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Overview
The authors hope that this book will be a valuable resource, not only for students, from school to university and beyond, but also for anyone with an interest in natural history, whether professional or recreational.
Product Details
| ISBN-13: | 9781907807077 |
|---|---|
| Publisher: | Pelagic Publishing |
| Publication date: | 04/15/2013 |
| Series: | Naturalists' Handbooks , #10 |
| Edition description: | Second Edition |
| Pages: | 142 |
| Product dimensions: | 5.80(w) x 8.20(h) x 0.50(d) |
| Age Range: | 13 - 18 Years |
About the Author
Helen combined research with teaching for 10 years before taking up a position (research scientist) with the Biological Records Centre (Centre for Ecology & Hydrology) where she is responsible for zoological data and research and works extensively with national zoological schemes and societies. Her research focuses on the effects of environmental change on insect populations and communities. She is particularly interested in the dynamics of invasive species and their effects on native biodiversity.
She coordinates the UK Ladybird Survey and is using the large-scale and long-term coccinellidae datasets (distribution and abundance) to understand and predict the effects of the arrival of the non-native harlequin ladybird (Harmonia axyridis) on native species. This work was selected for the 2009 Royal Society Summer Science Exhibition and the Moscow Science Festival in 2010. Helen has been invited to exhibit her research on ladybirds at the BBC Gardener's World Exhibition in June 2011.
Helen is working on a project to produce a comprehensive information portal on non-native species in Great Britain. She also leads a European study group within the International Organisation for Biological Control (IOBC) on the Risks and benefits of Exotic Biological Control Agents which uses the harlequin ladybird as a model species.
Helen continues with her research interests first initiated in her PhD on the ecology and dynamics of insect host-parasite interactions and has recently led an editorial team to produce a journal special edition (Ecology of Fungal Entomopathgens – Springer) which reflects her work in this field. She is currently working on another journal special issue (Invasive alien arthropod predators and parasitoids: an ecological approach), an activity through the IOBC WG that she leads.
The ecology of ladybirds is a subject that appeals to the public and throughout her career Helen has taken every opportunity to communicate her research to a wider audience. This has included natural history talks, school visits, bioblitz, popular science articles, podcasts and a significant number of interviews with the media. The arrival of the non-native harlequin ladybird in 2004 captured the imagination of the media and there has been sustained media interest in research on this species over the last six years.
Read an Excerpt
CHAPTER 1
Introduction
Biological science must stand on its foundations in basic observations of organisms in the field: what they do, when they do it, why they do it, and how they have come to do it.
Majerus, 1994
1.1 Introduction
Ladybirds are among the most attractive and popular of British insects. Many species are common. They may be found in almost any habitat from sea coast to mountain top, and from city wastelands to windswept heathlands. Almost every garden will have at least one species and many will have five or more species.
There are a number of reasons for the popularity of ladybirds. Firstly, ladybirds are charismatic insects. Many ladybirds have bright contrasting colour patterns, although not all are red with black spots. Some are black with red spots, others are yellow and black, or brown with cream spots. Some have stripes instead of spots and some no spots at all. Secondly, most species of ladybird are carnivorous. Both adults and larvae feed on aphids or other pest insects, which suck sap and damage many crops and garden plants. So, ladybirds are important predators of these pests and are considered 'beneficial insects'. Finally, ladybirds are connected with good fortune in many myths and legends. The name 'ladybird' is itself derived from the commonest species in Britain, the 7-spot ladybird, Coccinella septempunctata. The lady in question is Our Lady, the Virgin Mary. The red colour is said to represent her cloak, which in early paintings and sculptures was usually depicted as being red, and the seven black spots represent the seven joys and seven sorrows of Mary.
Yet, despite their popularity and important function as predators of pest insects, much is still unknown about the behaviour and ecology of British ladybirds.
This book aims to outline what is known about the species found in Britain, and to highlight areas worthy of scientific exploration. We hope that the book will encourage you to discover more for yourselves, particularly through your own natural history studies and scientific research. Ladybirds offer great scope for original observations and experiments, and their potential as biological control agents of plant pests makes new contributions to our knowledge of ladybirds even more worthwhile. Additionally although a few species of ladybird are increasing in number, there are some historically widespread and common species that are currently declining dramatically (Roy and others, 2011; Roy and others, 2012) and so increased understanding of their ecology and response to our rapidly changing environment is critical.
1.2 What are ladybirds?
Ladybirds are beetles and so belong to the largest order of organisms, the Coleoptera. There are two important characteristics that, taken together, distinguish ladybirds and most beetles from insects of other orders.
(i) The forewings are modified to form hard or leathery elytra (wing cases) that meet in the centre line, covering the abdomen.
(ii) The mouthparts are adapted for biting rather than sucking. Beetles generally and ladybirds in particular are unlikely to be confused with any other order except the Hemiptera (true bugs). The characters that distinguish ladybirds from bugs are shown in fig. 1.
Ladybirds are one family of beetles called the Coccinellidae. Coccinellids are small or medium-sized beetles, 1–10mm long; they are usually round or oval. The most obvious features of the upperside of a resting ladybird are the elytra, which in many species are brightly coloured and usually patterned with spots, bands or stripes. The elytra cover and protect the membranous flight wings (fig. 2) which are usually folded under the elytra when the ladybird is not flying. Between the elytra and the head is the pronotum. This is a plate which covers the upper surface of the thorax. It is broader than it is long and it extends forwards at the margins (fig. 3). The pronotum is often patterned, though not as brightly as the elytra. The head is retractable under the pronotum and the antennae are short and slightly clubbed. The legs are short and retract into grooves under the body. The feet (tarsi) have four segments, but because the third segment is small and hidden inside the deeply lobed second segment, only three segments are readily visible (fig. 4).
More than 4,500 species of coccinellid have been described worldwide. Forty-seven species are resident (established and reproducing) in Britain, and 27 of these also reside in Ireland. Various other species have been recorded on a few occasions in Britain, but they are not generally considered resident here (see chapter 7). Some of the British coccinellids (sub-family Coccidulinae) are small and unspotted, and would not normally be recognised as ladybirds. Whilst all 47 coccinellid species are listed in Table 1 and included in the key, this book primarily covers the 26 species that have received most research attention and were designated as conspicuous species in the first edition of this book.
Scientific names: In the entomological literature the numbers in the scientific names may be given in full (for example Subcoccinella vigintiquattuorpunctata), or may be simplified (for example Subcoccinella 24-punctata). Throughout this book we will refer to the ladybirds using the English name (where appropriate) but table 1 can be used as a cross-reference to the Latin name.
CHAPTER 2
Life history
2.1 General life cycle
Like all beetles, ladybirds pass through three stages - egg, larva and pupa - before reaching the adult state. So, like butterflies and moths (Lepidoptera), bees, wasps and ants (Hymenoptera) and true flies (Diptera), they are said to be holometabolous insects (undergoing complete metamorphosis).
For many ladybird species in Britain the full life cycle takes a year. Eggs are laid in spring or early summer. The larvae feed up over the next month or so, and the new generation of adults emerge from the pupae in mid to late summer. These adults feed but do not usually breed until the following spring, and so most species have just one generation a year (fig. 5). However, there are exceptions to this pattern. The rate at which larvae develop is affected by both temperature and food availability, and the development of eggs and pupae is also affected by the temperature. In some years, a number of species such as the 2-spot and 14-spot ladybirds have a second generation. Harlequin ladybirds in Britain generally have two generations, and sometimes a partial third generation, each year. For species completing more than one generation a year, individuals from both the early and late generations overwinter together. There are records of the 2-spot, 14-spot, cream-spot and eyed ladybirds surviving through a second winter. However, more information is needed if we are to be sure how common these exceptions to the normal pattern are, particularly in response to climate change.
2.2 Eggs
The eggs of most ladybird species are elongate and oval, and vary from a light yellow to a deep orange colour. They are laid on the leaves, stems and sometimes the bark of plants, often in the vicinity of prey. Most species fix their eggs at one end so they are found in an upright position (fig. 6), though the eggs of the pine ladybird are frequently laid on their sides. There is considerable variation in the number of eggs laid at one time, though most species lay batches of eggs, which are tightly packed together forming a cluster on the substrate. Females of the 2-spot ladybird typically lay between 20 and 50 eggs at a time. There is also substantial variation within a species in the number of eggs laid per female (fecundity). One of the more important influences is the type of food eaten by the adults. Hariri (1966) found that the 2-spot ladybird laid a lifetime total of 738 eggs per female, averaging 9.3 per day when fed on the black bean aphid (Aphis fabae), but when the pea aphid (Acyrthosiphon pisum) was used, the total was 1535 eggs, laid at an average of 20.4 per day. Fecundity is also affected by the quantity of food eaten, so, for example, there is a positive correlation between food consumption and egg production in the 11-spot ladybird (Ibrahim, 1955a, b). Influences on larval development also affect subsequent female fecundity. Sundby (1966) fed 7-spot ladybird larvae one third the normal amount of food and found the emerging adults were small and laid fewer eggs. It is also known that mating behaviour affects the number of eggs laid in the 2-spot ladybird; Sem'yanov (1970) showed that females increased their rate of egg laying after each mating. Ladybird eggs generally take about four days to hatch, though there can be considerable variation depending on ambient temperature. Table 2 shows that increasing ambient temperature reduces the length of time spent in the egg stage by the 7-spot ladybird, so that at 15°C it is 10.3 days, while at 35°C it is 1.8 days. However, there is an upper threshold temperature, above which the eggs will fail to hatch.
2.3 Larvae
Like all beetles, ladybirds have larvae lacking wingbuds (pl. 7 and 12). When the eggs hatch, the young larvae usually remain on or near their egg shells for about a day (fig. 7). They eat their egg shells and very often eat any later-hatching eggs, or any infertile eggs that have failed to hatch. After leaving their shells, the first-instar larvae must find food, which they do by actively hunting for prey. Mortality of newly-hatched larvae is high because capturing the first aphid is difficult for a first-instar larva, which is often smaller than many of the aphids it is trying to eat. The consumption of other eggs in the clutch (sibling egg cannibalism) provides vital nutrition to the cannibalistic larva and increases the chances of it surviving the first instar (Roy and others, 2007). Evidence suggests that larvae discover prey by physical contact rather than scent or sight although this undoubtedly needs further investigation (see 3.2). The way the food is taken depends largely on the relative sizes of prey and predator. A tiny first-instar larva can be found perched on the backs of relatively large aphids as if riding 'piggy-back' (fig. 8). Its jaws are embedded into the aphid and it feeds by sucking the body fluid of the aphid. Indeed all larval stages of the small and inconspicuous ladybird Platynaspis luteorubra suck the body fluid of aphids using the leg of the aphid as a straw to facilitate feeding. However, as the larvae of most species grow and become larger relative to their prey, they begin to eat solid parts such as the legs and antennae of the prey as well as the body fluid. Most larvae regurgitate fluid from the gut into the chewed aphid, allowing some pre-digestion before the aphid body fluid is sucked in.
A larva sheds its skin, or moults, three times before pupating, so there are four larval instars. The old skin splits on the upperside at the front and the larva frees itself over a period of about an hour. The new skin is initially pale and soft (fig. 9) but quickly darkens and hardens. The length of time spent as a larva depends to a large extent on environmental conditions. Prey density is important; the more prey is available, the faster the larvae can feed and grow. Very low prey density can lead to starvation. Banks (1957) calculated that the larvae of the 14-spot ladybird die unless they find food within 1–1.5 days after hatching. If prey levels are low, but above starvation levels, then development is slower than normal. Above a certain prey density, development rate is not increased but the resulting adults are larger. Prey type also influences development and reproduction of ladybirds and this is discussed in detail in chapter 3.
Temperature also affects development rate. In general, development is faster at higher temperatures but this relationship is not straightforward, as is obvious from table 2. As the temperature approaches the upper tolerance level, further increases produce only small increases in development rate. So, an increase from 15°C to 20°C reduces the larval phase of the 7-spot ladybird by 16.9 days; but a similar increase from 30°C to 35°C reduces it by only 1.3 days (table 2). Still higher temperatures will retard development, or even cause death.
2.4 Pupae
The fourth-instar larva stops feeding at least 24 hours before pupation. It becomes immobile as the tip of the abdomen is attached to the substrate, usually a leaf, stem or bark. It also adopts a characteristic hunched position (fig. 10). This stage is called the pre-pupa.
There are two main types of ladybird pupae. In the first, the final larval skin of the pre-pupa is shed right back to the point of attachment to the substrate, so that the pupa is naked. Most ladybird species have this type of pupa (pl. 12.1 and 12.2). The second type is found in ladybirds of the genera Exochomus (pine ladybird) and Chilocorus (kidney-spot and heather ladybirds), in which the skin splits lengthwise but is not shed (pl. 12.3). The duration of the pupal phase varies with the ambient temperature. Table 2 shows that in the 7-spot ladybird this stage lasts 15 days at 15°C but only 2.5 days at 35°C.
Although pupae are generally thought of as inactive, they are not completely immobile. If they are irritated there is an alarm response in which the fore region of the pupa is rapidly raised and lowered several times (fig. 11). This is probably a mechanism for dislodging parasites and startling predators. There appears to be considerable variation among pupae in the strength of this alarm response, and it is almost absent in some pupae. There is also variation in the strength of the response throughout the life of a single pupa, though a pupa can show the response almost immediately after pupation, and until a few hours before the adult emerges.
Pupal colouration is variable in some species. At least some of this variation is due to environmental conditions. Pupae of the 7-spot ladybird are light orange at high temperatures and much darker at low temperatures.
2.5 Adults
The adult ladybird emerges by splitting the front end of the pupal case. It takes several minutes to pull itself out, and then generally rests on the empty case to expand and dry the elytra and wings (fig. 12). At this stage the elytra and wings are very soft and contain very little pigment. The colour of the elytra is usually light yellow or orange in most species, although ladybirds with predominantly black elytra (such as the pine, kidney-spot and heather ladybirds) have red elytra on eclosion from the pupa. The basic adult pattern and colouration may take several hours or even days to develop, and more subtle changes take place over a longer period (pl. 8). This is most obvious in species such as the 2-spot and 7-spot ladybirds that have a red background colour, which gradually deepens over the weeks and months. One consequence is that newly-emerged adults are readily distinguishable from those that have overwintered, which are a much deeper shade of red. There are two main groups of pigments in ladybirds. The dark colours are the result of melanins, and the lighter orange, red and yellow pigments are derived from carotenes.
Most adults emerge in mid to late summer. They feed, perhaps for several weeks, before dispersing to their overwintering sites. In many species mating takes place in the spring. Ladybirds are most active in sunshine and mating pairs are a common sight in suitable habitats on warm, sunny spring days. In contrast to some insect species there is often no obvious courtship ritual. In many cases a male simply approaches a female and places his front legs on her elytra. If she accepts him he positions his genitalia and mating takes place (fig. 13). More elaborate behaviour occurs when a female rejects a male. The female may simply run or fly away. But if a male has a strong grip on the female she may raise her abdomen (fig. 14), or kick him with her hindlegs. If this does not deter the male, then the female rolls over to dislodge him (fig. 15), or in extreme cases she climbs up a plant stem and drops to the ground or attempts flight. A female may reject a male's advances if she is too young to mate, has recently mated, is about to lay eggs, or has a specific mating preference for a different type of male (see 6.4).
It is generally considered to be difficult to distinguish between male and female ladybirds. In most species the females are slightly larger than the males, and there may be small differences in shape, but these criteria are not totally reliable. Careful examination of the underside of the abdomen of 23 British species of ladybirds has revealed sexual differences in every case. These are best seen with a low-power dissecting microscope. Surprisingly, there is no single set of criteria that is applicable to all species, as each has its own distinctive features. These involve the size, shape and number of cuticular plates at the end of the underside of the abdomen. In some species the abdomen is pointed in the female, in others in the male, and in still others in both, or neither. The cuticular plates may be notched in the male, or undulating in the female. The only feature found in all males, and absent in all females we have examined, is three curved bands of thin flexible cuticle at the back margins of the abdominal segments. These are, for example, yellow in the eyed ladybird and glossy black in the 2-spot ladybird. They enable the abdomen of the male to be flexed at right angles during mating and hence provide a very useful diagnostic sexual feature. Figure 16 shows a representative selection of the sex differences in a variety of species.
(Continues…)
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Table of Contents
Editors' preface, iv,
Acknowledgements, v,
1 Introduction, 1,
2 Life history, 5,
3 Ladybirds in their environment, 15,
4 Ladybirds and their natural enemies, 30,
5 Variation in ladybirds, 42,
6 Population and evolutionary biology, 56,
7 Ladybird distribution, 74,
8 Identification of British ladybirds, 85,
9 Study techniques and materials, 115,
References, 132,
Index, 138,