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Overview
Topics cover the life history of leaf beetles, biology, their environment, natural enemies and interactions with humans. There is a thorough discussion about identification of British species, including detail on the juvenile stages (eggs, larvae, pupae) and a concise key to adults. A chapter is dedicated to study techniques and materials. The book is illustrated throughout with colour photographs and line drawings.
Leaf beetles is a vital resource for entomology students and educators, naturalists, nature conservationists, those involved in agriculture, horticulture and the management of stored produce.
Product Details
| ISBN-13: | 9781784271503 |
|---|---|
| Publisher: | Pelagic Publishing |
| Publication date: | 09/29/2017 |
| Series: | Naturalists' Handbooks |
| Pages: | 160 |
| Product dimensions: | 5.74(w) x 8.39(h) x 0.43(d) |
About the Author
Read an Excerpt
CHAPTER 1
Introduction
Leaf beetles are not as well known as ladybirds, but they too have many species that are distinctively patterned and dome-shaped. In addition, there are ten times more species and many of them are important crop pests. They are an attractive, fascinating and important group of beetles that deserves to be better known. The aim of this book is to introduce the leaf beetles and provide information to allow anyone to find and study them.
1.1 What are leaf beetles?
The leaf beetles are a large group with nearly 300 species in the British Isles (Table 1.1), and an estimated 40,000 worldwide in over 2,500 genera, making them one of the most diverse and regularly encountered beetle groups. As their common name suggests, they are all herbivorous and closely associated with plants, where they feed on various parts, not only leaves. Being beetles, they have biting mouthparts (unlike the piercing-sucking mouthparts of the Hemiptera or 'true bugs') and forewings that are hardened to form elytra (wing cases) covering the abdomen when viewed from above.
Most are in the family Chrysomelidae, with a small number of species in the closely related Megalopodidae and Orsodacnidae – collectively these three families constitute the leaf beetles and they are also generally referred to as 'chrysomelids', because previously, these three families were classified together in the Chrysomelidae. It is important to become familiar with the main features of the various subfamilies as this makes identification quicker, and a brief introduction to them follows.
Amblycerinae. These are similar in appearance to the Bruchinae and also associated with legumes. They were previously grouped with them as a separate family.
Bruchinae. Previously considered as part of a separate family, bruchines have been variously known as seed beetles, pea weevils, bean weevils, bean beetles and beanseed beetles because of their association with the seeds of leguminous plants. Many are pests of such crops, including dried and stored produce, especially in tropical and subtropical areas.
Cassidinae. Previously considered to be a tribe (Cassidini) of the subfamily Hispinae (which is not found in Britain), cassidines are now given subfamily status. They are commonly known as the 'tortoise beetles' owing to their dorsally flat-domed and more-or-less rounded appearance.
Chrysomelinae. Chrysomelines are the 'typical' (i.e. domed, relatively large and often metallic in colour) leaf beetles and include many of the more charismatic species within the British chrysomelid fauna, such as the rare tansy beetle Chrysolina graminis.
Criocerinae. In Britain, a small subfamily of relatively elongate, parallel-sided beetles represented by eight species including the introduced lily beetle Lilioceris lilii. All have a notch on the inner edge of the eye, although this may be slight.
Cryptocephalinae. A subfamily consisting of two tribes in Britain: the Clytrini (composed of one scarce and one recently extinct species) and the Cryptocephalini (all belonging to the genus Cryptocephalus). The genus Cryptocephalus includes several rare or endangered species and they are known colloquially as 'pot beetles' because of the appearance of the cocoons their larvae live in. These are initially built by the female during and immediately after egg laying, with the egg being held between the rear tarsi (feet) and covered by her faeces. Once covered, the pots are dropped to the ground among leaf litter, which often forms much of the larval diet, and the larvae add their own faecal material to their cocoons as they grow. The precise structure of these cocoons varies by species.
Donaciinae. Commonly known as the 'reed beetles' and associated with plants in or near water bodies and wetlands. Donaciines are more elongate than many other chrysomelids and many are distinctive, being brightly metallic in colour.
Eumolpinae. A small subfamily known by a single species in Britain, Bromius obscurus. They are superficially similar to the Chrysomelinae, but can be distinguished by features of the legs.
Galerucinae. A large subfamily comprising two tribes, the Galerucini and Alticini, both of which have previously been considered separate subfamilies. The Alticini are known as 'flea beetles' because of the well-developed flea-like jumping abilities of the adults and have been known as both the Halticinae and Alticinae. Examples include the cabbage-stem flea beetle, Psylliodes chrysocephala.
Lamprosomatinae. A small subfamily known by a single species in Britain, Oomorphus concolor. Like the Cryptocephalinae, they form cocoons from faecal matter.
Orsodacninae. In Britain, this is the only subfamily within the family Orsodacnidae. There is a single genus Orsodacne in Britain, consisting of two species of relatively elongate beetles. The subfamily was previously placed within the Chrysomelidae.
Zeugophorinae. In Britain, this is the only subfamily within the family Megalopodidae. There is a single genus Zeugophora in Britain, consisting of three species of relatively elongate beetles. It was previously placed within the Criocerinae and later the Orsodacninae, which is now a separate family, the Orsodacnidae. Some authors (e.g. Zaitsev & Medvedev, 2009*) place it (along with the Orsodacninae) within the Chrysomelidae.
1.2 Adult external morphology
Chrysomelids are small to medium-sized beetles – in Britain ranging from a little over 1 mm in length (Mniophila muscorum and Longitarsus minusculus) to 18 mm in large specimens of Timarcha tenebricosa. Their overall shape varies from being somewhat elongate (e.g. Donaciinae) to oval and domed (in many subfamilies) or rounded and flattened (Cassidinae) (Figs. 1.1–1.5).
The antennae are usually thread-like (filiform), although in some taxa (e.g. Bruchidius) they may be saw-toothed (serrate) or have modified and expanded segments (e.g. males of Phyllotreta exclamationis and Phyllotreta nodicornis) (Fig. 1.6). There are 11 antennal segments except in Psylliodes species, which have 10. The antennae are never clubbed, unlike those of the ladybirds, which some leaf beetles resemble.
In the flea beetles (Alticini), the hind femora are enlarged (Fig. 1.7) and contain a chitinous 'metafemoral spring' which allows them to jump by releasing stored energy generated by the tibial extensor muscle (Fig. 1.8). The shape of the spring varies between genera and therefore could, in principle, be useful in identification. The tarsi are all 4-segmented and in most species at least some are bilobed (if not, then the final tarsal segment is elongate as in Macroplea).
The head of a chrysomelid has a pair of compound eyes and mouthparts adapted for plant-feeding. The point of attachment of the antennae relative to the eyes can be useful in identification, as can the grooves and bulges of the head (Figs. 1.9, 1.10).
In most chrysomelids, the elytra cover the full length of the abdomen, although in the Bruchinae, the final segment (pygidium) remains exposed, and in some Galerucinae they may be shortened. Gravid (egg-bearing) females of some species, such as the common green dock beetle Gastrophysa viridula, have a swollen abdomen, which forces the elytra apart. The elytra may also be patterned as in a number of Cryptocephalus species. As well as aiding identification, these spotted patterns are likely to provide protection through mimicry of unpalatable species such as ladybirds (Chapter 4).
Although chrysomelids are ground and plant-dwellers rather than aerial specialists, the hindwings are used for flight in fully winged species with developed flight muscles and associated thoracic structures (Fig. 1.11). See Chapter 3 for more about variation in wing development and flight ability.
See Chapter 2 for details of juvenile morphology, and for morphology of reproductive structures.
1.3 Evolution and palaeontology
The earliest fossil chrysomelids date from the Jurassic (200–145 million years ago) and showed a great increase in diversity by the end of the Cretaceous (approximately 65 mya) at the same time as diversification of flowering plants. The Donaciinae were the largest group until the Pleistocene (approximately 1.8 mya), although the modern genera and species became established during the Palaeogene (65–23 mya) and Neogene (23–2.6 mya). The apparent dominance of donaciines may be due to their aquatic and riparian (water edge) habitats providing anoxic sediments favourable for fossilisation, although such habitats also remained stable for millions of years. This may explain why Plateumaris nitida (a North American species) is almost identical to Donacia primaeva from 30 mya, having changed little during this period (Elias & Kuzmina, 2008).
The more recent Quaternary (2.6 mya–present) fossil record has provided several hundred chrysomelid taxa (with some but not all recognised at species level) around the world, indicating its importance as a plant-feeding beetle group. Western Europe (especially Great Britain) is relatively well studied and around 150 fossil chrysomelid species are known from this area. The majority are associated with wetland habitats, although species from deciduous woodland, meadows and grasslands are also well represented. However, not all chrysomelid groups are equally represented, with only nine species of flea beetles known from fossil resin worldwide, most of which come from eastern Europe (Bukejs, 2014).
During the Pleistocene glaciations, it is likely that dry steppe and steppe-tundra formed the dominant landscape type in open, unglaciated parts of Europe. The fluctuating conditions associated with glacial cycles, including the bare soil left by retreating ice sheets would have favoured ruderal ('weedy') plants such as thistles (Cirsium) and plantains (Plantago), which are effective colonisers. These are also plants readily fed upon by a wide range of chrysomelids and thus the terrestrial beetle species may have been favoured by these conditions.
Despite the large numbers of fossils, these still only provide incomplete snapshots of the prehistoric fauna and the origins and spread of many of the Chrysomelidae are not well understood except in fairly broad terms. However, as the evolution of plant groups provides opportunities for new taxa of plant-feeding insects to arise, it is possible to follow this process to some extent. The first evolutionary branching of the Chrysomelidae probably occurred in the Upper Jurassic (161–145 mya) as cycad-feeders expanded to exploit monocots (early criocerines) and water-lilies (donaciines) as these evolved. Modern donaciines are still associated with wetland habitats and plants, while criocerines still feed on monocots e.g. Lilioceris (including the lily beetle Lilioceris lilii) on Liliacaeae and Crioceris (including the asparagus beetle Crioceris asparagi) on Asparagaceae, having specialised and diversified alongside these plant families. Along with this shift from generalist to specialist plant-feeding, it is possible that chrysomelids developed from pollen-feeders to being external and internal feeders on a range of plant parts.
Feeding relationships can therefore shed light on chrysomelid evolution, for example by looking at those that feed on crucifers (Nielsen, 1988). As with all chrysomelids, such beetles have had to adapt to the physical and chemical characteristics of their host plants, and although larvae and adults of any given species usually feed on the same plant, they tend to use different parts, with adults mainly on leaves (sometimes buds or flowers) and larvae often on or in (as miners) roots, stems and leaves. Crucifer-feeding has evolved separately several times with specialists developing early on in the tribe Phaedonini, and more recently in the Chrysomelini (both tribes within the subfamily Chrysomelinae). Plant–beetle relationships may also be used in evolutionary studies. For example, Poinar & Jolivet (2004) used information from host-plant biology and distribution and the fossil record to conclude that the ancestral food plant of Timarcha was a shrub in the family Ericaceae, because Vaccinium (an ericaceous genus including bilberry and cranberry for example) is the only host genus still used by both Old and New World beetles in this genus.
Considering the present-day fauna, there is no clear distinction between the Palaearctic and Oriental/Asian regions as many genera are shared and also found in tropical Africa. The long evolutionary history has led to significant spread and some British genera may have originated in what is now Australasia e.g. the genus Phaedon which shows similarities with Geomela (Australia) and Aphilon (New Zealand) (Reid, 1995; Daccordi, 1996). A number of strict relationships between chrysomelid groups and their host plants are known, such as that between Aphthona and spurges (Euphorbia), and the many specialist species-level relationships. A particularly well-defined example of the latter is the highly localised British endemic, bronze Lundy cabbage flea beetle Psylliodes luridipennis, which is found only on Lundy Island off the coast of north Devon, and feeds solely on Lundy cabbage Coincya wrightii, which is also endemic.
CHAPTER 2
Life history
2.1 General life history
The life cycles of leaf beetles go through the same stages as all other beetles – egg, larva, pupa, adult – and undergo full metamorphosis, a situation known as being holometabolous. In many British species, the life cycle takes one year, but this is highly variable within the leaf beetles, and even within a single species, and the following types of life cycle are known:
• One generation per year e.g. Oulema melanopus, Chrysolina banksi and Phyllotreta undulata (along with many other species, including most 'flea beetles') showing a peak of adults in spring or summer depending on emergence time, with one of the life cycle stages over-wintering. Some, such as the lily beetle Lilioceris lilii, have one generation per year in Britain but up to three in warmer areas of continental Europe. Some species can be variable, such as tansy leaf beetle Chrysolina graminis, which shows one generation per year but around 5% of the population surveyed near York undergo a long diapause of over a year, greatly lengthening the life cycle of those individuals (Cox, 2007).
• One generation taking more than one year. For example, Donacia dentata has a two-year life cycle while Donacia semicuprea has a three-year life cycle. Although not all are well understood, several species of Cryptocephalus (e.g. Cryptocephalus coryli) have life cycles lasting either two or three years.
• Aestivation e.g. the rosemary beetle Chrysolina americana. Adults can be found all year with a new generation emerging during May and June, then aestivating until July to September when they feed again and then overwinter. Breeding can occur from autumn through to April in southern England.
• Overlapping generations e.g. the bloody-nosed beetle Timarcha tenebricosa. Various generations of eggs and larvae may exist at the same time, along with adults throughout the year as they can live for 14 months or more.
The number of larval instars (stages) also varies within the chrysomelids, from three in Timarcha and Phyllotreta (among others) to six in Cryptocephalus nitidulus and C. coryli.
Most species mate frequently though some such as Plagiodera versicolora can mate later in the year and store sperm in order to fertilise eggs the next spring. In many chrysomelids, the life cycle is not fully understood, with the length of various stages unknown, manner of overwintering undetermined, or juvenile stages undescribed. In others, details are known from captivity but not in the wild. These are areas where useful study could be undertaken.
2.2 Eggs
Most chrysomelid eggs are elongate and oval (Fig. 2.1), ranging from approximately 0.15 mm wide and 0.3 mm long in small Alticini such as Phyllotreta undulata to around 1.9 × 3.5 mm in T. tenebricosa, the largest British species. Some are cylindrical with rounded ends (e.g. Chrysomela tremula), ellipsoidal (e.g. Sphaeroderma rubidum), highly elongate (e.g. Orsodacne cerasi), hemispherical (e.g. Phyllobrotica quadrimaculata) or globular to approximately spherical (e.g. Pyrrhalta viburni). Most are pale whitish to creamy or yellow, but the range of colours includes lime green, brown, orange-red, purplish, grey and black, with some more-or-less colourless and translucent. Some species show considerable variation in egg colour, which changes as they mature. For example, the eggs of Chrysolina staphylaea are a deep red-brown when freshly laid, becoming considerably paler, while those of Crioceris asparagi begin greenish-grey, darkening to become black. The eggs of most Cryptocephalus species are a typical yellowish colour, but this is obscured by their covering of dark faecal matter. The egg surface also varies from smooth to being extensively microsculptured or rough, with patterns and depth differing between species. The eggs of around 70 British species remain undescribed, something that careful observation of adults during oviposition could help to remedy.
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