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Several species of weevils whose larvae live in the canola stems are encountered in canola fields; for example, the small or spotted cabbage shoot weevil(Ceutorhynchus pallidactylus). Only the large canola stem weevil(Ceutorhynchus napi) is described below, as it causes the most extensive damage.

Biology

The adult beetles overwinter fully developed partly still in the pupal cradles of the previous year, but also in protected places, such as the ground litter of bushes, forest edges or hedges. As soon as air temperatures exceed 12 °C, the females begin to fly to rapeseed fields. This is usually the case from March onwards.

After about ten days of ripening feeding, the female gnaws holes in the canola stalk to lay her eggs, secreting a chemical substance that stimulates the plant tissue to form tissue growths. The female then lays one egg in each of these cavities and the tissue heals. The transparent eggs are completely hidden in the leaf tissue. Each female lays an average of 150 eggs. About six days after egg laying, the larvae hatch and begin feeding inside the stems. In the process, they create feeding tunnels in which they slowly work their way toward the soil.

To pupate, the larvae leave their host plant and transform into a pupa in a small, self-dug burrow. However, adult beetles do not leave this pupal cradle until the following spring in some cases.

Host plants

The pest lives on a wide variety of cruciferous species. Which ones are attacked depends primarily on their availability. For example, in early spring after overwintering, mainly winter rape and cabbage seed bearers are present. Cabbage vegetables, horseradish or radishes, on the other hand, are only attacked later by stragglers.

Distribution

The great canola stem weevil is widespread in Europe and is usually very common in all areas where canola, cabbage vegetables or horseradish are grown.

Propagation and transmission

The flight-capable beetles actively migrate into canola fields as soon as air temperatures exceed 12 °C. The time of flight into the canola fields can be determined by means of yellow trays.

Economic importance

Maturation feeding by females prior to oviposition on leaves is rather insignificant. However, the chemicals secreted during oviposition cause tissue growth on the stems, which can lead to cracking and buckling of the canola plants. The canola plants damaged in this way are further weakened by subsequent frost or drought and may thus be severely reduced in yield.

Prevention and control

• Control measures on rapeseed are carried out by chemical control of the adult beetles with agents against rapeseed stem weevils or biting insects on rapeseed (see list of plant protection products approved in Austria).
• The best time for application is in early spring as soon as the first beetles fly but no eggs have been laid yet. Eggs or larvae in the canola stems are very well protected and cannot be controlled chemically. If treatment is done too early and the influx from wintering grounds is not yet complete, a second treatment may need to be done.
• To detect the flight of the canola stem weevil, place yellow water-filled paint trays in the canola field. These "yellow trays" must be placed in a warm spot in the canola stand and checked daily for beetles. It is advisable to add a few drops of washing-up liquid and also some spirit. This will prevent the water from freezing on cold nights. During the inflow, the canola stem weevils can be caught well in yellow trays. With the onset of egg laying, this is no longer the case.
• The literature cites about three beetles per yellow pan per day as the threshold for spraying. However, the locally adapted threshold value may deviate from this and be considerably higher. It is recommended to keep accurate records also on the success of the measure, so that experience is preserved for the next years.

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