The front of the head of the 3 mm small cabbage stem weevil is elongated into a long, slender proboscis and has elbowed (geniculate) antennae that are made up of seven segments. Its dark-brown body is heavily sclerotised and covered in irregular scales of various hues of brown, giving the weevil its mottled look. The last leg segments are red-brown and bear divided claws. An ochre-coloured spot made of bright scales is visible behind the beetle’s scutum. The back part of the elytra, the hardened forewings, are covered with fine teeth. Adult beetles overwinter in protected locations, such as the litter of shrubs, edges of forests or hedges. As soon as the air temperature rises above 12 °C, the females begin to fly and visit their host plants. This usually happens from March onwards. Following a 10-day maturity feeding phase, the female beetles chew holes into leaf veins near the stem to deposit the eggs. Then, the female turns around and deposits 3-4 eggs in each of the holes. The holes are closed by new plant tissue. As a result, the translucent eggs are completely encapsulated in the leaf tissue. Every female beetle lays around 150 eggs. The larvae hatch after about six days. They have a brown head, no legs and begin to feed on the inside of the stems, creating boreholes and working their way slowly towards the ground.
Once it is time to pupate, the larvae leave their hosts and begin to pupate in a small hole in the ground they make themselves. The beetles hatch in the June of the same year, but only start reproducing the following year as the ovaries of the female beetles have not developed at the time and also require low winter temperatures to become fertile. While the majority of beetle larvae develop on oilseed rape in early spring, some late developers can be found on cabbage, radishes or horseradish a little later in the year.
Unlike the cabbage stem weevil, the rape stem weevil is considerably larger at up to 4 mm in length. Moreover, its body is covered in white, regularly arranged scales, giving it a grey striped appearance. This beetle begins to fly a few days earlier and deposits one egg per borehole into the stem of its host plant and not the petioles. While depositing the egg, each female secretes a substance which stimulates the plant tissue to grow quickly. The adult beetle hatches from its pupa following hibernation and not in early summer after larvae development. The cabbage seed pod weevil also lives in oilseed rape fields and is 3.5 mm long and grey in appearance thanks to its fine, white scales. In addition, it features a delicate line on its scutum. However, as this species begins to fly only at the time the oilseed rape flowers, confusion with other beetles is unlikely.
The type of damage depends on the weevil species, the season and the host plant’s development stage. Boreholes including larvae can be found on oilseed rape stems in April and May. In most cases, a number of larvae live in each stem, resulting in the shoots appearing hollow. Given the fact that the shoots will be bent and the stems burst in the case of a rape stem weevil infestation, the growth and yield of such plants will be impeded severely. The damage caused by the cabbage stem weevil is significantly less, by contrast. Occasionally, the petioles of cabbages, seldom radish bulbs, will be infested later in the year. However, the feeding damage by larvae caused in spring has little impact on most cabbage species. Unlike cabbage seed bearing plants, which are exposed to a much higher risk as they can easily begin to rot after being fed on by the larvae. Furthermore, the newly hatched, adult cabbage stem weevils can still cause severe damage in June. Cabbage plants that are grown near to ripening oilseed rape are especially at risk. The adult beetles eat holes and cavities into the veins of older leaves on younger leaves or still open heart leaves.