Ceutorhynchus pallidactylus (Marsham, 1802)
Cabbage Stem Weevil
This native and widespread Palaearctic species of weevil, notorious for the damage it causes to cultivated Brassica crops, is now virtually cosmopolitan and established as a pest wherever suitable crops are grown; it is a serious pest of spring-sown oilseed rape in many European countries, North Africa, Asia, Canada and America. In the UK it occurs commonly throughout England and Wales on both cultivated and wild Brassicas and has also been recorded on various Resedacaea and occasionally Hemp, Cannabis sativa. In the spring and early summer sweeping field margins where suitable crops are grown will produce the adults in abundance but beyond this they occur in a wide range of habitats; parkland, gardens and waste ground etc. although usually in small numbers. The adults overwinter among litter etc. near the host plants and become active in the spring when the soil temperature reaches 8 or 90C, at this time they may fly if necessary to find host plants but most of their time is spent on the ground or on host stems. They feed on tender stems and leaves but the damage they cause is insignificant, oviposition occurs during April and May when females bore small holes into the stems at the bases of petioles and insert small batches of 3 or 4 eggs; each beetle will produce about 40 eggs and the stems will discolour and enlarge around the oviposition sites. Larvae hatch after a week or so and bore down the stems, sometimes reaching the roots, and damaging the plants as they go; in severe infestations each stem will host a number of larvae and many will wilt and die off. Larval development is rapid and they are fully grown in about 3 weeks, during June and July they bore their way out of the stems, or sometimes enlarge the oviposition holes for the purpose, and make their way into the soil where they will pupate in a cell 4 or 5 cm below the surface. The pupal stage lasts about 2 weeks and new generation adults appear from late June to August, these will feed until the autumn when they will enter the leaf litter to overwinter. The economic significance of this species can be drastic; larvae may destroy seedlings and severely reduce the growth of young plants, and it has been shown that seed yield and seed growing capacity decreases from infected plants. Deep autumn ploughing is used to reduce their numbers but pesticides are still commonly used to control infestations.
Although several weevils occur on Brassica crops and many similar sized ceutorhynchine weevils occur in the U.K, the present species will soon become obvious; the overall shape, pale tarsi and random small patches of scales on the elytra are distinctive.
2.4-3.3mm. entirely black but for the red tarsi and dark red antennae. Head densely punctured, with fine erect setae and pale recumbent scales laterally between the eyes. Rostrum very long, glabrous and finely punctured; the scrobes not visible from above. Antennal scape gradually thickened from the middle, funiculus 7-segmented, the club small and pointed at the apex. Pronotum broadest at the base, narrowed to a raised anterior margin and with a strongly sinuate hind margin, strongly punctured throughout and raised laterally behind the middle. Elytra elongate, broadest at the base and narrowed to the obliquely truncate apex, and with a raised humeral prominence. Entire surface with small and pale semi-erect setae which are clearly visible laterally from above, and scattered creamy-white scales which are generally dense behind the scutellum. Stria narrow; each with a single row of scales, interstices rugose and shining; without tubercles except for a subapical group on 5-8 and smaller ones along the apical margin. Legs long and densely scaled; femora weakly toothed underneath, tibiae without apical spurs in the female; in the male there is a conspicuous and sharp apical spur on themed- and hind tibiae.