Ceutorhynchus assimilis Paykull Cabbage seed weevil

This weevil is a generally common pest of brassica seed crops; economic damage also occurs on certain vegetable brassicas. Brassica seed crops: adult feeding is of no direct significance, although feeding punctures may allow female brassica pod midges, Dasineura brassicae (see p. 171) to deposit eggs into pods. Most damage is caused by the larvae, which feed within the pods on the developing seeds; each larva (typically one per infested pod) usually destroys several seeds. Attacks are particularly common on oilseed rape and brown mustard (white mustard is immune to attack) but crops are often able to fully compensate for any damage caused. Vegetable brassicas: most significant damage is caused in summer, from mid-July to mid-August, by adults reared on nearby rape crops. Such individuals may feed on the buttons of Brussels sprout, producing brown markings on the outer leaves, and on developing cauliflower curds, causing the tissue to turn pinkish; the weevils may also feed on the young leaves of other vegetable brassicas, and check the growth of recent seedlings or transplants.

BIOLOGY

Overwintered adults emerge in May. They are attracted to flowering brassicaceous weeds, such as Sinapis arvensis, and also to brassica seed crops, especially oilseed rape and white mustard. Here, they feed on the buds, flowers and developing pods. After mating, the females deposit eggs singly in the young pods, usually from late May onwards. The weevils make distinct egg-laying punctures in the pod walls and then mark the pod with a pheromone to deter other ovipositing females. The eggs hatch in 1-2 weeks and the larvae then attack the developing seeds. Larvae feed for 3-6 weeks, depending upon temperature. When fully fed, usually in late June or July, each larva bites its way out of the pod, leaving a pinhead-sized exit hole in the wall (Plate 5a), and drops to the ground. Individuals then pupate in earthen cells a few centimetres below the surface. Young adult weevils appear about 2 weeks later, usually from mid-July to August. These weevils feed on brassicaceous weeds, and may also attack vegetable brassicas, before eventually taking up their winter quarters amongst debris in nearby woodlands or hedgerows.

DESCRIPTION

Adult 2.2-3.0 mm long, lead-grey, unicolo-urous, with a long, slender rostrum (Fig. 229). Egg 0.6 x 0.4 mm, oval, creamish-white and translucent. Larva up to 5 mm long; body creamish-white and sac-like; head light brown. NOTE Adults of Ceutorhynchus Horalis (Paykull). a greyish-black weevil, appear to be increasing in numbers on oilseed rape crops. Although of no pest status (it breeds on weeds such as Capsella bursa-pastoris and Erysimum cheirantholdes), the adults are sometimes mistaken for Ceutorhynchus assimdis and can, therefore, contribute during pest monitoring to overestimates of the number of seed weevils present in a crop. The weevils are most readily distinguished

Fig. 229 Cabbage seed weevil, Ceutorhynchus assimilis (x20).

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Fig. 229 Cabbage seed weevil, Ceutorhynchus assimilis (x20).

by their small size (1.5-1.8 mm long) and somewhat Apion-like appearance; the antennae of this species have just six funicular segments (there are seven in C. assimdis).

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