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The willow borer is one of the most feared pests. Wherever he settles, he indulges his immense hunger for wood. The consequences are almost always immense if he is not stopped. With the right guidance, you can combat the willow borer effectively. You will receive this in the following guide about this pest and you will also find out whether it is poisonous.

Dangerousness of the Cossus Cossus

wood damage

The cossus cossus, as the willow borer is called in science, is one of the moths. This one is not directly dangerous, but the caterpillarsthat develop from its eggs. Own these Cravings for wood and eat their way through entire tree trunks. As a result, they rob trees of all life energy, hollow them out from the inside and, in the worst case, destroy entire stands of trees in the garden.

fungal infections

The presence of the pests creates an optimal basis for fungal infections. Once the bark has been eaten away and the moist fibers are exposed, the wounds on the trees provide ideal conditions for fungal infestation. For this reason alone, the caterpillar is so feared by tree lovers and should be fought as soon as possible.

Dangerous for humans

The branches in particular quickly lose stability as a result of the wood being eaten away and the caterpillar hollowing out. They may break off at the slightest impact, such as a bird perched on them or snow. If people are below the affected branches, this can lead to serious injuries in the worst case. Damage to property is also not uncommon.

Reproduction and long as caterpillars on the way

The Wood Borer Pest lures in a certain way the fertilized butterflies on. It can happen that trees that have already been confiscated develop into a true breeding station. Considering that the moths lay around 700 eggs on the lower trunk of the tree, an unimaginable number of caterpillars can eat the wood, preferably fruit trees, birch or poplar.

detect pest

In order to combat willow borers or their caterpillars, they are to be recognized as such in the first instance. This can be done by identifying visible caterpillars or by the typical damage pattern. Ideally, both factors can be combined and a definitive conclusion drawn about a willow borer infestation. Fruit trees, birches and poplars are particularly affected, although other deciduous trees are also affected.


The caterpillars usually go unnoticed when they are tampering with your trees. It's less because of your size than because you nocturnal are. During the day they hide and are therefore rarely discovered. During the day they usually stay in the tree cavities or cave passages they have created. Here you continue to nibble despite a lot of sleep, which can often be heard as a light "creaking" of the tree. If there is a suspicion of an infestation, this could be another sign that it is the caterpillar of the Cossus Cossus.
Other identifying features are:


  • Moths in close proximity to trees
  • Moths have an enormous wingspan of between eight and ten centimetres
  • Gray to grey-brown body
  • Light gray wings with dark gray marbling


  • Length of about ten centimeters
  • Black in color from head to neck
  • Red back area
  • The lower trunk area is orange to brown-reddish
Willow Borer, Cossus Cossus as moth and caterpillar

damage picture

In order to be able to identify a wood borer or its caterpillars, the existing damage to trees is usually sufficient. In most cases, several characteristics apply, as they occur in the case of an infestation:

  • Unpleasant odor surrounds the trees, reminiscent of vinegar
  • At first only a few withered branches in one area, then the remaining regions were also affected
  • Leaves wither, dry up and fall off
  • Typical boreholes in the tree bark
  • Wood chip on the ground near the tree trunk
  • Drilling chips are reddish in colour
  • If you look closely, you can see feces in the shavings
  • Older drill holes turn black
  • Brittle tree bark forms and crumbles
  • Tree seems weaker and weaker

Combat: Instructions

Fighting is imperative briskly recommended as soon as the cossus cossus is discovered or all signs point to an infestation, so that the affected trees are saved and the infestation does not spread to neighboring trees. This is usually the case if control is carried out too late. For this reason, trees should always be checked regularly for a willow borer infestation so that control can take place as quickly as possible.

Since the caterpillars stay inside the tree, they are difficult to catch and fighting them is even more difficult. This is only possible when the caterpillars are on the tree surface, unless you try it with a wire or use a special manure.

Remove affected plant parts

If branches and twigs show signs of infestation, they should be cut down to the healthy wood. In this way you remove caterpillars sitting in the twigs and branches and at least reduce their number. This increases the chances of success of the treatment with quassia wood manure and should be considered accordingly first measure take place. The cut plant parts are to be burned and must not be disposed of in the compost under any circumstances.

Impaling with wire

The attempt to impale the caterpillars with a wire inside the tree is not really effective and is also very tedious and time-consuming. To do this, insert it into a drilled hole and push it in all directions in quick movements until you meet the resistance of the inside of the tree.

This method does not have much chance of success, although the caterpillars are certainly easy to reach due to their size. But you won't be able to kill them all, but you shouldn't be deprived of this method.

quasia slop

Fighting with quasia sludge has significantly more chances of success. This is made from quassia wood, which is available as bitter wood in well-stocked pharmacies. It is effective against eggs and caterpillars.

Instructions for production and use

  • Place 150 grams of quassia wood in a pot with two liters of water
  • Leave the pot to steep for 24 hours
  • Then boil the water for an hour
  • Sifting wood out of the water
  • Add 250 grams of soft soap to the manure and stir well
  • Pour slurry into a spray bottle or pump
  • Spray directly into boreholes and spray onto branches and the area close to the trunk
  • Treated trees should be rinsed vigorously with clear water after about two to three days
  • Be careful when spraying: the liquid manure is also poisonous to other insects
  • Slurry can be kept for a few months and should be used several times as a preventive measure from spring to autumn

tip: If you let the quassia wood dry after the first use, you can use it three more times to make new manure.

Chemical control

So far, no chemical product is available or approved that could be used effectively against the willow borer caterpillar.

Natural predators

Caterpillars usually have some natural predators. With these, this is more difficult, because the willow borer caterpillars are not only quite large, but also defend themselves violently. You can still try with us parasitic wasps make. Plenty of supplies should be obtained from the garden trade. Caterpillars are one of their delicacies and with larger numbers of parasitic wasps there is a good chance that the repellent methods will not be sufficient to prevent feeding.

Last instance: tree felling

If the tree poses a danger due to extensive hollowing out or if a spread to neighboring trees is to be prevented immediately and without compromise, then a tree felling is a solution. You may need a felling permit here. After felling the tree, all parts are to be burned.

Is the wood drill poisonous?

Although the willow borer caterpillars are quite aggressive and also have a mandible, they are not poisonous and could cause symptoms of poisoning. If you have children in the house/garden or dogs and cats running around, caution should still be exercised. The bite can be very painful. Children may react to this with skin irritation. Dogs and cats are usually kept at a distance by spraying out the vinegar-smelling secretion of the caterpillar when they come too close. If they are bitten anyway, nothing more than skin irritation is to be expected.

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