A portrait of the local willow borer

A portrait of the local willow borer

the essentials in brief

  • The feeding activity of the willow caterpillar can easily be confused with the corridors of other wood-dwelling species. However, the willow borer is not notifiable. The native species attacks willows and poplars in damp trees.
  • Caterpillars eat their way into the wood of ailing trees and leave behind up to two centimeters thick and one meter long feeding tunnels. Typical are drill holes in the bark, drill dust and crumbs of excrement at the base of the trunk and the intense smell of vinegar. Caterpillars are not poisonous.
  • The first step is to cut back into the healthy wood. A quassia solution is the most effective control agent against the caterpillars. Once they have eaten their way into the wood, fighting them is almost impossible.
  • Females lay their eggs on rough bark so that the caterpillars can dig into the wood. They take several years to develop and pupate either in the wood or in the substrate.

Are willow bores notifiable?

Willow Borer

The caterpillars of the willow borer can significantly impair the stability of the infested tree. They can destroy entire trees within a few months. If avenue trees are infested, there is an increased risk of wind breakage. The species is widespread in Europe and prefers to nestle in old willow trees. It can also occur in the home garden.

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However, it is not necessary to report Willow Borers. There is no obligation to report. If you spot a caterpillar feeding tunnels, you should identify the species carefully. Often it is not possible to clearly distinguish the corridors of the willow drill from those of other reportable wood pests.

Shape of the feeding ductsSpecialtyLarvaenotifiable
Willow BorercircularBoring dust is carried outside with the manuredark red back, sides yellow, head capsule blackNo
Asiatic hardwood longhorn beetleovalCoarse drilling chips remain in the corridorslegless, with a brown chitin plate on the front chestYes
Chestnut drillroundonly in branches less than 10 cm in diameterfirst pink, later light yellow with black dotsNo
Poplar buckovallimited to poplars and willowsspherical, front plate granularNo

How to recognize damage from Willow Borers

The willow borer is considered a pest because its caterpillars destroy already weakened shrubs and trees. At first glance, the damage patterns cannot be clearly attributed to this species. There are numerous wood pests that leave holes in the wood. Therefore, pay attention to small traces that reveal the cause.

Typical damage to a willow borer caterpillar


Willow bores preferentially attack older and already diseased trees, to which they can be dangerous. Their irregular bores usually lead down the trunk and can extend to a meter in length. The tunnels reach a diameter of up to two centimeters.

Affected trees suffer from a disturbed supply of nutrients and water, so that branches and leaves dry up. The heavily hollowed wood can easily break in the wind. In the boreholes, rot fungi often spread, which further weaken the tree. If the infestation is severe, there is a risk that the entire tree will die.

Recognize infestation

The willow borer caterpillars' feeding pattern includes large holes in the bark through which excrement and drill dust are disposed of. At the base of an infested tree you will often find reddish-colored drill chips and excrement remains. You can see sap flow by older holes.

Activities of a Willow Borer caterpillar can be recognized by the typical vinegar smell that envelops the tree. In the case of a strong infestation, cracking or rasping feeding noises can be heard at night, which penetrate from inside the tree to the outside. Occasionally the caterpillars move clearly visible on the tree bark.

Willow borer caterpillars' bores are irregularly shaped, up to two centimeters in diameter and usually lead from top to bottom.

This is what caterpillars look like

Willow Borer

While the damage pattern can easily be confused with the activity of other wood-eating caterpillars, the caterpillar is clearly recognizable. It has a yellow colored body and develops a dark red marked back in the later stages of development. The fact that the caterpillars are actually yellow in color becomes clear in wintering individuals. If you find a caterpillar in the uppermost substrate layer in winter, it has lost its typical red color and appears completely yellow.

Typical characteristics:

  • usually 60 to 100 millimeters long
  • Head and parts of the neck shield black all year round
  • very shiny body
  • Warts covered with short white hairs
  • strongly developed mouthparts

Why are trees being attacked

Willow bores are among those wood bores whose caterpillars develop in fresh wood. The adult moths have stunted proboscis and cannot eat. Their only reason for existence consists in reproduction and thus in the maintenance of their species.


The Willow Borer is a butterfly that is widespread and nocturnal. It mainly inhabits moist trees with willows. Therefore, the moths are often to be found in flowing waters but also in parks or meadows. Occasionally, the species are widespread in mixed forests or in house gardens and orchards with old trees.

Females seek out ailing and older trees to lay their eggs, so that in nature they initiate and encourage the decomposition of the old wood. They are likely attracted to the acetic acid-like odor that older larvae exude in the wood.


Frequently infested trees

Willow borers prefer deciduous trees and occur predominantly on various native willow species or ornamental shrubs such as harlequin willow. When woody plants are severely weakened, the moths also lay their eggs in the wood of other tree species. Caterpillars can be found in maple, beech, ash, poplar, oak, walnut, linden and elm.

Preferred tree species:

  • Salix : common willow
  • Betula : Silver birch
  • Alnus : black alder
  • Pyrus : pear
  • Malus : apple

Is the Willow Borer Caterpillar poisonous?

The Willow Borer's caterpillar is aggressive and can use its biting tools when it feels threatened. Children, dogs and cats should keep a distance from the caterpillars. If the caterpillar bites the willow borer, it can be very painful. A bite poses no health risks and the moths are not considered to be toxic to humans.


Wood drill bits and their food value

Numerous wood drill caterpillars have been regarded as food insects since ancient times. Greeks and Romans enjoyed the caterpillars as a delicacy. To get a protein-rich treat, the caterpillars were fattened with flour. In Mexico, the caterpillars of a wood drill, which prefers to live in agaves, have been considered edible since the Aztec times. The caterpillars are still eaten today. The Aborigines living in Australia love this delicacy and eat the caterpillars together with other wood-dwelling caterpillars of the rootworm or longhorn beetle.

First aid by pruning

Willow Borer

Pruning is the most effective way to get rid of the willow borer at an early stage of the infestation. The longer the caterpillars live in the wood, the more they expand. The bore holes of the moth caterpillars can be up to one meter long. Cut back the infested tree until healthy and non-pierced wood is visible. When there are no more caterpillars living in the wood, the wood can recover and sprout again.


Burn the pruning, because otherwise the willow borer caterpillars will continue to develop in the wood and can trigger new infestation.

Refined varieties

If a grafted harlequin willow is attacked by the willow borer, pruning is more difficult. You have to make sure that the finishing point remains intact. This is located in the upper third of the trunk and is significantly thickened, as a processing rice was grafted onto a typical willow species. If you cut off this point, your harlequin willow will no longer sprout but the actual substrate.

Fight Willow Borers

There is usually no need to combat the Willow Borer. Since it is an important part of a functioning ecosystem and is native to Europe, it is not necessarily a massive wood pest. There are no chemical sprays that are approved to control the Willow Borer. Nevertheless, the species does not meet with recognition in the home garden. Elimination measures make sense for trees with ornamental value.

Early control necessary

If the caterpillars have eaten their way into the wood, fighting them is almost impossible. It is therefore important to kill the young caterpillars in the early stages when they are still feeding on the bark. The females prefer to lay their eggs at the base of the trunk, where the caterpillars hatch after a short time.


Quassin is a strong bitter substance that is said to have an insecticidal effect. It is found in some types of bitterwood, such as the Brazilian quassia tree (Quassia amara), and is available as a powder in pharmacies. An extract from the bitter substances helps against willow borer caterpillars if these are sprayed directly. You can also spray the solution on the trees from spring to autumn as a preventive measure. Note that quassin is not beneficial to beneficial insects.

Preparation of the Quassia solution:

  1. Pour 150 grams of bitterwood or quassia powder with two liters of water
  2. Let it steep overnight and then bring to the boil
  3. Let the brew simmer for about an hour
  4. Sieve off the liquid and dilute with ten liters of water

If you use bitterwood, you can dry the pieces after boiling and reuse them. Quassin has a bitter value of 13,000,000. After a 13 million-fold dilution, the solution still tastes bitter. You can also dissolve 250 grams of soft soap (€ 17.27 on Amazon *) in the boiling stock. It ensures that the liquid adheres better to the bark. After two to three days, the residue on the tree should be rinsed off with clean water.

Chrysanthemum extract

Willow Borer

The Dalmatian insect flower produces the natural poison pyrethrum. It serves as a contact insecticide and is used against common pests, their eggs and larvae. Since the active ingredient also endangers beneficial organisms, it should be used outdoors with extreme caution. The knock-down effect occurs within a few minutes. Some insects manage to break down the pyrethrins in the body.

Wire method useful?

It is often recommended to push a sturdy wire into the drill holes. The caterpillar is supposed to be impaled lengthways through the continuous poking. However, the success of this measure is not very great. The feeding tunnels sometimes extend up to a meter into the wood, so that you cannot detect the caterpillar living in it with a wire that is too short. Severely damaged trees can be inhabited by multiple caterpillars, so you never get all of them.


Willow borers need deciduous trees to lay their eggs, the bark of which is moist, rough and furrowed. The caterpillars drill into wood that has been damaged by drought or weakened by diseases and fungi. A regular water supply and potash fertilization are the most important measures to prevent infestation, because they support the vitality of the tree. After cutting the tree, cut wounds should be immediately sealed with a wound closure. Open wounds offer the caterpillar an ideal gateway.


To prevent oviposition, you should regularly lime the trunk. Before doing this, the bark is removed with a stem scraper or a brush, which makes the stem unattractive for the moths.

Way of life and development

Willow borer belong to the family of the wood borer (Cossidae). Males have a wingspan of 80 millimeters. Females reach a size of 100 millimeters, which is why the species is considered the largest small butterfly in Central Europe. As a nocturnal butterfly, Cossus cossus has an inconspicuous brown color. There are black transverse lines on the forewings. The main flight time extends from June to July, with the butterfly being observed from late May to early August.

Egg laying

Females produce around 700 eggs after mating. They place several egg packages, each containing 20 to 50 eggs, in the furrows of rough tree trunks. Weakened deciduous trees such as willows and poplars are preferred for laying eggs. The eggs are protected from drying out by a sticky secretion.


After the larva hatches, it penetrates the bark. It feeds on tree sap and wood fiber and goes through several moults. In the second year, the caterpillars eat their way deeper into the wood and move up the trunk through the tree. Shortly before pupation, the larvae have reached a length of 100 millimeters. Their development takes between two and four years because wood fibers have hardly any nutrients. The young moths hatch from their pupae in summer.


Pupation usually takes place after the third winter in the wood. The caterpillars produce a firm cocoon that is about six inches tall. This consists of web threads and is enveloped by drilling chips when it is in the feeding duct. Pupae in the substrate are often interspersed with earth particles. There are three ways in which willow borers can pupate:

  • Variant 1 : dig into the substrate in autumn to overwinter and pupate in spring
  • Variant 2 : overwinter as a caterpillar in the tree and pupate in the spring in the litter layer
  • Variant 3 : pupate behind an opening in a chip cocoon that is blocked with drill chips

frequently asked Questions

Is the Willow Borer Useful?

Willow Borer

This species, which prefers to settle in pastures, is an important part of a functioning ecosystem. She is one of those wood drills who target fresh and living wood from already sick or weakened trees.

The willow borer accelerates the death of diseased trees by drilling tunnels in the wood that can run through the entire tree. The fungi that then settle in the drill holes also ensure that the wood decomposes more quickly. Thus this moth clears up nature and makes room for fresh seedlings.

How many Willow Borer caterpillars live in an aisle?

As soon as the young caterpillars have hatched from their eggs, they look for a gateway into the wood. They live gregariously under the bark in the first year. This so-called space drain ends in the second year of development. The caterpillars are distributed in the wood in individual corridor systems that are separated from each other. Most of their feeding activity takes place down the trunk, as they pupate at the base of the trunk or in one of the lower feeding holes.

What does a willow borer caterpillar look like?

The larvae of the Willow Borer are between six and ten centimeters long. When looking for a place to overwinter or pupate, the caterpillars can be observed in autumn on the bark of infested trees or on the ground. At this time, your back is covered by a dark to wine red band and the sides appear yellow.

The body is bright and has a few white hairs. The black head and the partly black neck area create a signal drawing. The wood vinegar scent, which is somewhat reminiscent of the goat smell, is typical. The pupae are about six inches long and colored reddish yellow. The individual abdominal segments are provided with dark rows of thorns.

Why do overwintering caterpillars look different than summer caterpillars?

The fact that the caterpillars have a noticeable red color on their back in the late stage of development has evolutionary reasons. It often happens that the caterpillars can be found on the bark or crawl across the ground in search of a pupation place. In these cases they are easy prey.

Using the signal colors black and red, they try to inform potential predators that their bodies are inedible or poisonous. In fact, the caterpillars do not contain any poisonous ingredients, so the red color is an important protection against voracious birds and other predators. During the winter months, they lose this color and appear completely yellow.

What do willow bores look like?

The moths can hardly be seen when they sit on the bark of deciduous trees. Their plump bodies have light gray colored wings that are marbled in dark gray. Parts of the wings appear brownish. The wings imitate the coloring of tree bark, which creates an optimal camouflage. The legs also fit into this camouflage pattern, because they are ringed in black and white. Females are slightly thicker than males. They reach a wingspan between 65 and 100 millimeters.