Ceutorhynchus obstrictus (Marsham, 1802)
Cabbage Seedpod Weevil
Native to and generally common across almost the entire Palaearctic region, this species occurs throughout Europe from the Mediterranean north to the UK and southern parts of Fennoscandia, it is also widespread across Asia Minor and North West Africa and was first reported from North America in 1931, presumably from specimens accidentally imported from Europe, and is now established, widespread and a serious pest of oilseed rape and its commercially grown varieties in both the United States and Canada. In the UK the species currently known as C. assimilis (Paykull, 1792), the root collar-gall weevil, was formerly known as obstrictus and so older records need to be treated with caution, the present species is, however, known to be generally common across England and Wales and widespread, though much more local, in Scotland and Ireland. The species is widely polyphagous; in general on Brassicaceae and particularly on species of Brassica L. and Sinapis L., with more than 30 wild and cultivated forms recorded as hosts, but the list of wild species is likely to be more extensive. Habitats include wherever the hosts are common, especially on disturbed sites and waste ground, and adults often appear on wild plants in the spring before migrating to crops where they may become serious pests. Adults overwinter among litter or in the soil and become active in the spring when the daily temperature reaches 7 or 8°C, they feed for a while on stems, buds, stamens and unfolding foliage, gnawing small holes into plants but doing little damage, before mating and migration begins. UK populations peak during May and June but this may vary with latitude in Europe. Oviposition begins in May and continues into late June, females bore a small hole into a seed pod and lay a single egg or exceptionally two eggs and each will lay between 40 and 150 eggs over a few weeks, Larvae emerge after a week or so and each will consume between 3 and 7 developing seeds, they are fully-grown after about 4 weeks and now they bore through the pod and drop to the ground to pupate. Pupation occurs in a subterranean cell 2-3cm below the surface and new-generation adults occur during July and August. These adults will feed into the autumn when the temperature falls but
Ceutorhynchus obstrictus 1
Ceutorhynchus obstrictus 2
will not mate until the following spring. In warmer parts of Europe mating and oviposition may begin in late summer and larvae may overwinter and pupate the following spring to produce adults from May. A wide range of crops may be damaged during heavy infestations, most commonly cabbage, turnip, radish, mustard and rape, although some crops such as rape can partly compensate by producing more seed pods and large populations generally tend to attract abundant parasite and predators. Adult beetles feed extensively and sometimes feeding damage is visible on young plants but it is the larvae that can significantly diminish yields e.g. a single female ovipositing on 4 rape plants may cause a 5-10% reduction in yield. Further and equally serious damage may follow as pod midges (Dasineura brassicae (Winnertz, 1853)) may oviposit in holes left by adults and their larvae may effectively destroy the entire yield by causing pod shatter. Pesticides are still being used to control the weevils but less destructive methods such as growing crops further apart, using traps and drying out seeds soon after harvesting are being widely adopted in Europe.
2-3 mm. A small and rather nondescript weevil with a broad and elongate body and long rostrum, entirely black but appearing grey due to extensive pale pubescence, most specimens have paler scales forming a longitudinal stripe on the pronotum and sometimes continuing onto the elytra just past the scutellum and on the epimera which are visible from above in front of the elytral humeri. Head with large convex eyes and diverging temples, the cuticle between punctured and evenly convex, rostrum long, narrow and evenly down curved. Antennae long and slender, the scape gradually thickened in the apical third, the funiculus 7-segmented and the club long and pointed, in the male inserted about the middle, in the female behind the middle. Pronotum transverse, broadest near the base and narrowed to a sub apical constriction, in side view the apical ‘collar’ only weakly raised, surface convex with a small tubercle either side of the disc. Elytra weakly curved from sloping shoulders to separately rounded apical margins, in side view gently curved to a shallow apical declivity, striae narrow, each with a row of very small pale scales, interstices with two or (in places) three rows of elongate pale scales. Legs long and robust, middle and hind femora without a ventral tooth, male mid and hind tibiae with a sharp internal terminal tooth, female tibiae unarmed, claws without a basal tooth.