THE ECOLOGY OF INSECT PEST POPULATIONS IN MAIZE STORAGE CRIBS IN NIGERIA by Richard Hugh Markham, B.A. (Nat. Sci.) A thesis submitted for the degree of Doctor of Philosophy of the University of London and the Diploma of Imperial College. Tropical Stored Products Centre, (Overseas Development Administration), London Road, Slough, Berkshire. March 1981 THE ECOLOGY OF INSECT PEST POPULATIONS IN MAIZE STORAGE CRIBS IN NIGERIA Richard Hugh Markham Abstract This study considered the insect populations infesting white dent maize stored in well-ventilated cribs at two localities in South West Nigeria. The pest complex was dominated by Sitophilus zeamais (Col.: Curculionidae) but included a great diversity of other pest species and natural enemies. The incidence of individual species was studied from pre-harvest infestation through six to ten months of storage and was shown to follow a consistent succession. The spatial distribution of insects within a crib was not uniform and individual species showed consistent patterns of distribution at a particular time. The seasonal incidence and distribution patterns of major species are discussed in terms of observed changes in grain moisture content, temperature and grain damage. The roles of intra- and interspecific relationships in limiting populations are considered. Sitophilus populations rapidly reach a 'plateau' and it is concluded that further significant increase is prevented by this insect's responses to its own high population density. The relationship between the field infestation and subsequent pest population increase in store is considered with particular reference to the effects of time of harvest, removal or retention of the husks and of damage* caused in the field by Lepidoptera larvae. Colonisation was found to be mainly by active migration of insects to the newly- loaded crib. Storage of maize in the husk provided no protection against insect damage although it did affect the distribution of insects between cobs. Techniques for sampling of insects from cribs are considered and the results of the study are discussed in terms of their implications for pest control strategies. iii. TABLE OF CONTENTS ABSTRACT ii TABLE OF CONTENTS iii LIST OF TABLES v LIST OF FIGURES viii CHAPTER 1 Introduction 1 CHAPTER 2 THE PHYSICAL ENVIRONMENT 2.1 The Crib as a Drying Structure 6 2.2 Macroclimate at Ibadan 11 2.3 Microclimate and Physical Conditions within the Crib 16 2.3.1 Grain Moisture Content 17 2.3.2 Temperatures within the grain bulk 30 2.4 Maize as a substrate for Insect Development 36 CHAPTER 3 SAMPLING TECHNIQUES 3.1 Introduction 39 3.2 Insect Sampling:Constraints and Considerations 41 3.2.1 General Objectives 41 3.2.2 Choice of Sampling Universe 42 3.2.3 Insect Mobility . 42 3.2.4 Sampling Units and Variability 44 3.2.5 Sample Size in Relation to the Size of Cribs 52 3.2.6 Sample Size and Handling Time 53 3.3 Assessment of Insect Sampling Techniques 55 3.3.1 Iowa Corn Probe 56 3.3.2 Destructive Sampling of Whole Cribs 62 3.3.3 Replacement Sampling 63 3.3.4 Sampling to Estimate Recruitment 65 3.4 Extraction of Insects from Grain Samples 66 3.5 Damage Assessment 70 CHAPTER 4 INSECT DISTRIBUTION WITHIN THE CRIB 4.1 Introduction 73 4.2 Sampling and Analysis for Insect Distribution 75 4.3 Preliminary Distribution Trial 82 4.4 Long-Term Changes in Insect Distribution 85 4.5 Short-Term Changes in Insect Distribution 96 4.6 Distribution of Losses within the Cribs 115 4.7 Species Interaction and Habitat Selection 121 iv. CHAPTER 5 THE INITIATION OF INFESTATION 5.1 Introduction 131 5.2 Pre-Harvest Infestation 135 5.3 The Effects of Harvesting Practice on Infestation 145 5.4 Persistent Effects of Pre-Harvest Damage 162 5.5 Sources of Storage Infestation 171 CHAPTER 6 THE INSECT COMMUNITY:COMPOSITION AND SUCCESSIONAL CHANGES 6.1 Introduction 175 6.2 Treatments and Sampling Techniques 177 6.3 The Abundance of Major Insect Groups and Changes in the Physical Environment 182 6.4 Incidence and Role of Individual Insect Species 189 6.4.1 Primary Pest Species 189 6.4.2 Secondary Pest Species - Coleoptera 196 6.4.3 Predatory Coleoptera 204 6.4.4 Heteroptera 204 6.4.5 Hymenoptera 207 6.4.6 Diptera 213 6.4.7 Psocoptera 213 6.4.8 Other Insect Groups 214 6.5 Other Arthropods 214 6.6 Vertebrates 216 6.7 Grain Weight Losses 217 6.8 Conclusions 220 CHAPTER 7 DISCUSSION: CHARACTERISTICS OF THE MAIZE CRIB SYSTEM AND • IMPLICATIONS FOR CONTROL STRATEGIES 223 APPENDIX .1 An annotated list of species of insects and 235 mites recorded from maize cribs at Ibadan and Ilora APPENDIX II Collated data: Distribution Studies 250 APPENDIX III Collated data: Succession Studies 259 APPENDIX IV Methods for estimation of moisture content of ' 288 grain and cores APPENDIX V Methods for analysis of grain weight loss 295 APPENDIX VI Collated analysis of variance tables 298 ACKNOWLEDGEMENTS 305 BIBLIOGRAPHY 306 LIST OF TABLES CHAPTER 2 2.1 Probability levels from 3-factor analysis of variance of moisture contents from different positions in a crib, at four different stages during the storage season 2.2 Summary results of analysis: effects of time of day and position in crib on grain moisture content (Short-Term Distribution Trial) (a) probability levels from analysis of variance (b) treatment means 2.3 Summary results of analysis: effects of position in crib on grain moisture content (Termination of Short-Term Distribution Trial) (a) probability levels from analysis of variance (b) treatment means 2.4 Summary results of analysis: effects of time of day and position in crib on grain temperature (a) outline design (b) probability levels from analysis of variance (c) treatment means CHAPTER 3 3.1 Difference in adult insect abundance in probe samples from different positions in 3 cribs (Carpophilus) of 4 cribs (Sitophilus) (a) probability levels from a single factor analysis of variance (b) treatment means '3.2 Insect abundance in probe samples from different parts of a single crib (a) probability levels from a 3-factor anovar (b) treatment means 3.3 Effect of time after collection on numbers of insects emerging from grain samples CHAPTER 4 4.1 Effect of sampling position within crib on insect numbers for (a) Sitophilus zeamais (b) Carpophilus dimidiatus 4.2 Results of Factorial Analysis of Variance for dispersion of Sitophilus zeamais in Long-Term Distribution Trial for: (a) adults (b) emergences 4.3 Results of Factorial Analysis of Variance for dispersion in Long-Term Distribution Trial of: (a) Carpophilus dimidiatus (b) Cathartus quadricollis vi. 4.4 Results of Factorial Analysis of Variance for dispersion of insects in Sample 4 (Long-Term Distribution Trial) 95 4.5 Analysis of crib totals from Short-Term Distribution Trial 100 4.6 Dispersion of adult insects within cribs: effect of time of day and sampling position on insect abundance (Short-Term Distribution Trial) 106 (a) probability levels from 3-factor analysis of variance (b) treatment means from factorial analysis of variance 4.7 Summary of adult insect distribution pattern (Short-Term Distribution Trial) 107 4.8 Distribution pattern of emergences (Short-Term Distribution Trial) based on 2-factor analysis of variance 112 (a) probability levels (b) treatment means 4.9 Summary results of analysis of variance: effect of position within crib on grain weight loss 119 (a) summary anovar table (b) treatment means 4.10 Correlation matrices showing associations between insect species and with two environmental parameters 124 CHAPTER 5 5.1 Summary results of field samples: infestation by major pest groups, grain damage and husk cover 138 5.2 Percentage of cobs infested by each species (or group) 139 5.3 Summary of sampling regime for Harvesting-Practice Trial 146 5.4 Selection of damaged and sound cobs for storage - Harvesting- Practice Trial 149 5.5 Effects of harvesting on adult insect populations comparison of adult insect numbers before harvest (field samples) and 24 hours after harvest (snapped and husked) ' 152 5.6 Effects of early harvesting on insect.populations 153 (a) .comparison of mean moisture contents (b) comparison of emergent insects (c) comparison of adult insects from early and late harvested maize 5.7 Effect of time of harvest and retention of husks on grain moisture content 155 5.8 Probability levels from analysis of variance: effects of time of harvest, removal of husks and position in crib on insect numbers 157 (a) adult insects - first storage sample (b) emergences - first storage sample (c) emergences - second storage sample 5.9 Effects of time of harvest and removal or retention of husks on adult insect infestation (first storage sample) 158 (a) Cathartus quadricollis (b) Oryzaephilus mercator (c) Gnatocerus maxillosus - (d) Palorus subdepressus 5.10 Effects ori emergences of time of harvest and removal/retention of husks at the time of: 159 (a) the first storage sample (b) the second storage sample 5.11 Effects of time of harvest on losses 160 5.12 Effects of field damage by Lepidoptera on initial infestation of maize by storage pests (Pre-Harvest Damage Trial) 167 (a) probability levels from 3-factor analysis of variance (b) mean number of insects emerging from samples 5.13 Effects of field damage on subsequent infestation in store 168 5.14 Effects of field damage on subsequent infestation in store 169 5.15 Progressive changes in mean moisture content and mean dry weight over storage period 170 5.16 Final weight loss (4 months in store) for maize damaged in the field by Lepidoptera (Pre-Harvest Damage Trial) 170 CHAPTER 6 6.1 Summary of crib 'treatments1 used for Succession Studies 181 6.2 Mean loss in dry weight at the end of the storage period 218 6.3 Weight loss in dry season cribs at termination: effect of position/sampling: 219 (a) mean .weight loss for different positions (b) probability levels from a 2-factor analysis of variance viii.