farmland, experimental plots and a lake. The work was carried out in an area known as the Rush Meadow by the northern shore of Silwood Lake. The study site is located at the East'ern end of Rush Meadow where Juncus effusus L.