Insect Herbivore-Host Dynamics: Tree-Dwelling Aphids

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Overview

Literature currently available on the population dynamics of insect herbivores tends to favour a topdown regulation of abundance, owing much to the action of natural enemies. This unique volume challenges this paradigm and shows that tree-dwelling species of aphids, through competition for resources, regulate their own abundance.

The biology of tree-dwelling aphids - particularly their adaptation to the seasonal development of their host plants - is examined. When host plant quality is favourable, aphids, by telescoping generations, can achieve prodigious rates of increase that their natural enemies are unable to match.

Using analyses from long-term population census and experiments, this book is designed to introduce students and research workers to insect herbivore-host dynamics using the interaction between aphids and trees as a model.

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Editorial Reviews

From the Publisher
Review of the hardback: '... short, uncluttered and digestible ...' TRENDS in Ecology & Evolution

Review of the hardback: 'I am always pleased and excited when a book by Tony Dixon arrives on my desk. This latest offering is no exception. It is well up to his usual standard and a worthy companion to his two recent books on related subjects ... fluid writing, lucid arguments and well chosen examples ... an excellent book that covers most of ecological theory based on examples not only just from tree aphids, but mainly from the interaction of one species of aphid, the sycamore aphid Drepanosiphum platanoidis and its natural enemies and host plant the sycamore Acer pseudoplatanus. This is a major tour de force ... a must-buy item for any ecologist ... destined to be a classic.' Journal of Insect Conservation

Review of the hardback: '... well written and clearly structured ... it will be of value to many aphid researchers and herbivore ecologists.' Entomologia Generalis

Review of the hardback: '... it gives the reader not only the insight to the partial problem of the selected aphid species and their hosts but it serves also as a model for studying other ecological tasks.' Thaiszia: Journal of Botany

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Product Details

  • ISBN-13: 9780521802321
  • Publisher: Cambridge University Press
  • Publication date: 3/21/2005
  • Pages: 208
  • Product dimensions: 5.98 (w) x 8.98 (h) x 0.79 (d)

Meet the Author

A. F. G.Dixon is an Emeritus Professor in the School of Biological Sciences at the University of East Anglia. He has written over 200 papers on aphids and their natural enemies in scientific journals, and has written or edited 10 books. In 1992, he was awarded the Gregor Mendel Gold Medal by the Czech Academy of Science, in 2000 a medal of honour by Akademia Podlaska, Poland, and in 2001 became Laureate of the University of South Bohemia, Czech Republic.

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Read an Excerpt


Cambridge University Press
0521802326 - Insect Herbivore-Host Dynamics - Tree-Dwelling Aphids - by A. F. G. Dixon
Excerpt



1
Introduction



The prodigious rate of increase of aphids has fascinated entomologists for centuries. Réaumur (1737), like Leeuwenhoek, thought aphids were hermaphrodite and calculated that one aphid may give rise to 5.9 billion over a period of six weeks. Bonnet (1745) was the first to appreciate that aphids were bisexual but could produce a succession of broods without males, a phenomenon that later became known as parthenogenesis (Owen, 1849). Huxley (1858) was also fascinated by parthenogenesis in aphids and calculated that after 10 generations, if they all survive, an aphid can give rise to a biomass equivalent in weight to 500 million stout men. Occasionally these extraordinary rates of increase are realized. White (1887), of Selborne, records that at about 3 p.m. on 1 August 1774, showers of hop aphids fell from the sky and covered people walking in the streets and blackened vegetation where they alighted in Selborne and adjoining towns. Similarly, enormous numbers of cereal aphids plagued people in England in 1790 (Curtis, 1845), and in September 1834 an immense cloud of the peach potato aphid swept across the river, covered the quays and streets of Gent, and darkened the sky in both Brugge and Antwerpen (Morren, 1836). Thus, aphids are potentially capable of becoming very abundant over a wide area. Fortunately such plagues are rare. The implication of this is that aphid abundance is normally regulated well below plague levels.

My interest in tree-dwelling aphids started from an observation when experimenting with ladybirds in the field in 1956. Contrary to my expectation the aphid population on sycamore in summer consisted almost entirely of adults. This provoked the question why, and resulted in my studying reproductive aestivation in the sycamore aphid Drepanosiphum platanoidis (Schrank), and ultimately the population dynamics of three species of tree-dwelling aphids. That is, tree-dwelling aphids were selected not because they are model organisms for studying population dynamics but because I found them interesting, and having studied under George Varley facilitated my acceptance of the then widely held view, aptly summarized in the statement: 'The world is green'. This implies that herbivores are seldom food limited, appear to be natural enemy limited and therefore not likely to compete for food (Hairston et al., 1960). In addition, because the ladybird Rodolia cardinalis had proved such an effective biological control agent, I expected my studies to reveal that ladybirds in particular would be important in the regulation of aphid abundance.

The first aphid I studied was the sycamore aphid. Several students helped me in this task: Robert Russel (1968), Paddy Hamilton (1969) and Martyn Collins (1981) with the natural enemies; Jack Jackson (1970) and David Mercer (1979) with migration; John Shearer (1976) with the other common aphid on sycamore Pemphigus testudinaceus (Fernie); Forbes McNaughton (1970) and Peter White (1970) with the effect of the aphid on the growth of sycamore; and Richard Chambers (1979) and Paul Wellings (1980) with quantifying the role of qualitative changes in the aphid in its population regulation. When it became obvious that natural enemies were not important in regulating sycamore aphid abundance I also started to study the lime aphid Eucallipterus tiliae (L.). This aphid was included in the study because I saw large numbers of ladybird larvae pupating on the trunks of lime trees in years when lime aphids were abundant. As with the sycamore aphid several students helped me: David Glen (1971) and Steve Wratten (1971) with the natural enemies; Margaret Brown (1975) and Neil Kidd (1975) with quantifying the role of qualititative changes in the aphid; the late Mike Llewellyn (1970) and Peter White (1970) with the effect of the aphid on the growth of lime; and the late Nigel Barlow (1977) with a simulation study of this aphid's population dynamics. Large numbers of ladybird pupae on the foliage of Turkey oak similarly drew my attention to the aphid Myzocallis boerneri Stroyan, living on this tree. It was also sampled, along with the continuing census of sycamore and lime aphids, for four years in Glasgow. On my moving to Norwich in 1974 the population census study of the lime and sycamore aphids was discontinued but that of the Turkey oak continued for another 19 years. Aulay McKenzie and Richard Sequeira assisted me with the study of this aphid.

The analysis of the sycamore aphid population census data owes much to Richard Chambers, who introduced me to the concept that the rates of development and growth could be differentially affected by temperature and food quality, and both Richard and Paul Wellings for demonstrating the effect of density-induced qualitative changes in the aphid. That of the lime aphid was very dependent on the simulation study of Nigel Barlow (Barlow & Dixon, 1980), and that of the Turkey oak aphid on the auto-regression analysis of the census data by Richard Sequeira (Sequeira & Dixon, 1997). In terms of the overall synthesis, however, it is a pleasure to acknowledge Pavel Kindlmann's enormous contribution to the understanding of aphid population dynamics and biology. He is a great listener and excellent at expressing ideas in mathematical terms. Seamus Ward, similarly, made an equally important contribution to the analysis and above all the understanding of tree aphid population biology, in particular risky dispersal (Chapter 8) and seasonal sex allocation (Chapter 9). Finally, Vojta Jarosik made a great and valued statistical contribution in exploring the role of density independent processes in the regulation of the Turkey oak aphid.

Starting with the population census of sycamore aphid and then adding the lime aphid, and finally the Turkey oak aphid in my quest for support for the view that natural enemies regulate the numbers of tree-dwelling aphids proved enlightening. The fact that in all three cases, natural enemies did not regulate aphid abundance but density dependent induced changes in the aphids apparently did, convinced me of the generality of the phenomenon. This was necessary because the climate of opinion favoured natural enemies, particularly parasitoids. Although the Natural Environment Research Council (NERC) supposedly favoured long-term population studies, it proved very difficult to obtain funding to do this work. On one occasion a NERC 'visiting group' questioned us about the progress of our research. Although it was not clear why we were so privileged we were left with the feeling that it might be because of the minor role we attributed to natural enemies. The visiting group recommended continued support for the long-term census work providing there was greater emphasis on the effect of aphids on tree growth. In retrospect this directive diverted attention from the main problem - what regulates the abundance of these aphids? Similarly, the time and effort my group subsequently spent studying cereal aphids was also largely wasted. In the early 1970s, Bill Murdoch suggested I apply my ecological understanding of aphids to a more practical problem. A series of outbreaks of cereal aphids resulted in pressure being placed on politicians by the farming community, which resulted in more of the government funding given for agriculture research being used for studies on cereal aphids (Dixon, 1987a). That is, the moral pressure and the availability of funding made it easy for me to work on cereal aphids. Unfortunately, the enormous research effort expended on these studies by several groups lacked a clearly defined applied objective and the availability of funds was determined by political rather than scientific criteria.

As stated by Andrewartha and Birch (1954) population ecology should be based on the study of living organisms in their natural environments. The theoretical framework of population ecology has been more intensively studied than most other aspects of ecology, but is still far from complete. Advances are dependent on insight and imagination, followed by a mathematical formulation of the process under consideration. The resultant mathematical models may reveal other patterns, which were not previously appreciated. It is important that these ideas should be tested experimentally as 'whether or no anything can be known, can be settled not by arguing but by trying' (Bacon, 1620). However, the tendency to present studies in the form of an hypothesis-testing exercise often lacks conviction and rigour. Is it not possible to arrive at an understanding without hypothesis testing - it is reputed that Newton did! It is possibly more important to ask an impertinent question as by so doing one is on the way to a pertinent answer (Bronowski, 1973).

The importance of using long-term population censuses to test ecological theory is widely acknowledged and frequently stressed. However, such studies are tedious to do, difficult to sustain for long periods and initially very unproductive. The methodology of such studies also rarely escapes destructive criticism from population theorists. Therefore, it is not surprising that there are very few long-term studies of insect populations. In addition, convincing others that the accepted dogma might be wrong is far more difficult than to conform. If one assumes all that is necessary is to present the evidence and leave truth to persuade, then one is being very naïve about people's motives. The establishment of the day, which in the case of insect population dynamics favours regulation by natural enemies, is a powerful force with considerable inertia. It is likely in this case that monstrous certainty has impeded rather than aided the search for truth.

Early in 1974 George Varley sent me a copy of his book: Insect Population Ecology and in the accompanying letter wrote: 'The aphid population problem, with overlapping generations, is of a different order of complexity from the simple things we now are beginning to understand'. However, if one accepts that the life cycle of an insect can be viewed as beginning and ending with an egg then the supposed complexity largely disappears. On hatching from an egg an aphid clone grows by parthenogenesis and at the end of the vegetative season switches to sexual reproduction and produces eggs. The fact that the 'body' of each aphid clone does not exist in space as a discrete unit but may be scattered widely in space and always consists of many units does not invalidate the life cycle concept. Parthenogenesis enabled aphids to achieve the prodigious rates of increase alluded to above, which have been a major factor in shaping their population dynamics. The objective of this book is to present the case for self-regulation rather than top-down regulation in this group of plant-sucking herbivores. That is, the aphid population problem can be greatly simplified by ignoring the complexity of aphid life cycles. This can be done by focusing on the relationships between the numbers at the beginning and the end of the different phases of increase and decline in a season, and between the end of one season and the beginning of the next. This simple approach was adopted because it resulted in a better system of prediction than more complicated approaches, and was arrived at only after achieving some understanding of the life cycles of aphids.


2
Tree-dwelling aphids



This book mainly deals with the population biology of deciduous tree-dwelling aphids, in particular, the sycamore aphid Drepanosiphum platanoidis. Information on the lime aphid Eucallipterus tiliae and Turkey oak aphid Myzocallis boerneri, which have not been studied in the same detail, is also presented. These aphids all belong to the subfamily Drepanosiphinae, fossils of which are first recorded in the late Cretaceous and make up a large proportion of the early Tertiary aphid fauna. Extant species of this subfamily are mainly host specific and most live on trees belonging to the Fagaceae, Ulmaceae, Aceraceae and Betulaceae. This subfamily of aphids is taxonomically well defined and not as polymorphic as the Aphidinae and Lachninae, which make up most of the present-day aphid fauna. From an ecological point of view this is important as it means they are easily identified, even in the field.

LIFE CYCLE

Most aphids live on only one species of host plant. The sycamore, lime and Turkey oak aphids belong to this group. They spend the winter months as eggs. In spring these hatch and give rise to nymphs that develop into the winged adults of the first generation, known as fundatrices. These adults are parthenogenetic and viviparous, and their offspring develop into other parthenogenetic viviparae. Several parthenogenetic generations occur in succession until the onset of autumn when the nymphs develop into unwinged egg-laying females and winged males. This is the sexual generation, which mate and produce the overwintering eggs (Figure 2.1). Bonnet (1745) was the first to show that aphids may propagate without fertilization for as many as 10 generations. Then, under certain conditions, winged or wingless males appear and copulate with wingless egg-laying females. This

Figure 2.1. Life cycle of the sycamore aphid (A, fundatrix; B, alate virginopara; C, male; D, ovipara; and E, egg).

cyclical parthenogenesis, in which periods of parthenogenetic reproduction alternate with sexual reproduction is thought to have evolved in a seasonal climate, possibly that associated with a glacial period in the Lower Permian. However, live-bearing or viviparity, another characteristic feature of aphids, must have evolved later as the ancestors of aphids, and the closely related adelgids and phylloxerids are egg-laying or oviparous (Heie, 1967).

The occurrence within a species of different forms or morphs: unwinged and winged parthenogenetic viviparae, males and egg-laying sexual females, is also characteristic of aphids. This polyphenism reaches its greatest development in host-alternating species of aphids, which may have as many as eight morphs that differ at least in their external morphology. Most deciduous tree-dwelling aphids have far fewer morphs, usually only winged parthenogenetic females, winged males and wingless egg-laying females, although the parthenogenetic females may show differences in external and internal morphology associated with generation-specific strategies (p. 28).

EVOLUTIONARY INDIVIDUAL

As the aphids that hatch from overwintering eggs are parthenogenetic, populations are made up of clones - the 'evolutionary individuals' in Janzen's (1977) terminology. How a clone allocates resources to particular functions is likely to determine its fitness (Dixon, 1977). Individuals in each clone are involved to a varying degree in defence, dispersal, reproduction and aestivation/hibernation (Figure 2.2). However, specialization in one or other of these functions imposes constraints in terms of resource allocation for carrying out other functions. At certain times particular functions are more important than others for the overall fitness of a clone, and this has resulted in a division of labour within a clone, which is reflected in its polyphenism. This implies that altruism is common in aphids. However, the general view is that only the soldier caste is altruistic. That most soldier aphids are sterile and are likely to die defending and safeguarding the survival of their clone mates supports the claim that they act altruistically. Winged aphids similarly have a greatly reduced fecundity and a very low probability of surviving to reproduce, but in dispersing they benefit the overall fitness of their clones. Thus it would appear that winged forms also act altruistically. If this is true, then contrary to Hamilton's (1987) claim altruism is common in aphids.

TELESCOPING OF GENERATIONS

As a parthenogenetic egg does not require fertilization, it can begin to develop as soon as it is ovulated. Under congenial conditions aphids take approximately one week to develop from birth to maturity, whereas other similar sized insects take approximately three weeks. However, if one takes into consideration that an aphid starts developing inside its grandmother (Figure 2.3), then the actual development time is 2.5 times longer than it takes an aphid to develop from birth to maturity, i.e. approximately three weeks. Comparison of the generation times of organisms of a wide range of sizes indicates that the larger and more complex have much longer generation times than smaller organisms (Bonner, 1988). This trend suggests that organisms the size of aphids

Figure 2.2. Functional aspects of polyphenism and the allocation of resources to dispersal (dotted area), reproduction (hatched area) and survival (filled area) by winged (alate), unwinged (aptera) and aestivating, hibernating and soldier morphs of aphids. (After Dixon, 1985.)

should have generation times in the order of one month and mites in the order of one week (Figure 2.4).

Therefore, there appears to be a minimum 'time' required for development, which is a function of the size/complexity of organisms. Parthenogenesis has enabled aphids to start developing at ovulation and more importantly inside immature or even embryonic mothers. Then, given that there is a constraint on the rate of development, there

Figure 2.3. Diagrammatic representation of the telescoping of generations; the aphid has developing within it its daughters, which have aphids developing within them - the granddaughters.

are great advantages in telescoping generations (Kindlmann & Dixon, 1989), which has given aphids approximately a threefold reproductive advantage (Dixon, 1990a). In addition, in being viviparous, aphids avoid the heavy mortality experienced in the egg stage in other insects. In this way aphids have been able to achieve rates of population increase normally associated with much smaller organisms such as mites.

Their short generation time also enables aphids to track very closely the seasonal changes in their resources. Individuals in each generation must be able to survive the worst conditions they are likely to

Figure 2.4. The relationship between generation time and size. (After Bonner, 1988.)

experience, which in long-lived individuals is likely to constrain their performance when conditions are favourable. Aphids generally are not so constrained (see p. 28). The seasonal sequence of short-lived generations have generation-specific strategies, by which they anticipate in terms of morphology and physiology the seasonal changes in conditions (p. 28). This close tracking in time, and matching morphology and physiology to seasonal changes in resources, is important in determining the great abundance of many species of aphids relative to their resources.



© Cambridge University Press
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Table of Contents

1 Introduction 1
2 Tree-dwelling aphids 6
3 Trees as a habitat : relations of aphids to trees 18
4 Trees as a habitat : relations of aphids to their natural enemies 43
5 Carrying capacity of trees 54
6 Aphid abundance 64
7 Population dynamics 93
8 Risky dispersal 122
9 Seasonal sex allocation 136
10 Aphids and tree fitness 147
11 Rarity, conservation and global warming 162
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