Cabbage seedpod weevil (Ceutorhynchus obstrictus)

Cabbage seedpod weevil biology & life cycle

Takes about eight weeks for the cabbage seedpod weevil to develop from egg to adult stage. There is only one generation per year. The adults overwinter in leaf litter in shelterbelts, ditches and woodlots. Adults are usually found on the buds and flowers of the plant. In the spring the adults can be found on other crucifer plants (stinkweed, wild mustard, flixweed, and volunteer canola) prior to crop flowering. Mating occurs in the spring until early pod developing, when the females can lay eggs in the seed pod. Once hatched, the larvae feed on the developing seeds. When mature, the larvae chew round exit holes in the pod and drop to the ground. Pods inhabited with the larvae will appear distorted.

Identifying Cabbage seedpod weevil

Cabbage seedpod weevil: eggs

The eggs are very small, oval and opaque white. Usually only one egg will be found in a pod, however two or more eggs can be found during times of serious outbreak.

Cabbage seedpod weevil: larvae

Larvae are white and grub-like, without legs or eyes.

Cabbage seedpod weevil: Pupae

Mature larvae.

Cabbage seedpod weevil: adult

Adults are ash-grey and approximately 3 to 4 mm in length. They have a prominent curved snout similar to most weevils.

Cabbage seedpod weevil: management

The earliest flowering canola crops tend to have the highest risk from cabbage seedpod weevil and should be monitored very closely.

Cabbage seedpod weevil adult abundance is monitored by taking sweep net samples. Sampling should begin when the crop first enters the bud stage and continues through the flowering period.

Select ten locations within each field, and at each location, count the number of weevils from ten 180° sweeps. Sampling locations should include both the perimeter and interior of the field to obtain an accurate estimate of weevil numbers throughout the field. At the crop’s very early flower stage, weevils congregate along the edges, and it may be possible to simply spray the edge if done early enough.

A few other weevil species may also be found occasionally in canola, but these do not require control measures. The most common of these is a closely related species, Ceutorhynchus neglectus, about one-half the size of the cabbage seedpod weevil that will feed on canola but prefers flixweed.

Beneficial insects may be present, and their conservation should be taken into account before spraying insecticides. If the land was planted to cereal the previous year, parasitoids of key pests such as the wheat stem sawfly, cereal leaf beetle or wheat midge may be present, and spraying could reduce their populations and lead to future pest problems.

Economic thresholds
Chemical control is recommended when an average of 3 to 4 adult weevils are collected per one 180° degree sweep sample at 10 to 20 per cent flowering.

Environmental control
Cooler temperatures and rainfall in August favour the development of the new generation of weevils and may lead to higher numbers in the following year.

Source: Government of Alberta