Biological control of the cabbage seedpod weevil
Controlling the cabbage seedpod weevil in an environmenally friendly way
The cabbage seedpod weevil, Ceutorhynchus assimilis, is a somewhat pear-shaped beetle, about 2-3 mm long. Its colour is black with grayish-white hairs covering its body and legs.
A cabbage seed weevil
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The very long, narrow snout is slightly bent and its antennae are clubbed. It is especially the feeding larvae that can cause damage by consuming seeds in the pods of a variety of crucifers.
Biology of Ceutorhynchus assimilis
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The first adults of the cabbage seedpod weevil, Ceutorhynchus assimilis (Orde: Coleoptera; Familie: Curculionidae) appear in spring and feed from flower buds or young fruits. They reach their highest flight activity when their host plants - oilseed rape, cabbages, swede and other crucifers - are in full flower, from late April to June. Females cut a hole in the wall of a pod and lay in it one white, oval egg. The larva hatches after 8-9 days and feeds from the seed buds. After about 4 weeks, during which the larva matures, it destroys 3 to 5 seeds. The mature larva cuts a hole of about 1 mm diameter in the pod wall through which it leaves the pod and drops to the ground. It pupates 5-10 cm below the surface and emerges as a new adult about 3 weeks later, to feed from cultivated and wild crucifers. In August, the adults hide in leaf litter to overwinter.
Damage by the cabbage seedpod weevil
Although adults feed from flowers, buds, pods and stems of their host plants, they do not normally cause any serious damage. Feeding larvae, on the other hand, can cause damage by consuming seeds in the pods. However, female beetles that feed and cut holes for egg-laying makes it easier for the damaging brassica pod midge, Dasineura brassicae (Order: Diptera; Family: Cecidomyiidae), to enter the pods. The larvae of these gall midges also feed from the seeds as well as from the internal wall of the pod. In addition, feeding causes serious spillage of seed because of deformation, yellowing and bursting of the pods. The presence of the brassica pod midge, therefore, makes the cabbage seed weevil a much more serious pest.
Sites that give additional information
- Growing Small Farms: Cabbage Seedpod Weevil
More information on and beautiful photo's of the cabbage seed weevil
Natural enemies of C. assimilis
The cabbage seed weevil has a variety of natural enemies. Tiny wasps of the family Mymaridae, Antoniella declinata and Anaphes autumnalis, are egg-parasites. Other wasps of the families Pteromalidae (Trichomalus perfectus) and Braconidae (Microtonus melanopus), are parasites of the larvae. Of these, T. perfectus is the most widespread and abundant in the UK and can attack over half the larvae in plots untreated by insecticides.
Treatment - for those gardeners or smallholders that do not grow crucifers for their seed
- pull up brassicas, swedes etc. before they start flowering, to prevent seed spillage in the garden.
Treatment - for those gardeners or smallholders that are interested in seed
- place yellow traps filled with soap water to kill adult weevils.
- check plants regularly for the presence of weevils, by shaking the plants (the weevils drop to the ground when disturbed) and only if a threshold level of two weevils per plant is exceeded should you use insecticides. It is recommended to use a pyrethroid during the flowering period, as this will not harm the most important parasitoid, T. perfectus, which arrives after the main flowering period to search for larvae of C. assimilis in which to oviposit. However, pyrethroids are harmful to bees as well as to parasitoids of the brassica pod midge and should therefore only be used as a last resort.
- another often recommended insecticide against the cabbage seed weevil is triazophos. As this has a detrimental effect on T. perfectus, I advise strongly against its use.