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Aspects of the Ecology of the Common Banded Mosquito, Culex Annulirostris, a Major Vector of Murray Valley Encephalitis Virus

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Aspects of the Ecology of the Common Banded Mosquito, Culex Annulirostris, a Major Vector of Murray Valley Encephalitis Virus ASPECTS OF lHE ECOLOGY OF lHE CCJrvM>N BANDED r-t:>SQUITOJ CULEX ANNULIROSTRIS~ A MAJOR VECTOR OF MURRAY VALLEY ENCEPHALITIS VIRUS by GARRICK McDONALD Dip.Ag.Sci. (Dookie), B.Sc. (La Trobe} A thesis presented in fulfilment of the requirements of the degree of Master of Agricultural Science, in the Faculty of Agriculture and Forestry, University of Melbourne. December, 1981. TABLE OF CONTENTS TABLE OF CONTENTS i ABSTRACT iii ACKNOWLEDGEMENTS AND STATEMENT OF SOURCES vi LIST OF FIGURES viii LIST OF TABLES xi CHAPTER 1: INTRODUCTION AND LITERATURE REVIEW 1 1.1 CuZex annuZirostris Skuse 1 1.1.1 Importance 1 1.1.2 Distribution 3 1.1.3 Description 4 1.1.4 Life cycle and biological studies 8 1.1.5 Habitat preferences 10 1.1.6 Relative abundance 11 1.1.7 Host preferences 12 1.1.8 Vector status and disease epidemiology 12 1.2 Age specific life and fecundity tables and PoPUlation growth potential 15 1.2.1 The theory 15 1.2.2 Use in mosquito studies 18 CHAPTER 2: EXPERIMENTAL 20 General Introduction 20 2.1 The influence of temperature on development, survival and fecundity of CuZex annuZirostris in the laboratory 21 (a) Juvenile development and survival 22 (b) Adult survival and fecundity 23 (c) ·Age specific life and fecundity tables, and population" statistics 27 2.2 The behaviour of Culex annuZirostris larvae in the laboratory 42 2.3 Field studies of juvenile CuZex annuZirostris 47 2.3.1 .Ecology of Culex annulirostris within a fresh water pond - Mildura 48 (a) Colonization, larval survival and development 50 (b) Time-specific life tables 50 (c) Larval distribution 51 2.3.2 Ecology of Culex annulirostris within a fresh water swamp - Shepparton 63 (a) Development and survival 63 (b) Larval distribution 64 2.4 Field studies of adult Culex annulirostris 68 2.4.1 Seasonal abundance of mosquito populations 69 (a) Culex annulirostris 71 (b) Other species 75 2.4.2 Nightly activity and abundance of Culex annulirostris 78 CHAPTER 3 : GENERAL DISCUSS ION 81 3.1 The bionomics of juvenile Cx annulirostris 81 3.1.1 Larval development 81 3.1.2 Larval survival and predation 82 3.1.3 Colonization 84 3.1.4 · Mosquito control 85 3.2 Population ecology of Cx annulirostPis 88 3.2.1 The effect of temperature on population growth 89 3.2.2 Population development of Cx annulirostris in relation to the transmission of M.V.E. virus 91 3.3 Alternative vectors of M.V.E. virus in the Murray Valley 102 3.4 Conclusions 103 REFERENCES 106 APPENDICES 114 ABSTRACT In the Mildura area of the Murray Valley, the mosquito Culex annuZiroatris is the dominant summer species. It becomes active in mid-spring and reaches a peak during late February or early March, declining rapidly during April. To examine the population dynamics of Cx annuZirostris in detail, a laboratory colony was established to measure population growth potential at various constant temperatures, and the results were related to adult and juvenile populations in the field, monitored during 1975-78. In the laboratory, population growth potential was determined by compiling age specific life and fecundity tables at 15°, 20°, 25° and 30°C. Population growth was positive at 20°, 25°, and 30°C, greatest at 25°C, and negative at 15°C. All larvae died at 10° and 40°C, and survival was greatest at 25°C. Graphic interpolation of the population growth statistic r (the intrinsic rate of natural increase), at r = O, m m provided a means of estimating the temperature threshold of population growth. This temperature was 17.50 c. Field observations confirmed this result as the mean daily temperature at which spring populations commenced growth, and autumn populations ceased activity, was approximately 17.50 c. Field.studies 'of colonization and juvenile development and survival within an experimental pond and· an established swamp wer.e conducted over two years. Within the pond, colonization by Cx annulirostris occurred within 24 hours of it being filled with water, and larval densities were greatest six or seven days after filling. A gradual decline over the remaining six to eight weeks of the study was attributed to increasing predation and, during the second year, also to declining temperatures. Larval mortality between egg and adult eclosion was 89% (average) , daily rates of mortality being reasonably consistent throughout development. Similar studies in the swamp demonstrated that 75% of larvae survived to eclosion in the absence of predators, and less than 1% survived in the presence of predators. Developmental rates of larvae in the pond and swamp varied considerably between about eight and eleven days. These results did not appear to comply with laboratory observations, possibly due to inaccuracies in field temperature records. In the laboratory, studies of larval behaviour indicated that larvae spent at least 87% of their time suspended from the water surface. It was suggested, in the absence of detailed field studies, that developmental rates of larvae may be influenced strongly by the surface temperature of the water, particularly if the larvae changed this behaviour in the event of surface temperature extremes. Cx annu~irostris was consistently found breeding in shallow and fresh bodies of water containing dense vegetation. In fresh water, the occurrence of vegetation was the major influence on larval distribution. The differences in population growth of Cx annulirostria observed over the study seasons, and also reported from years of M.V.E. epidemics, were explained using models compiled from laboratory and field data. The first model was based on the finite rate of natural increase (antilog e r ,. or A.) and provided a prediction of population growth for the three m seasons over which monitoring took place. The actual data used included the size of the initial spring catches and the mean weekly temperatures recorded for each season. In each case, the predicted population increased at a similar rate to that of the observed populations during spring and early summer of each year. However, beyond December, the predicted rate of growth was substantially greater than that of the observed populations. This model indicated that (i) the size o.f overwintering populations, (ii) the date of onset of activity in spring and (iii) the availability of breeding grounds during summer may be significant influences on the growth rate and size of summer populations. A second model indicated how various combinations of temperature and water availability may give rise to abnormally large populations of Cx annulirostris, and hence assist in triggering epidemics of M.V.E. Although conjectural, these models may provide a starting point for the design of a programme to predict M.V.E. epidemics in the Murray Valley. A total of 13 species were identified from adult trapping studies, with Culex pipiens australious being the second most common species. Circumstantial evidence indicates that this species may be an important amplification vector of M.V.E. virus prior to epidemics. ACKNOWLEDGEMENTS AND STATEMENT OF SOURCES The results presented in this thesis are from original research conducted at the Plant Research Institute, Burnley, and at the Horticultural Research Station, Mildura. I am indebted to my supervisors, Dr. I.W. McLaren, Mr. G.A. Buchanan and Mr. G.P. Shelden for their helpful advice and guidance throughout this research project, and for their criticisms of the manuscript. I am also grateful to Dr. B.H. Kay, Entomologist, Queensland Institute of Medical Research, and Mr. P.M. Ridland and Mr. A.M. Smith, Entomologists, Plant Research Institute, Burnley for their constructive comments on the manuscript; and Dr. E.N. Marks, Entomologist, Q.I.M.R., and Mr. J. Blyth, National Museum, Victoria for their identification of mosquitoes and predatory insects. I am grateful for the valuable technical assistance and co-operation received from Ms. c. Ricke and Mr. I.R. Smith in performing the laboratory studies. I would also like to thank Mr. R. Jardine, Biometrician, for assistance with the statistical content of the thesis, Mr. P. Hardy and Mr. F. Zudich for assistance in the Shepparton swamp studies; Ms .. D. Dittmer and Ms. J .. Kelly for their help in preparing the graphs and Ms. s. Broughton and Mrs. C. Ridland for typing the manuscript. Finally, I wish to thank Jenny McDonald for her assistance, patience and support during all operations of this res.earch project. The light t~aps were placed on the properties of the Victorian Forest Commission, Mildura, and Mr. J. Whiting of Gol Gol (N.S.W.). All of the work described was my own except for the section of the thesis dealing with juvenile development and survival in the laboratory, which was done in co-operation with Mr. I. Smith. Aspects of this project have been submitted and accepted for publication, and these are cited in Chapter 2 and Appendices I and II. Garrick McDonald LIST OF FIGURES No. Page 1 Cx annuZirostris egg raft containing approximately 320 eggs. 5 2 Cx annuZirostris larvae. Fourth and third instar. 6 3 Cx annuZirostris adult. 7 4 Water bath designed to maintain a constant temperature for the rearing of mosquito larvae in glass beakers. 24 5 A guinea pig held in a retaining stock to allow uninterrupted feeding of Cx annutirostris females. 26 6 Cages used to study the longevity and fecundity of Cx annuZirostris. 26 7 The relationship between water temperature and rate of juvenile development of Cx annuZirostris. 29 8 The effect of seven constant water temperatures on the survival of the juvenile stages of Cx annuZirostris. 32 9 Age-specific longevity (1 b and fecundity (m ) of Cx annuZirostris at (a) 2~ , {b) 25° and (c)x30°C.
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