THE BIOLOGY AND POPULATION ECOLOGY OF STENOCRANUS I4INUTUS (FABVICIUS) (DELPHACIDAE-HEMIPTERA) by YIN YIN MAY, B.Sc. (Hons.), M.Sc. (Rgn.) A thesis submitted in partial fulfilment for the Doctor of Philosophy in the Faculty k..f Science University of London . Imperial College of Science and Technology, Silwood Park December, 1970. Sunninghill, Ascot, Berkshire. 1 o (2) ABSTRACT The biology and population; dynamics of Stenocranus minutus (Fab.;) were studied during the period from March 1965 to September 1970 in a small area of cocksfoot grass- land (Dactyliq plomerata, L;) arSilwood nark, Berkshire: minutus is univeltine and overwinters in the adult stage; The adults emerge at the end of August to the begin- ning of October: 'Ile adult female is di-morphic with a non- -migrant form and an obligatory migrant. form; the former is sub-mq.cropterous and the latter macropterous. Emigration occurs on emergence: for a short period• only; The females and males are not, sexually mature on emergence; In winter, ,the adults hibernate among the litter underneath the host plant and re-emerge, when the temperature becomes favourable in spring. Oviposition takes place from end of April to beginning of July; The eggs are laid on the green parts of the grass tussocks: There are five nymphal instars, appearing in the second half of Juile and disappearing in the second half "of September: Adult, and nymphal numbers were estimated by sampling large, medium and small tussocks in each plot, by means of various methods which are mentioned in detail in the thesis; The egg population was estimated by cutting_one-eighth of large tussocks, one-quaiter of medium tussocks and one-half of 1 small tussocks; bringing them back into the laboratory, and examining each stem for the numbers of eggs laid and converting these results into the total egg population estimate. Losses of the adult population were caused mainly by predation by spiders and nabids, by parasitism by pipunculids and mites, by emigration which was partly related tr. deteriora- -tion of the habitat and by winter deaths: Nymphal mortality 1. (b) was also due to predation by spiders, nabids, mirids, anthoco- -rids and mites, by weather factors such as rainfall, fluctua- -tions in temperature and relative humidity, and by deterio- -ration of habitat; Egg mortality resulted from sterility, predation by Heteroptera and from parasitism by MYmaridae; Extreme fluctuation in climatic conditions also conbituted to death of the eggs; Population budgets are presented for the years 1969 and 1970„ 2. TABLE OF CONTENTS page ABSTRACT 1 TABLE OF CONANTS 2 INTRODUCTION 8 SECTION I. BIOLOGY OF STENOCRANUS MINUTUS 11 1. Description of the habitat and the host plant. 11 (i)The habitat 11 (ii)The host plant 13 2. Methods of rearing 14 3. Life history of Stenocranus minutus 15 (i) General observations on the life history of S. minutus 15 (ii) The egg stage 17 A. Description of the egg stage 17 B. Numbers arid distribution of oviposition sites 18 C. Effect of temperature on the egg 19 D. Effect of different humidities on the egg 21 (iii) The nymphal stages 22 A. Eclosion and behaviour of nymphs 22 B. Durations of nymphal instars 25 C. Rates of growth of different parts of the body of Stenocranus, estimated by their linear measurements in successive instars 26 D. Development of other characters throughout successive instars 30 E. Key for separating successive instars of S. minutus 35 3• Page (iv) The adult stage 37 A. Introduction 37 B. Sub-macropterous forms 37 C,. Macropterous forms 38 D. The female insect 39 E. The male insect 44 F. The main differences between the two forms 45 Sexual maturation 46 (i) Internal male reproductive organs 46 (ii)Internal female reproductive organs 46 (iii)Changes in the reproductive organs in both sexes 48 (±v) Rate of maturation in the macropterous forms 53 Observations on female reproduction 54 (i) Laboratory studies on pre-oviposition period 57 (ii)Fecundity in the field 58 (iii)laboratory studies on oviposition 60 A. Fecundity of sub-macropters in the outdoor insectary 60 B. Fecundity of macropters in the outdoor insectary 63 C. Fecundity of sub-macropters in the constant temperature rooms 64 DC; Fecundity of macropters in the constant: 66 temperature rooms 66 E. Rate of oviposition in the sub-macropters Page F. Correlation between number of eggs laid by Stenocranus and duration of oviposition period 67 G. Effects of overcrowding on oviposition 68 (iv) ,-.7117eight changes 74 (v) Changes in the fat body throughout the season 79 6,; Flight activity of the adult 81 (1) Dispersal of the adult 81 (ii) Methods used to study the flight activity 87 A. Examinations of weekly samples of adults to determine the proportion of macropterous forms 87 B.; By means of water traps 88 By means of sticky traps 89 E; By the use of suction traps 89 SECTION II. BIOLOGY OF THE P'RASITES AND PREDATORS OF STENOCRANUS MINUTUS' 1; Methods for rearing parasites 94 2 Life history of the egg parasite, Anagrus incarnatus Haliday (Nymaridae) 94 (i) Life history of A; incarnatus 94 (ii) External morphology of the different stages 98 A: The egg stage 98 B. First larval stage 99 C. Second larval stage 99 D. Pupal stage 99 E. Adult stage: 100 5. Page (iii) Effects of parasitism on ,the eggs 103 3: Life history of the nymphal and adult parasite, Cephaloris curtifrons Coa 103 •(1) Introduction 103 (ii) Methods 104 (iii)Life history of Cephalops curtifrons Coe 105 (iv) External morphology of the different stages 111 A. The egg stage 111 B. First instar larva 111 C. Second instar larva 112 D; The pRpal3stage 114 E. The adult female and male 115 F. Effects of parasitism on external features of sub-macropterous forms 118 G; Effect of parasitism on reproductive organs of sub-macropterous forms 121 11; Effect on host movement 122 I; Effect of parasitism on macropters 123 4; The ectoparasite of adults and nymphs, Charletonia singularis (Oudeman) 124 5: Methods for rearing predators T26 6:Total fauna of predators in the field 127 7, Predators of the egg stage 129 8; Predators of the nymphal and adult stage 133 SECTION III. POPULATION STUDIES T; Methods of sampling the population 148 (1) The egg stage 148 A; Method I 148 B; Method II 149 C. Method III 150 6,, page. (ii) The nymphal and adult stages- 151 A. Method I 151 B. Method II 151 C. Method III 152 D. Method IV 152 E. Method V 153 2, Analysis and comparison of different methods of sampling 154 (i)The egg stage 134 (ii)The nymphal and adult stages 157 3. Analysis of population data obtained from different methods 162 (i)Distribution of adult S. minutus in the field 162 (ii)Survivorship and mortality of the adults from peak numbers 165 (iii)Sex ratio 165 (iv)Fecundity 166 (v)Estimation of recruitment and mortality in different stages in the life cycle of S. minutus 168 (v1) Fluctuations in the numbers of macropterous forms 177 4. Causes of mortality 177 (i) Mortality in the egg stage 177 A. Sterility of eggs 180 B. Egg parasites 180 C. Egg predators 181 D. Stem rot 183 E. Other factors 184 7 • page () Mortality in the nymphal stages 185 A. Failure to hatch 186 B. Weather factors 186 C. Predation 186 D. Parasitism 187 E. Numbers taken for dissection in the laboratory 187 (iii) Mortality in the adult stages A. Parasitism 188 B. Predation 189 C. Emigration 189 D. "Winter disappearance" 189 E. Numbers taken for dissection 190 SUMMARY AND DISCUSSION 192 GRAPHS AND FIGURES 212 ACKNOWTEDGEMENTS 265 REFERENCES 26/ APPENDIX I 299 APPENDIX II 300 INTRODUCTION This is primarily a study of the seasonal and annual fluctuations in the population of the delphacid,- Stenocranus minutus (Fabricus) on cocksfoot, Dactylis glomerata L. An attempt has been made to estimate the changes in the population, to assess and. interpret natality and mortality quantitatively and, finally, to construct a population. budget. The work was done from Larch 1968 to September 1970. The population of Stenocranus was a small one, composed-of approximately eight to nine thousand adults when they were at their 2 • maximum, living in a more or less closed habitat of 260 m containing approximately 1171 grass tussocks. This delphacid is specifioto Dactvlis glamerata•although Whittaker (1969) found Stenocranus an Brachypodium pinnatum (L) Beauv. The estimates of the adult population, nymphal instars and eggs were obtained by several methods which acted as independent and mutual checks to one another. Causes of mortality were studied quantitatively whenever possible. Some observations were made. on the biology of the mymarid paraSite, Anagrus sp. which was responsible for a proportion of the mortality of eggs and also to the pipunoulid parasite, Cenhalons curtifrons Coe which was responsible for part, of the adult mortality. Only the male individual of Cephalops curtifrons has been described by Coe (1966), and that was based on a single male specimen. The female pipunculid has not been described. description of the female is included in this thesis. Dulng the course of, these investigations, it became evident that certain other aspects of the biology of Stenocranus needed detailed study. Therefore, a fair portion of time was assigned to various, aspects of the biology of the delphacid. These include adult emergence, nymphal emergence, emigration out of the breeding site reproduction and. egg distribution. 9. Although much work has been done on the systematics of Stenocranus, there are only a few published papers on the biology of Stenocranus minutus. The first paper by Hassan (1939) deals with the general bionomics of some British Delphacidae, including Stenocranus minutus and their parasites with special reference to the Strepsiptera.