Cabbage Stem Weevil

Ceutorhynchus pallidactylus

Changed on: 21.05.2019
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There are several weevil species the larvae of which live in the stems of cruciferous plants. The most common one is Ceutorhynchus pallidactylus (old name C. quadridens [Panzer 1795]), which is commonly known as the cabbage stem weevil. Additionally, there is also Ceutorhynchus napi in varying numbers, also referred to as rape stem weevil. Moreover, there are occasional mentions of Ceutorhynchus picitarsis, the black winter stem weevil. Given that the life of these species is similar, we will discuss them as one in this article.

There are several weevil species the larvae of which live in the stems of cruciferous plants. The most common one is Ceutorhynchus pallidactylus (old name C. quadridens [Panzer 1795]), which is commonly known as the cabbage stem weevil. Additionally, there is also Ceutorhynchus napi in varying numbers, also referred to as rape stem weevil. Moreover, there are occasional mentions of Ceutorhynchus picitarsis, the black winter stem weevil. Given that the life of these species is similar, we will discuss them as one in this article.

Pest

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.

 

Symptoms

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.

Control

Spread and Host Plants

Numbers of cabbage and rape stem weevils vary frequently in all regions where oilseed rape, cabbages and horseradish are grown.

These weevils feed of various species of crucifers. The plant that is infested is mostly determined by its availability: winter oilseed rape and cabbage seed bearers are mainly available in early spring after the overwintering period, for instance. Cabbages, horseradish or radishes are usually affected by latecomers.

Prevention and Control

  • Control measures for oilseed rape include the use of chemicals for rape stem weevils or biting insects on oilseed rape to combat adult beetles. At present, these chemicals include various pyrethroids at present. (see Register of authorised plant protection products in Austria).
  • The best time for application is in early spring, as soon as the first beetles have started flying, but have yet to deposit any eggs. Eggs or larvae in rape stems are very well protected and cannot be controlled using chemicals. Applying the insecticide too early when the beetles have not yet arrived on their hosts,  might require a second treatment.
  • Yellow bowls with water should be placed in the oilseed rape fields to determine the flight of the cabbage stem weevils. These yellow bowls must be placed in the field at a warm spot and checked for cabbage stem weevils on a daily basis. It is recommended to add a few drops of washing-up liquid and rectified spirit to avoid freezing on colder nights.
  • Expert literature writes about three beetles per bowl per day as a threshold value for using chemical sprays. The value can vary from region to region and be much higher. It is recommended to keep exact records even on the success of the measures applied, to have experience values for future years.
  • The ideal point of time to combat the pest will also be announced via the alert service provided by the local Chambers of Agriculture (https://www.warndienst.at). In any case, there must be a check whether cabbage stem weevils are actually present in the crop culture in question.
  • Cabbages should be grown as far away as possible from oilseed rape, as this is the beetle’s main host. Additionally, a later growing season is recommended to avoid the flight period of the beetles. 
  • Damage caused by the larvae’s spring feeding on the petioles of cabbages is usually insignificant. However, the feeding of newly hatched beetles of the new generation in early spring (when the oilseed rape ripens) should be watched for. These beetles infest adjacent cabbages, but won’t deposit any eggs, though.
  • Use close-meshed fly screens or fleece to keep the beetles from getting to the crops, when sowing seed bearers.

Photo credits: Image1, Image 2, Image 3: © P. P. Kohlhaas/AGES, Vienna; Image 4,Image 7: © I-PGH/AGES, Vienna; Image 5, Image 6: © W. Dukat/AGES, Vienna.

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Cabbage stem weevil
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Cabbage stem weevil larva
Larve des Kohltriebrüsslers
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Rape stem weevil
Rapsstängelrüssler
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Damage to stem
Schadbild Stängel
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Borehole on the vein of a cabbage leaf
Bohrloch und Bohrgang an der Rippe eines Kohlblattes
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Cavity made by beetles on a cabbage leaf
Fraßgrube der Käfer an einem Kohlblatt
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Boreholes in a radish bulb starting from the leaf veins
Bohrgänge in Radieschenknolle von den Blattadern ausgehend
 
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