Bromius obscurus (Linnaeus, 1758)
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POLYPHAGA Emery, 1886
CHRYSOMELOIDEA Latreille, 1802
EUMOLPINAE Hope, 1840
BROMIUS Dejean, 1836
This is the only U.K. member of the Eumolpinae. It is limited to a single population and is regarded as endangered. It was first recorded as British in 1831 by Stephens from specimens found in Lincolnshire. Then it was not recorded again until 1979 when a colony was found on a disused railway embankment beside a river near Bosley on the Cheshire-Staffordshire borders. Globally Bromius obscurus has a very wide Holarctic distribution, occurring throughout Europe to the very north of Fennoscandia and into northern Siberia. Further south it occurs throughout Turkey, Kazakhstan, Russia, Mongolia, China and Japan. It is widespread through the U.S.A. and across Canada to the far north. In the U.K. it is associated with Rosebay Willowherb, Chamerion angustifolium, as well as other willowherbs, Epilobium spp. They have also been recorded on vines, Vitis viniflora and Great Butterbur, Aetasites japonicas. Adults feed on the foliage, cutting ragged holes in the leaves. They occur from May to October. Eggs are laid either on the ground or low down on stems in small batches. The distinctive larvae feed in groups on the roots, when full grown they are about 7mm long, C-shaped white grubs with a yellowish brown head, brown or black mouthparts and short legs placed near the head. Feeding continues through the summer and autumn and, when full grown, the larvae tunnel deep into the ground to overwinter. Pupation occurs during the following March or April in a subterranean cocoon. Formerly the species was a pest of grapes in Europe but it has now been largely eradicated by insecticides in such situations. However it remains a pest in North America and Canada where it is referred to as the Western Grape Rootworm. There it is also a well known pest of floribunda roses. Public information notices warn growers of its potential presence and contact numbers are given for identification and horticultural services. It seems that, given the species former pest status in Europe and California, growers further north are keen not to let it become establishes in sensitive areas. Hosts in the northern parts of the Americas are given as grapes, fireweed and roses. In the U.K the species is flightless and may be parthenogenetic. On the continent they are recorded feeding on a range of Oenotheraceae.
Among the U.K. fauna the species is distinctive. 5-6mm. Entirely black or dark grey with short, pale pubescence. Legs long and robust. Head with dense, coarse puncturation which is confluent over much of the surface. Eyes entire, large and protruding. Frons longitudinally furrowed. Clypeus smooth. Basal segments of palps and antennae pale. Antennal insertions separated by about 1.5 times the length of the first segment. Antennae gradually and weakly broadened to apex-in our specimen they appear 12-segmented; the terminal segment is divided into a small apical segment. Pronotum very convex and evenly rounded laterally, broadest about middle and with a furrow laterally across the base so that the hind angles appear toothed. Surface with large punctures which are not confluent. Scutellum large and pentagonal. Elytra parallel, depressed behind prominent humeri, with weakly impressed broad and sometimes obliterated striae. Puncturation a little stronger than that on the pronotum; random, but with a tendency to run into rows, especially laterally. Side margins weak, epipleura narrowed to apex. Pygidium exposed below the elytral apices and strongly punctured. Underside entirely dark, finely punctured and pubescent. All tibiae with 2 small spines on inner margin at apex.
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EUMOLPINAE HOPE, 1840
After Galerucinae and Cassidinae this is the third largest chrysomelid subfamily. It includes more than 500 genera and 7000 species and more are added to the list every year. The number of tribes is still being worked out; numbers vary from 9 to 14 or 17. They are very widely distributed and most speciose in tropical Africa, tropical America and the pacific rim including New Caledonia. Neotropical fauna are mostly Eumolpini while old world fauna is mostly Typophorini and adoxini. In many areas, eg. New Caledonia and New Guinea, the Eumolpinae is the largest chrysomelid subfamily.
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The form of the adults varies within the typical chrysomelid appearance. Oblong, compact and globose to elongate oval. Antennal insertions generally widely separated. Antennae filiform or slightly thickened. Mandibles with 2 or 3 teeth. Maxillary palps 3 segmented. Similar to chrysomelinae but differ in wing venation and tarsal structure:
-Eumolpinae have two cubital cells while chrysomelinae have only one.
-In Eumolpinae the third tarsal segment is deeply bilobed and extends alongside the last segment. In chrysomelinae it is (viewed from below) either entire apically or has a small median notch.
Claws are generally toothed at the base or bifid but sometimes simple. Elytra completely covering the abdomen. Adults are usually smooth and many are shiny and metallic. Some tropical genera are densely pubescent with long or short hairs, randomly distributed or seriate on the elytra. Some genera have dense pale scales.
In general they oviposit in soil around the foodplant. Larvae all look similar; C-shaped and without ocelli. They develop in soil and feed on roots, often in numbers together. Pupation occurs in the soil. Species vary from strict monophages to oligophages where different populations of a species may choose a different foodplant from a narrow range. For many of the impressive Neotropical species the foodplants are often members of the solonaceae or apocynaceae. In Asia, asclepiadaceae, and in Africa, euphorbiaceae. Very often the larvae feed on the same host as the adults. Some polyphagous species are pests of cocoa and bananas in South America. Angiosperma and gymnosperms also host eumolpines but none are known to feed on ferns which are hosts for a range of alticinae. Species of Syagrus are attracted to Malvaceae. Various species of eumolpinae in America, Southeast Asia and Africa have become specialist feeders on a range of latex producing plants; they have been noticed cutting major ribs of leaves to stop the flow of latex before feeding on the peripheral areas. Many of these species show a typical behaviour of dragging the mouthparts side to side across the leaf to remove accumulated latex.
Although the Eumolpinae are distributed worldwide, they are essentially tropical insects and the diversity decreases away from these regions:
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Madagascar. 39 genera and 256 species (27 endemic.)
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South Africa. 38 genera and 175 species.
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Malaysia. 31 genera and 170 species.
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Australia. 30 genera and 500 species.
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New Guinea. 29 genera and 137 species.
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France. 5 genera and 13 species.
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Fennoscandinavia. 3 genera and 4 species.
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Northern Siberia. 3 genera and 4 species.
They are absent from the isolated pacific islands but there is a single species on the Galapagos.