Biology and Population Ecology of the Mustard Beetle
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BIOLOGY AND POPULATION ECOLOGY OF THE MUSTARD BEETLE Phaedon cochleariae FABRICIUS by Maria Rosa S. de Paiva, Licenciada in Biology (Portugal) A Thesis submitted in part fulfilment of the requirements for the Degree of Doctor of Philosophy in the University of London. Imperial College Field Station Silwood Park, Ascot, July, 1977 Berkshire. 2. ABSTRACT The biology and population ecology of the mustard beetle Phaedon cochleariae Pabricius were studied under laboratory and field conditions. In the laboratory, the relationships between temperature and fecundity, longevity, weight cycle and food consumption of adults were investigated. The food preferences of the adults were tested and related to the nitrogen content of four species of cruciferous plants. The relationship between temperature and development was studied for all stages. The number of larval instars was inversely correlated with temperature. Development thresholds were found to be higher for the eggs and larvae than for the pupae. Measurements and diagrams of internal reproductive organs at different stages of maturity were made and could be used in assessment of ages of field populations. As an aid to the interpretation of mortality in a field population, a laboratory population was set up and its fate was followed in the absence of natural enemies. The highest mortality in the laboratory occurred in the eggs and last larval instar. Pupal mortality was very low. The field population originated from reared adults, released onto a crop of turnips in Spring 1974. This population was studied for the following three seasons. Adults, eggs and larvae were sampled at regular intervals, while the rate of pupation was estimated independently. Mortality occurred in complete absence of parasitism, but predation by carabid beetles and spiders was detected. Density independent factors 3. were the main causes of mortality. Thus, in 1974 and 1976, the normal sequence of life cycles was upset by extreme weather conditions. Popula- tion budgets are given for the three years of this study. The behaviour of the adults was investigated both in an insectary and in the field. The ability of the beetles to fly, and some of the factors that influence dispersal were studied. Experiments with marked adults showed that movements were over a short distance and that dispersal by walking took place only under conditions of acute food shortage. 4. CONTENTS ABSTRACT CONTENTS 4 INTRODUCTION 11 Geographical distribution of Phaedon cochleariae. 13 History of pest attacks in Britain 14 SECTION I - Biology of P. cochleariae 17 1. Life history 17 2. Separation of sexes based on weight and size 20 3. Methods of rearing 22 4. Effect of temperature on fecundity, longevity and weight cycle of adults 22 4.1 Introduction 22 4.2 Materials and methods 24 4.3 Results 25 a) Effects of two temperatures on a single variable 25 i) Fecundity 25 ii) Pre-oviposition, oviposition and post-oviposition periods 25 iii) Longevity 27 iv) Mature weight 27 v) Rate of oviposition 28 b) Relations between pairs of variables at a constant temperature 32 i) Longevity and fecundity 32 . 5. Page ii) Longevity and mature weight 32 iii) Fecundity and mature weight 32 c) Relation between weight rise from emergence to maturity and 33 i) Fecundity 33 ii) Longevity 33 d) Effect of weight at emergence on its increase to maturation. 33 4.4 Summary and conclusions 35 5. Effect of plant species on fecundity, longevity and weight cycle 37 5.1 Introduction to 5. and 6.1 37 5.2 Methods and host plants 38 5.3 Results 39 a) On mustard 39 b) On chinese cabbage 39 c) On Brussels sprouts 39 i) Old plants 39 ii) Young plants 39 5.4 Summary and conclusions 42 6. Experiments on food consumption by adults 43 6.1 Introduction (See 5.1) 43 6.2 Effect of plant species on amount of food consumed 43 a) Methods and host plants 43 b) Results 43 c) Total nitrogen content of host plants 43 6. Page e) Summary and conclusions 45 6.2 Effect of temperature on amount of food consumed. 47 a) Methods 47 b) Results 47 c) Summary and conclusions 50 7 Effect of temperature on developmental rates of immature stages and mortality 50 7.1 Aim of experiments 50 7.2 Experimental methods 51 a) Eggs 51 b) Larvae 51 c) Pupae 51 7.3 Analysis of results 52 a) Eggs 52 b) Larvae 58 c) Pupae 60 7.4 Summary and conclusions 60 8 Lower lethal limits of temperature 61 8.1 Developmental thresholds of immature stages 61 8.2 Survival of adults at low temperatures 62 a) Methods 62 b) Results 63 c) Discussion 63 8.3 Summary and conclusions 65 7. Page 9. Reproductive organs and sexual maturation 65 9.1 Aim of observations and experiments 65 9.2 Methods 66 9.3 Results and description of the reproductive organs 67 a) Males 67 b) Females 68 9.4 Summary and conclusions 73 SECTION II - Laboratory studies on adult behaviour 79 1.1 Introduction to SectiOn II and Section III - 11. 79 1.2 Ability to fly 80 1.3 Effect of starvation on locomotory activity and flight 80 1.4 Effect of density on dispersal 82 1.5 Effect of the quality of food on dispersal 8.3 1.6 Effect of the water content of soil on locomotory activity 83 1.7 Summary and conclusions 86 SECTION III - Population studies 88 A - Laboratory experiments 88 1. Materials and methods 88 2. Population budgets 89 2.1 Introduction 89 2.2 Estimation of population parameters 93 2.3 Budget of a population in controlled conditions 97 2.4 Analysis of the budget data 97 8. Page Field experiments 101. 1. Description of the study area 101 1.1 Plot A 10.1 1.2 Plots B and C 103 2. Sequence of field experiments 104 2.1 1974 104 2.2 1975 104 2.3 1976 105 3. Sampling methods _106 3.1 Eggs, larvae and adults 106 3.2 Pupae 107 i) Estimation of the rate of pupation 107 3.3 Summary and conclusions 109 4. Methods of ageing the population 111 4.1 Examination of reproductive organs 111 4.2 Cuticular rings 111 4.3 Summary 111 5. Examination for parasitism 112 6. Predation 112 6.2 Arthropod predators 112 a) Field observations 112 b) Pitfall traps and sticky traps 113 c) Feeding trails in the laboratory 116 d) Field collections 116 e) Summary and conclusions 117 9. Page 7. Estimates of field populations 121 7.1 Estimation of population parameters 121 a) Natality 121 b) Durations of immature stages 121 c) Survival rates and recruits to stages 122 7.2 Population budgets 133 a) Description 133 b) Analysis of the budget data 133 7.3 Summary and conclusions 135 8. Population patterns of distribution 138 8.1 Introduction 138 8.2 Type of distribution 139 a) Eggs 139 b) Young larvae 143 c) Old larvae 143 d) Adults 144 8.3 Summary and conclusions 144 9. Studies on dispersal 145 9.1 Use of marked adults 145 a) Home range 148 b) Rate of dispersal 148 10. Page 9.2 Survey of adjacent areas 148 9.3 Invasion of a crop 149 9.4 Experiment on mixed cropping 149 9.5 Summary and conclusions 154 General Discussion and Summary 156 Acknowledgements 164 References 166 Appendices 176 INTRODUCTION The mustard beetle, Phaedon cochleariae Fabricius (Coleoptera, Chrysomelidae) is a pest of the family Cruciferae. In the United Kingdom, during the first half of this century, it has often been reported to be of economic importance, particularly to water- cress growers. However, since 1960 it has been observed less frequently and at levels below the threshold of economic injury. Over the last decade, this decline has been accentuated to such an extent that nowadays it is difficult to find the species out of doors. No references to attacks in other countries, in recent years, could be traced. Apparently, the decline of this species is not restricted to Britain. One of the latest references where P. cochleariae is still con- sidered as causing losses of economic importance, dates back to 1962 (publ. 1965). In this paper, Stepanova states that the mustard beetle, together with nine other insect species "....are the principal pests of cruciferous crops in the North-West of the Soviet Union.". The aim of the present work was to understand the population ecology of this beetle. It was expected that this investigation would reveal some of the reasons for the reduction of its numbers. The paucity of existing data on the biology of this species, determined that a major section of this project was devoted to the study of parameters such as fecundity and rates of development, under laboratory conditions. A culture of P. cochleariae was kindly supplied by the Insecticide Department of Rothamsted Experimental Station. The plants used in all laboratory experiments were grown in the 12. greenhouses of Silwood Park. Under these circumstances, outbreaks of aphids, especially Myzus persicae and Brevicoryne brassicae, were frequent and difficult to control. Several measures of control were tried such as spraying the plant with a 10% soap solution, but without success. During October 1974, spraying the turnip plants with a nicotine solution, resulted in over 70% mortality of the adult beetles. At this stage, help was sought from the Horticultural Department of Reading University, who kindly offered a new batch of insects. The field work extended for three seasons. Over this period, extreme weather conditions recorded during the Summers of 1974 and 1976, influenced the outcome of the experiments. The first season was cold and wet, in contrast to the last one, which was excessively hot and dry.