DRYOPHTHORINAE Schönherr, 1825
Grain Weevils
Our three Sitophilus species are stored grain pests, while Dryophthorus corticalis is a very local saproxylic species.
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Introduction
This is a large subfamily of more than 1200 species in 150 genera and 5 tribes although the status is of the group is not settled (and there will probably be polarized opinions on this for a long time); the group is characterized by the form of the antennae, and more especially the club, the unique structure of the last tarsomere and the male genitalia; here there is a lateral line dividing the aedeagus into an upper tectum and a lower pedon. This divided form of the aedeagus is also seen in many Brachycerinae and is the basis for removing the groups from the ‘true weevils’ or Curculionidae and raising each to family status, this would help validate the group as monophyletic but would not be the final word as, eventually, molecular data will also need to be taken into consideration. It is cosmopolitan in distribution with by far the greatest diversity in tropical regions and temperate faunas tend to be small; about 70 species of 15 genera and 3 tribes are Nearctic while all the tribes are represented in the Palaearctic region as follows. The monogeneric Cryptodermatini Bovie, 1908 is represented by 4 species of Cryptoderma Ritsema, 1885 from China and Japan; it is otherwise an Old-World, mostly tropical group of about 30 species. Dryophthorini Schoenherr, 1825 includes 2 species of Stenommatus Wollaston, 1873 from eastern Asia, and 6 species of Dryophthorus Germar, 1823; 4 from Japan, a single more widespread eastern species and the Eurasian D. corticalis Paykull, 1792) which extends to the UK, a saproxylic species also present in Africa, Australia and the Nearctic region. Orthognathini Lacordaire, 1865 includes 2 species of the widespread old world genus Sipalinus Marshall, G.A.K., 1943 from eastern Asia. The tribe otherwise includes about 40 species in 6 genera and is Holarctic, with 2 species of 2 genera occurring in the United States, as well as extending into tropical Africa and Australasia. Species of the pantropical genus Yuccaborus LeConte, 1876 are commonly known as yucca-weevils because some are pests, but otherwise the group includes some truly bizarre-looking species. Rhynchophorini Schoenherr, 1833 is the largest group with about 32 genera of 6 subtribes occurring in the Palaearctic region. Diocaladrina Zimmerman, 1993 includes 2 genera and 5 species from Eastern Asia and the Oriental region, of which one has been introduced to France. Litosomina Lacordaire, 1865 includes 5 genera and 15 species including 10 species of the familiar Sitophilus Schoenherr, 1838;
5 of these occur in Europe of which 3, virtually cosmopolitan stored-product pests, extend to the UK. The group otherwise includes more than 30 genera and is mostly tropical. Ommatolampina Lacordaire, 1865 includes a single Palaearctic species from eastern Asia and the Oriental regions. Polytina Zimmerman, 1993 is represented in Asia by the invasive Small Banana Weevil, Polytus mellerborgi Bohenam, 1838, a species otherwise widespread in warmer Old World regions. Rhynchophorina Schoenherr, 1833 includes 6 genera and 34 Palaearctic species; with one exception all are Asian or Oriental but the widespread and notorious Asian Palm-Weevil, Rhynchophorus ferrugineus Olivier, 1791 has become established around the Mediterranean since the 1980s. Sphenophorina Lacordaire, 1865 is well-represented in the Palaearctic region with 50 species of 18 genera, most are from eastern Asia and Oriental regions but 5 species of Sphenophorus Schoenherr, 1838 are widespread in Europe while a further one has been introduced (Corsica), and two further species have become established, Cosmopolites sordidus Germar, 1823, the almost cosmopolitan banana root borer, in Southern Spain, and Scyphophorus acupunctatus Schoenherr, 1838, one of the most important pests of cultivated agaves, around the Mediterranean. The Strombocerini Lacordaire, 1865 includes 13 Palaearctic species of 5 genera but is not represented in Europe. The European fauna is therefore very small a significant proportion of invasive species while that of the UK includes 3 invasive species of Sitophilus and the very widespread old world Dryophthorus corticalis.
The bamboo weevil - Cyrtotrachelus dux
China
Biology
Weevils of this subfamily cannot be appreciated from a familiarity of the UK or even the European fauna; there are many large and bizarre tropical species e.g. Cyrtotrachelus dux Boheman in Schoenherr, 1845, a serious pest of commercially grown bamboo and among the largest species of the subfamily, reaches 40 mm and has extraordinarily long front legs and rostrum, the Bottlebrush Weevil, Rhinostomus barbirostris (Fabricius, 1775), the notorious coconut palm pest from Central and South America, is similarly large and the males have a very long and dorsally dentate rostrum with dense and long golden pubescence lining the lateral margins and also around the pro-thorax. The largest species is probably the Australasian Protocerius collossus Schoenherr, 1838 which has been reported at 60mm (excluding the rostrum), and among the strange morphological features of the subfamily are a spatulate or ventrally developed rostrum and unusually modified tarsi or antennal clubs. But several features characterize the group as a whole; the antennal scape is long and placed towards the base of the rostrum so that, in repose, it reaches at least to the anterior margin of the eye and usually far beyond it, the antennal club is composed of two distinct parts; the first segment, which is usually elongate or otherwise enlarged, is glabrous and shiny while the remainder are pubescent or otherwise strongly contrasting, and the form of the terminal tarsomere which is extended both dorsally and ventrally and curve to meet between the base of the claws, these extensions have been termed ‘dermal lobes’. The British species are all small <4.5mm, compact and more or less continuous in outline with distinctly geniculate antennae and robust and moderately long legs. In general appearance they superficially most resemble some of the Cossoninae, otherwise despite the size they are typical of much of the family; head small with a variable but long and prominent rostrum, weakly convex eyes and short antenna placed near the base of the rostrum; the funiculus 4 or 6-segmented and the club 2-segmented. Pronotum elongate, rounded laterally and constricted before the anterior margin, surface shiny and strongly and moderately densely punctured. Elytra elongate and continuously or separately rounded apically, with complete and strongly punctured striae and variously raised interstices. Pro-tibiae with a strong and sharp, inwardly-facing tooth which appears to be continuous with the external edge. Tarsi 5-segmented; of typical curculionid form in Sitophilus i.e. with the third segment bilobed and the fourth tiny or, in Dryophthorus, without lobed segments and apparently 4-segmented.
This subfamily is primarily a tropical group and mostly associated with monocotyledonous plants of the families Arecaceae, Liliaceae, Cyperaceae and Poaceae, and there are many notorious pest species of both stored products and crops, especially grains, bananas, coconuts and commercially grown bromeliads. Dryophthorus Germar, 1824, a mostly old world genus of more than 60 species of which one is listed as British, is unusual in occurring among decaying wood in the wild. Larvae generally develop in fleshy roots, stems or fruits, sometimes in aquatic habitats. Completing the British list are 3 species of Sitophilus, all are cosmopolitan pests of stored grains etc. and all are established introductions.
UK Species
Sitophilus zeamais