Ceutorhynchus assimilis (Paykull, 1792)
Turnip Gall Weevil
For many years the name assimilis was applied to the Cabbage Seed Weevil, C. obstrictus, and so older literature and records must be treated with caution, this can cause confusion but the life-histories of the species are very different and because both are sometimes serious crop pests they may thus be recognized from the many detailed accounts on line and in the literature. To add further confusion the present species is widely referred to as C. pleurostigma (Marsham, 1802). C. assimilis occurs throughout most of the Palaearctic region and in Europe it is generally common from the Mediterranean to the UK and some southern provinces of Fennoscandia, it is widespread in North Africa and is often quoted as an established adventive in North America but these records probably refer to C. obstrictus as the present species does not occur there. In the UK it is locally common throughout England and Wales, although almost certainly less so than during the first half of the 20th Century, and less so throughout Ireland and Scotland north to Orkney. Host plants include a very wide range of both wild and cultivated Brassicaceae (>30 species and cultivars are recorded) and a few other species are occasionally used e.g. Hemp (Cannabis sativa L., Cannabaceae), Wild Mignonette (Reseda lutea L., Resedaceae) and Nasturtium (Tropaeolum majus L., Tropaeolaceae). Large populations sometimes occur and the species is an occasional serious pest of both domestic and commercial brassica crops, especially turnips, kale, Brussels, various types of cabbage and occasionally, though no less destructive, on winter oilseed rape. Adults are active from April until September but specimens are sometimes recorded much earlier or later, they peak in abundance during July and August and sometimes again in September; there are two overlapping generations and so adults tend to be common throughout the spring and summer. In the first case overwintered females feed and oviposit from March until May and the resulting adults appear in July and August, these will feed before overwintering under plant debris or in the soil and repeating the cycle, they become active from early spring and in this case development from egg to adult takes about two months. In the later brood females oviposit from August until September or November and these larvae will feed before overwintering, completing their development through the spring and producing adults from the end of May, in
this case the development from egg to adult takes up to 10 months. Females bore small holes into roots or root collars just under the soil and deposit small batches of eggs which will hatch after a week or two, they continue laying over two or three months and each will produce between 120 and 150 eggs. Larvae mine through lower parts of stems and roots, producing conspicuous pale and hollow galls as they go, sometimes several mines will merge and produce large and complex galls up to 6 cm long and 2 cm wide or these may merge and form an irregular swollen mass. Pupation occurs in the galls and this stage lasts about two weeks, overwintering larvae remain within the galls, they resume development early in the spring although feeding may continue during mild winter spells. Adult feeding may become locally extensive within a crop but in general does not affect the plant too adversely, the most significant damage is caused by larvae to young plants during autumn and early winter and at this time severe infestations may severely retard plant development, older and well-established crops are rarely damaged and may even be avoided by ovipositing females but certain crops, especially Swedes and turnips, tend to be more vulnerable.
2.3-3.0 mm. Elongate and broadly-oval, body entirely black or dark grey with scattered long pale scales, often obvious along the centre of the head and pronotum, underside densely covered with pale scales that are obvious from above on the epimera in front of the shoulders, legs substantially dark, usually with the tarsal apices reddish, antennae pale to dark brown with dark grey clubs. Head with small, weakly convex eyes and diverging temples, surface flat and strongly punctured throughout. Antennae long and slender; scape thickened in the apical third, funiculus 7-segmented and the club long and pointed. Pronotum transverse, broadest towards the base and evenly curved to a (sometimes weak) sub-apical constriction and weakly-raised apical margin, surface strongly and densely punctured, a little more strongly so that the head, and with a small tubercle either side of the disc. Prosternum between the coxae, mesosternum and metasternum not impressed to form a rostral channel. Elytra broadest behind sloping shoulders and evenly curved to continuous apical margin, striae deep and narrow, each with a single row of pale adpressed scales, interstices roughly and mostly transversely sculptured, with dark recumbent pubescence which is hardly noticeable and scattered elongate pale scales. Legs long, robust and with elongate pale scales throughout, middle and hind femora with a distinct ventral tooth, male middle and hind tibiae with a sharp internal tooth at the apex, in the female unarmed. All claws with a distinct tooth on the internal margin.