Cabbage stem weevil and cabbage seed weevil
Ceutorhynchus pallidactylus, Ceutorhynchus assimilis
The head of the Small Cabbage Weevil (Ceutorhynchus pallidactylus), which measures only 2-3 mm, is elongated in a proboscis shape, as is typical of weevils, and bears knobbed, seven-limbed antennae. Its dark brown body surface is strongly hardened (sclerotized) and irregularly covered by brownish scale hairs in various shades. This gives it its mottled gray appearance, which is why it also bears the German name Gefleckter Kohltriebrüssler. The presence of a yellow-brown spot behind the neck shield and reddish-brown foot limbs make it easy to distinguish from other species, such as the closely related cabbage stem weevil.
The larvae, up to 5 mm long, are whitish in color, bear a light brown head capsule and are legless.
There are several species of weevils whose larvae live on a wide variety of cruciferous plants. The most common is the small cabbage stem weevil.
In addition, the large cabbage stem weevil(Ceutorhynchus napi), the cabbage pod weevil(Ceutorhynchus assimilis) and the cabbage gall weevil(Ceutorhynchus pleurostigma) also occur. The black cabbage shoot weevil(Ceutorhynchus picitarsis) is also occasionally mentioned. The lifestyle of these species is similar and they produce only one generation per year.
Small cabbage weevil
The adult beetles overwinter in sheltered places, such as in the ground litter of shrubs, forest edges, or hedgerows. As soon as the air temperature exceeds 12 °C in March, the females begin to fly and seek out their host plants. After about ten days of maturing feeding, holes are gnawed into the stem-near area of leaf veins for egg laying. Three to four eggs are then laid in each of these cavities. Afterwards the tissue heals again. The transparent eggs are completely hidden in the leaf tissue. Each female lays an average of 150 eggs in this manner.
After about six days, 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. The larvae leave their host plant and pupate in a small, self-dug burrow. In June of the same year, beetles hatch from them, but they do not reproduce until the following year. While the main mass of weevil larvae develops on canola in early spring, stragglers may be found on cabbage vegetables, radishes, or horseradish somewhat later in the year.
Cabbage pod weevil
This weevil species does not fly until canola is in bloom. It lays eggs in the pods of cruciferous vegetables after a ripening feeding. The larvae feed on the seeds and, after developing, bore out of the pod wall to pupate in the soil. After hatching in the summer, the beetles feed on flowering crucifers before going to their wintering grounds. The cabbage pod weevil favors infestation by the cabbage pod gall midge(Dasineura brassicae).
Large cabbage shoot weevil
In contrast, the greater cabbage shoot weevil flies several days earlier and lays one egg in each feeding hole in the stem of its host plant, not in the petioles. When laying eggs, each female secretes a substance that stimulates the plant tissue to grow tissue. The beetles hatch after overwintering rather than in early summer.
Cabbage gall weevil
Unlike its related species, the larvae of the cabbage gall weevil live in pea-sized growths (galls) in the root neck or main root of young plants. Usually the beetle hibernates here to lay eggs in the spring after a maturing feeding. However, it is also said that the females lay eggs in the fall, so that the larvae overwinter in their galls.
Black cabbage shoot weevil
The black cabbage shoot weevil begins laying eggs as early as fall. Damage to the plants is caused by the larvae already over the winter months.
The feeding tunnels of the larvae of the small cabbage shoot weevil are found in April and May in the petioles and stems of cabbage vegetables, rarely also in the bulb of radish. In June, the adult beetles are often found on young heart leaves that are still exposed or also on the leaf veins of older leaves, where they cause pit-shaped feeding holes. Pods damaged by the cabbage pod weevil have bright spots and pinhead-sized boring holes.
Yield losses from these weevils can be 20-30%, depending on the weevil species and the condition of the crop. Severe damage can occur, especially to cabbage seed bearers, after spring planting. The puncture site caused by the oviposition as well as the borehole of the larvae can also serve as entry ports into the plant for other pathogens such as fungi or bacteria.
Cabbage shoot weevils feed on a wide variety of cruciferous plants (canola, cabbage, mustard, radish, turnip, radish, garden cress, and many others). However, which ones are attacked depends primarily on their availability: for example, in early spring after overwintering, mainly winter rape and cabbage seed bearers are available. Plants such as cabbage vegetables, horseradish or radishes, on the other hand, are only attacked later by "stragglers".
Prevention and control
- Chemical control of the adult beetles is possible with agents against the large and spotted cabbage shoot weevil/canola stem weevil/cabbage pod weevil or biting insects (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 have not yet laid eggs. Eggs or larvae in the canola stems are very well protected and cannot be controlled chemically. If treatment is done too early, a second treatment may be needed.
- Yellow trap trays or yellow glue boards are used to determine the start of beetle flight:
- Set up along field edges and in the crop and check daily for beetles.
- Adding a few drops of dishwashing liquid and methylated spirits is recommended - this will reduce freezing on cold nights.
- Observe thresholds for spraying.
- As a preventive measure, plant cabbage as far away from canola as possible and choose later planting dates to avoid the beetle flight period.
- When planting seed bearers in early spring, wrapping them in tightly meshed fly screen or fleece has proven effective in preventing the beetles from flying in.
- Cultivation of early-shooting canola varieties.
- Encourage canola development in the juvenile stage.
Last updated: 24.11.2021