Bruchinae | uk beetles

BRUCHINAE Latreille, 1802

Seed Beetles

Includes wild species as well stored bean and pea pests. In either situation adults may occur in huge populations.

Introduction

Historically treated as a family and often placed next to the weevils, no doubt due to the short and broad rostrum and the overall different appearance from most other Chrysomelid groups, some species are still referred to in this way e.g. The Broad-Bean Weevil, Bruchus rufimanus Linnaeus, 1767 or the Cowpea Weevil Callosobruchus maculatus (Fabricius, 1775). The group is now firmly established as a subfamily of the Chrysomelidae and the former subfamilies of Bruchidae are classified as tribes. In this modern sense there are about 1700 described species in 6 tribes and about 70 genera although there has been no thorough modern system devised and some of the old ‘dumping ground’ genera e.g. Acanthoscelides Schilsky, 1905 or Bruchus Linnaeus, 1767 will no doubt continue to be split; originally most of the Palaearctic species were assigned to Bruchus but in 1905 Schilsky split some off to form Bruchidius, and a catalogue of the group was produced by Pic in 1913, and many other genera have since been devised e.g. Tuberculobruchus Decelle, 1951 and Conicobruchus Decelle, 1951. But despite the many genera described in the 20th century, large numbers still remain in a few genera and many species await description. The group is cosmopolitan with the greatest diversity in dry tropical and subtropical regions and many species are widespread, more especially so since they are readily transported with foodstuffs; with the exception of Madagascar, the oceanic islands host only more widespread species, and the continental islands tend to have low diversity. There are more than 760 New World species of which about 150 occur in North America, central Europe hosts about 50 species and the U.K. fauna includes almost 20 established species in 4 genera with several more recorded as occasional introductions. Of the 6 tribes, 3 contain only a single genus; Rhaebus Fischer von Waldheim, 1824, the only genus of the Rhaebini Chapuis, 1874, contains 7 Palaearctic species which are atypical of the subfamily; elongate and brilliant metallic with almost filiform antennae, resembling more typical Chrysomelids. Eubaptini Bridwell, 1932 contains 4 species of Eubaptus Lacordaire, 1945 and is native to the Neotropical region, these are typical of the subfamily with robust antennae and the dorsal surface finely pubescent throughout, E. palliates Lacordaire, 1845 has strikingly bicoloured elytra. Kytorhinini Bridwell, 1832 includes about 16 species of the mostly Asian genus Kytorhinus; typical of the group

POLYPHAGA Emery, 1886

CHRYSOMELIDAE Latreille, 1802

1.7-5.3mm

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but some males have strongly pectinate antennae. A single species, the tiny, elongate and rather drab K. proxilus Fall, 1926 also occurs in the northern United States. Pachymerini Bridwell, 1929 includes three subtribes; the Pachymerina Bridwell, 1929 with 20 or so species in 4 genera are mostly Neotropical although two species also occur in the southern United States; Caryobruchus gleditsiae (Linnaeus, 1767), the palm seed weevil is a drab and large species, 5-10mm, with greatly expanded hind femora, and Pachymerus nucleorum (Fabricius, 1792), the coconut or kernel borer, another large and drab species. The Caryedonina Bridwell, 1929 includes 5 genera and about 90 species, and the Caryopemonini Bridwell, 1929 includes 3 genera and 20 or so species, these are Palaearctic, Oriental and African bruchids, all are typical of the subfamily and many have expanded hind femora e.g. species of Carydon Schoenherr, 1823, a native African genus now also established in the southern United States and pests of tamarind and other seeds. The Amblycerini Bridwell, 1932 includes two subtribes. The Amblycerina Bridwell, 1932 includes more than 100 species of the New World genus Amblycerus Thunberg, 1815, the majority are Neotropical, and only 7 species occur in the United States. These are readily recognized as bruchids but lack the expanded hind femora seen in some groups and many have narrow, almost filiform, antennae. Spermophagina Borowiec, 1987 includes about 30 species of the New World genus Zabrotus Horn, 1885, and about 150 species of Spermophagus Schoenherr, 1833 are distributed throughout the old world with the exception of Australia, about 20 species occur in Europe but not the U.K. All members are typical of the group. The Mexican Bean Bruchid, Z. Subfasciatus (Boheman, 1833) is now pantropical, having been distributed along with foodstuffs. The vast majority of species (>75%) are included in the Bruchini Latreille, 1802 and three subtribes are recognized. They are generally broadly similar and many were at one time included in the nominate genus. The Megacerina Bridwell, 1946 contains about 60 species of the New World genus Megacerus Fahraeus, 1839; these are the most distinctive group, sometimes referred to as long-horned bruchids because of the strongly pectinate antennae of the males, see e.g. M. cubiculus (Casey, 1884). Colour and size vary widely in the genus but most are small, <5mm, and drab or patterned with grey or brown streaks although the bicoloured M. discoideus Say, 1824 is striking. The Bruchina Latreille, 1802 includes the single large genus Bruchus Linnaeus, 1767 with about 40 Palaearctic species by modern standards but many other species remain included from earlier classifications that will no doubt be reorganized, these include species from farther afield; Africa, Asia and Australia that will in all likelihood be transferred to the next subtribe. The Acanthoscelidina Bridwell, 1946 includes the majority of the genera, more than 40, and a large number of species both described and awaiting description. All are broadly similar and typical of the subfamily with dark bodies, often patterned with white or creamy scales or setae, campanulate pronotum and strongly striate and broad elytra. Most of the species are included in two large genera; Bruchidius Schilsky, 1905, with more than 300 species and many more awaiting description, are now generally cosmopolitan but native to the Old World, and Acanthoscelides Schilsky, 1905, with more than 300 species so far described, is native to the New World but not now more widespread; about a third of the species occur in Mexico. It is very likely that these genera will eventually be split. Other speciose genera, all native to the New World, include Caryedes Hummel, 1827 with more than 40, mostly Neotropical species, Sennius Bridwell, 1946, with about 40 species in North and South America, Stator Bridwell, 1946 with about 35 species, and Merobruchus Bridwell, with 25 species.

Bruchus rufimanus Boheman, 1833

Description

The habitus of the group as a whole is very characteristic and so a familiarity with the U.K. species will allow them to be recognized generally. The smallest members include some species of Acanthoscelides at a little under 1mm while the largest are the Palm Seed Weevils of the Neotropical genus Speciomerus Nilsson e.g. S. giganteus (Chevrolat, 1877) which may reach 17mm, but the majority of species are less than 5mm.

Most are black or dark brown but there are many pale brown species and some are distinctly red, orange, yellow or discreetly and strikingly bicoloured, the appendages vary from black to yellow and in many the front legs are lighter than the others. Relatively few species are metallic e.g. all members of Rhaebus and some Meibomeus Bridwell, 1946 and Bruchidius Schilsky, 1905. Most are overall broadly elongate although some are almost quadrate as in some Spermophagini, and sometimes distinctly so e.g. Dahlibruchus Bridwell, 1931 or Rhaebus. All are strongly convex and distinctly pubescent although this varies from sparsely and finely to densely clothed and conspicuously patterned with broad scale-like pubescence which may be diagnostic for some genera or species e.g. see Bruchus ulicis Mulsant & Rey, 1858. Some species have glabrous areas on the pronotum and/or the elytra. The dorsal surface is generally microsculptured and punctured, often densely so and with punctures of varying sizes. The head is strongly declined although easily drawn out to ‘set’ so that it is visible from above; it is elongate with a short and broad rostrum. The vertex and frons are generally short and convex although many species have a longitudinal median ridge towards the front; the temples are strongly contracted from behind the eyes to a short and relatively narrow neck. The antennae are 11-segmented with the scape distinctly longer than the pedicel, the flagellum is usually serrate and often strongly so although in some groups it is pectinate and usually sexually dimorphic, and in the Rhaebini it is almost filiform. The insertions are visible from above and situated within, or very close to, the anterior margin of the eyes. The clypeus is well developed and at least partially covers the mandibles. Apical segments of the palpi fusiform. Eyes convex and proportionally very large, strongly protruding laterally, and deeply notched anteriorly, sometimes almost divided and often appearing crescent-shaped, the facets usually fine. The pronotum varies from quadrate to strongly transverse and is generally narrowed from the base to a rounded front margin, or angled and/or toothed laterally, the margins may be weakly bordered or partially bordered, and the dorsal surface usually lacks sculpture, being simply flat to strongly convex, and only rarely are there tubercles or distinct depressions. The scutellum is obvious, usually transverse or triangular, and sometimes has an incision at the base. The elytra are distinctly wider than the base of the pronotum and vary from quadrate to about twice as long as wide, the surface is usually microsculptured and punctured and in some groups there are small tubercles or spines towards the base, there are 10 well-impressed and often punctured striae which may be abbreviated towards the base, especially before the humeral prominence, and are often joined before the apex. In the vast majority of species the apices are separately rounded and at least to some degree truncate, leaving a heavily sclerotized pygidium and often one or more tergites exposed. Abdomen with five free ventrites, the first more than twice as long as the second and lacking post-coxal lines. The intercoxal process at the base of the first ventrite varies from acute to broadly rounded. Most species are fully winged and strong fliers. The legs are well-developed and robust, especially the hind pair. The trocanters are joined obliquely to the base of the femora so that these touch, or almost touch, the coxae. The fore legs are usually ‘normal’ with straight or weakly curved and narrow femora and tibiae that appear agile and are often unarmed. The pro- and mesotibiae are often sexually dimorphic; in males they may be more curved or adorned with carinae or spurs. The hind legs are greatly modified; the metafemora laterally compressed or ventrally flattened, and often enlarged, sometimes greatly so often furnished ventrally or apically with teeth that may be diagnostic for genera and species. The metatibiae are long, flattened and often expanded towards the apex, and usually armed with apical teeth or spines. Tarsi 5-5-5 in both sexes; segments 1 and 2 are elongate, segment three is bilobed and usually strongly so, and segment four is reduced and often hardly visible. The basal segment is often modified; greatly elongate, curved and flattened, and may be sexually dimorphic. The terminal segment is elongate.

Ecology

Many adult bruchids feed on pollen, not necessarily that of the host, and many have specialized maxillae to facilitate this, it is likely that a period of feeding is necessary before some species become sexually mature but many breed and lay eggs without feeding at all. In temperate zones most species are univoltine, breeding and ovipositing in the spring with larvae developing through the summer. Eggs are laid individually or in small groups along the suture of the seed pod although in some, mostly tropical species, they are glued to the host with secretions which will prevent desiccation. A Neotropical species which has now become established in Europe, Pseudopachymerina spinnipes (Erichson, 1834), lays groups of overlapping eggs which are thought to be at least partly protected from parasitic wasps. Most species oviposit on newly developing seed pods but a few, especially those laying eggs on trees, select matured pods. This is the only subfamily of the chrysomelidae to develop within seeds and the only exception to this seems to be the European Bruchidius cinarascens (Gyllenhal, 1833) which develops within stems of Apiaceae. The host plants of many species are known and about 70% of these are legumes, some are pest of cultivated crops, especially in the genera Callosobruchus, Bruchus, Acanthoscelides and Zabrotes, but this holds for most groups within the subfamily and is not strictly the case as more than 30 plant families have been recorded as hosts. Some groups are more specialized; the Rhaebini develop within the seeds of Nitraria L. (Nitrariaceae), and members of the Eubaptini develop in the seeds of Ruellia L. (Ruellieae, wild petunias). Pachymerini develop in palm seeds (Arecaceae) and most Megacerina within seeds of Convolvulaceae. Cistaceae (rock-roses) host species of Bruchidius and Acanthoscelides, and seeds of various Lamiaceae are consumed by species of Stator and Salviabruchus. Species of Caryedon develop within peanuts and tamarinds. The seeds of palm-oil plants, fast becoming one of the world’s dominant cultivated species, are consumes by species of Pachymerus. Many bruchids are being considered as biological control agents to help control invasive legume species such as Cytisus, Genista and Mimosa as well as bindweeds of the genus Ipomoea (Convolvulaceae).

Larvae develop entirely within seeds and the first instar bears a specialized prothoracic structure which is used to penetrate the pod; they do not eat their way in or consume the pod as many contain poisonous defensive chemicals. They initially tunnel into the seed and then enlarge a feeding cavity towards the centre; they grow rapidly, pass through 3 or 4 instars and most are fully grown within a month or less. When fully grown they move to the inside of the seed pod and scrape away the tissue leaving only a translucent circular ‘window’ which will assist the adult in escaping the pod. They then return to the inside of the seed to pupate, this stage is also rapid taking between one and three weeks. All stages can aestivate within the pod if necessary e.g. stages still developing during the onset of cold weather will spend the winter in the pod and continue developing in the spring. Under artificial conditions e.g. among seeds stored at suitable temperatures etc. they may bred continuously and produce huge populations. Some species always diapause in the pods after they eclose e.g. adults of Caryedon serratus (Olivier, 1790), the groundnut bruchid, remain for several months, ensuring that when they emerge the next generation of pods are available. In a few Old World species the larvae leave the pods when fully grown to pupate in cocoons on the pods or adjacent foliage. The larvae of at least one Southern African species of Caryedon leave the pods to pupate in the soil under the host.

Among the species regularly recorded in the U.K. are Acanthoscelides obtectus (Say, 1831), which is found among stored products and sometimes also on flowers outside, and two species of Callosobruchus Pic, 1902 which are similarly only rarely found outside. Several others occur occasionally among stored products and two have only recently been found in the wild; Bruchidius siliquastri Delobel, 2007 originally described from China and now present in the southeast, and B. imbricornis (Panzer, 1795), a European species recorded from the southeast. Among the widespread species B. villosus (Fabricius, 1793) occurs on Cytissus, and B. cisti (Fabricius, 1775) and B. varius (Olivier, 1795) occur on a range of legumes. Five species of Bruchus Linnaeus, 1767 occur in the wild and several others have been recorded among imported food. They occur on vetches, Vicia and Lathyrus; widespread and common species include B. loti Paykull, 1800, B. rufipes Herbst, 1783, B. rufimanus Boheman, 1833 and B. atomarius (Linnaeus, 1767). At least some of the widespread species will be recorded from general sweeping.