THE BIOLOGY of the LEEK MOTH, ACROLEPIA ASSECTELLA (ZELLER) by John Stuart Noyes B.Sc., A.R.C.S. a Thesis Submitted for the Degr
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THE BIOLOGY OF THE LEEK MOTH, ACROLEPIA ASSECTELLA (ZELLER) by John Stuart Noyes B.Sc., A.R.C.S. A thesis submitted for the degree of Doctor of Philosophy in the University of London th May 10 1974 From the Department of Zoology, Imperial College of Science and Technology Field Station, Silwood Park, Sunninghill, Berkshire. 2. TABLE OF CONTENTS Title Page 1 Table of contents 2 Abstract 6 Chapter 1: Introduction 8 1.1) Distribution of Acrolepia 8 1,2) Economic damage 8 1.3) Life history of Acrolepia 11 1.4) Parasitism 13 1.5) The study area 14 1.6) Aims of the study 14 Chapter 2: Laboratory experiments with Acrolepia 16 2.1) Review of the literature on Acrolepia 16 2.2) Olfactometer experiments 21 0 Location of females by males 21 ii) Host plant location 25 2.3) Host plant preference for oviposition 26 2.4) Periodicity of oviposition 29 2.5) Effect of temperature on fecundity and oviposition rate 29 2.6) Effect of temperature and age on fertility 33 2.7) Distribution of eggs on the host plant 36 2.8) The effect of temperature on the longevity of femalei3 37 2.9) Sex ratio and the influence of temperature on the rate of development 39 3. 2.10) Discussion 42 i)Location of females by males 42 ii)Location of host plant and oviposition 45 iii)Effect of temperature 47 Chapter 3: Field experiments with Acrolepia 53 3.1) The estimation of mortality 53 3.2) Investigation of possible predators 63 3.3) Effect of density on mortality 64 3.4) Winter survival and subsequent oviposition 65 3.5) Discussion 72 i)Mortality 72 ii)Oviposition after overwintering 74 Chapter 4: A simulation model of an Acrolepia population 76 4.1) Introduction 76 4.2) Construction of the model 77 i)Selection of parameters 77 ii)The model 80 4.3) Analysis of output 90 i)Stability of the model 91 ii)The effect of temperature on the population size of Acrolepia 93 4.4) Discussion 99 i)Limitations of the model 99 ii)Uses of the model 101 4. Chapter 5: The interaction between Acrolepia and its pupal parasite, Diadromus pulchellus Wesm. 104 5.1) Review of the literature on the parasites of Acrole 104 5.2) Location of a mate. by Diadromus males 107 i)Olfactometer experiments 108 ii)Arena experiments 110 iii)Observations 113 5.3) Olfactometer experiments on host location 114 5.4) Behavioural responses to host pupae 117 i)Oviposition behaviour 117 ii)Searching behaviour after host contact 118 5.5) Recognition of trail odours by Diadromus 120 5.6) The response of Diadromus to host or parasite density 124 i)Closed arena experiments 127 ii)Open arena experiments 134 iii)The response of Diadromus to contact with another individual 140 5.7) Discussion 140 Chapter 6: General discussion 145 Summary 152 Acknowledgements, 156 References 157 5. Appendix 1: The annual life cycle of Acrolepia in different European countries 175 Appendix 2: Systematic position and taxonomic notes on Acrolepia assectella (Zeller) 181 Appendix 3: The raw data obtained from experiments on Acrolepia 188 Appendix 4: Calculations for the population model of Acrolepia and listing of the programme 209 4.1) Calculation of the daydegree thresholds of the different stages 209 4.2) Construction of the temperature-related fecundity curves 215 4.3) Calculation of the theoretical spring fecundity of Acrolepia 219 The flow diagram of the simulation model of an Acrolepia population 222 Programme listing of the model 232 Appendix 5: Results of experiments investigating the interaction between Acrolepia and Diadromus 237 6. ABSTRACT Several aspects of the biology of Acrolepia were studied, in particular its interaction with the pupal parasite, Diadromus pulchellus Wesm. An olfactometer was used to investigate the location of female Acrolepia by males and the location of the food plant by females. Oviposition behaviour was studied in the laboratory. This included experiments to determine whether leek or onion is the preferred oviposition site and if this could be altered by breeding the moth on the less favoured plant for several generations. Further experiments included the effect of temperature on the fecundity, fertility and longevity of female Acrolepia and also its effect on the rate of development of the moth. Field experiments were conducted to assess the survival of the immature stages of the moth under natural conditions and also to determine whether there was any density dependent mortality. This involved the planting of known numbers of eggs on leeks and sampling at intervals. ,Winter survival of the moth and subsequent oviposition were also studied, The interaction between Acrolepia and its pupal parasite was studied under laboratory conditions. An olfactometer and an arena were used to investigate the location of Diadromus females by males and the response of females to the host and to areas which have teen previously searched by themselves or other females. The effect of different host and parasite densities on the behaviour of Diadromus females was studied in open and enclosed areas. A simple model was developed to simulate the effect of temperature on the rate of increase of an Acrolepia population. The results were used to determine the oeason of the year in which temperature fl'ictuations may have 7. the greatest affect on the population. A possible means of using this output as a basis for the control of Acrolepia is discussed. 8. CHAPTER 1 INTRODUCTION 1.1) Distribution of Acrolepia The leek moth, Acrolepia assectella (Zeller), is a widespread member of the Yponomeutidae, whose larvae mine in several species of Allium. The first published observation of Acrolepia was by Zeller in his taxonomic description of the species in 1839, when larvae were found boring in the stems of onion in Berlin and Frankfurt (see appendix 2). Since then the moth has been found in all European countries extending from Italy (Frediani 1954) to Sweden (Tullgren 1918) and from Spain (Gaedike 1970) to European Russia (Velitchkevitch 1922) and Norway (Fjelddalen et al. 1960). The latter noted that it had been found as far north as latitude 70°N in 1957 (fig. 1). It has also been found in Hawaii (Notes & Exhibitions 1944, 1945). In Britain, the moth was first noted in Corfe (Dorset) in 1901 when it was found in large numbers in a plot of leeks. Extensive damage was later reported from Sussex in the late summer of 1943 (Jary & Rolfe 1945). and more widely in Kent in 1944. Since then it has been reported from most costal areas from the Isle of Wight to Norfolk and the Thames Valley. Recently (1969) it has been reported from Devon (fig. 2). 1.2) Economic damage Acrolepia is of particular importance as a pest of leek, onion, garlic and shallots in countries such as Italy, Spain, France and Holland and to a lesser extent in Norway, Great Britain and the remaining European countries. In Britain, the most serious damage is done to leeks. The larval mines split open when the leaves grow forming elongated holes which give the plant a very .ragged appearance. After a very heavy attack there may 9. The distribution of Acrolepia in Europe. 10. Fig. 2 The knovffi distribution of Acrolepia in Great Britain. 11. be extensive rotting as occurred at the Royal Horticultural Society trial ground at Wisley in 1959 when 80% of the leeks were attacked (Baker 1961). The plant may die away completely due to secondary attacks by other insects such as Drosophila phalerata Meig. (Tullgren 1918), Hylemya antioua (Meig.) (Rahn 1966), Fannia sp. (Diptera: Nuscidae) and Megaselia sp. (Diptera: Phoridae). The damage to onions is usually less severe. Occasionally the larvae burrow into the bulb near the leaf bases and allow entry of organisms which cause it to rot. Further injury is sustained by the plant when the seed head is formed, as caterpillars bore through the flower shoot and work upwards into the top where the flower stalks arise. Feeding in this area may cause the flowering head to become loose or even fall away, especially in windy conditions. - Consequently, the formation of seeds is greatly impaired or even prevented. 1.3) Life history of Acrolepia The life history of the leek moth in Great Britain has been described briefly by Jary, Rolfe & Carpenter (1948). More detailed descriptions have been published by Bovien (1932) in Denmark, Del Guercio (1897) in Italy, Siegrist (1945) in Switzerland, Tullgren (1918) in Sweden, Velitchkevitch (1922) in Russia, Fjelddalen et al.(1960) in Norway, Frediani (1954) in Italy and Labeyrie (1956) in France. The female lays approximately 100-200 eggs (chapter 2). Each egg is about 0.57 mm long and 0.33 mm wide (iaan 1943); it is broadly oval and slightly irridescent with low ridges on its upper surface. After a period of 5-14 days the greyish first instar larvae hatch and may burrow directly into the tissue of the plant or wander for a short distance before mining into the leaves. As the larvae grow they gradually become yellowish-green, eventually attaining a length of about 1.2 cm. In leeks, the fourth 12. and fifth instar larvae leave the mines working their way to the centre of the plant where they feed between the bases of the leaves. They give the leek a shot-hole appearance when they emerge. If the larvae are feeding on onions they remain almost entirely within the hollow leaves. There is, therefore, little evidence of mines, but during a heavy attack the larvae will eventually work their way into the bulb. When the fifth instar larvae emerge from the plant they wander around before finally pupating inside a delicate, lattice-work cocoon spun on the plant itself, or in debris on the strface of the soil.