The willow borer is a large moth that is rarely seen. Its caterpillars, on the other hand, are not uncommon, these are found quite often. The willow borer lives mainly in alluvial forests, but is also found in settlement areas. It is most often found near pastures, on flowing water, in meadows and in parks. June and July are the preferred flight times for butterflies, which include moths. But they already fly from the end of May to the beginning of August. The caterpillar can be found from August to May. It is big, actually not to be overlooked.

Appearance and way of life of the willow borer

Both the caterpillar and the moth are drawn very characteristically. The adult animals reach a wingspan of up to 80 mm. Females are larger and thicker than males. The physique of both is rather plump. On the light gray wings you can see a dark gray marbling or fine lines. However, some parts of the wings are more of a brownish color. Overall, the overall color of the butterfly is reminiscent of tree bark, so the perfect camouflage. Willow borers only have stunted proboscis so they cannot ingest food. That’s why they don’t get old, more than a few weeks, up to three months at most, they don’t get any older.

The caterpillars of the willow borer are considered pests because they eat their way through the wood of many deciduous trees. Although they prefer willows, which is clear from the name, they also do not disdain poplar, birch, black alder and fruit trees, which makes them the declared enemy of home gardeners. Apple and pear trees are particularly affected. The caterpillars are in the garden, you should be able to tell by the clearly noticeable smell of vinegar. The caterpillars themselves are approx. 100 mm long, so really big. They have a yellow body, but with a broadly colored dark red back. The head and part of the neck plate are black. The whole body is very shiny. Occasionally one can find short white hair. In the caterpillar stage it can be overwintered two to four times.

The females of the Willow Borer lays their eggs (up to 700) preferentially in sick or dead trees. Our gardens are much too tidy, however, so that such wood can hardly be found there. That is why healthy trees are also used. The caterpillars live and eat in the bark and moult several times. They then penetrate deeper into the wood and run through it with their corridors. These can be up to 2 cm wide. If there are many caterpillars in a tree, it can die. The development of the caterpillars is complete after two to four years. They pupate. The pupae are red-yellow in color and have a dark row of thorns along the abdominal segments. Around June the finished willow borer hatches and the cycle starts all over again.

Recognize infestation – damage

The infestation can be recognized by the vinegar smell. There is a smell of goats in the immediate vicinity of the tree. From the outside you can see feces and drilling chips, both in the immediate vicinity of the drill holes. These are wider than they are tall and quite large. The problem is that an infected tree, which gives off the typical smell, attracts more females to lay eggs. This leads to a strong infestation. You can find many caterpillars, often in very different stages of development. The caterpillars also like to migrate to neighboring trees. They also cover relatively long distances, even crossing streets if need be.

If you cut off branches, you will find fungal infections on and in the feeding ducts. This is typical of the willow borer. The caterpillars trigger these infections to make the wood easier to digest.


In areas known to have the willow borer there, trees must be inspected for infestation in June and July. Then the tree trunks are especially important. The bark should be examined from July to September, on the trunk and on the thicker branches. Feeding marks indicate the caterpillars.


Fighting it is difficult because you often cannot get to the caterpillars inside the tree. Early detection helps. The young and only the very young caterpillars, which are still eating the bark, can be fought by means of a targeted chemical treatment before they drill into the wood. Spruzit® pest-free from Neudorff should help. However, I have not found anything to confirm this. The agent works against sucking and biting insects. Both of these do not apply to the Willow Borer’s caterpillars.

  • The method of piercing the feeding tunnels with a wire and trying to kill the pests works quite well. But the idea alone makes me uncomfortable. In addition, it is unlikely that you can get all of the caterpillars. Here, too, it is important to discover the infestation in good time and then act immediately.
  • Remove infected branches and burn them before the moths hatch. It is essential to cut into healthy wood.
  • Cut wounds should be treated with wound closure agents.
  • If the infestation is severe, it often makes more sense to fell the tree and burn everything. If necessary, the roots must also be cleared if there are still traces of feeding down there. It is important to fell the caterpillars before pupation.
  • Chemical control is almost impossible. Plant protection products are not available due to their approval.
  • I found a recommendation that related to a systemic agent, “Confidor”, mainly because of the active ingredient imidacloprid. This is a chemical from Bayer AG and probably the most widely used against insects worldwide. Imidacloprid occurs in various products available from us commercially. It is a contact and food poison and therefore helps against sucking and biting insects. In the event of contact, however, it should help against significantly more pests. Imidacloprid acts on the nerve cells of insects.

Alternative tip

A few times I have read that there is a cure for the caterpillars. It’s called Quassia and an extract from Quassia amara, the Surinam bitterwood. This agent has a feeding and a contact effect against biting and sucking insects. In any case, it is effective against apple blossom pickers and apple saw wasp, as well as against cherry fruit flies, plum moths and numerous species of aphids. Quassia is often found in stores under quassia powder or bitterwood powder. Quassia extract or quassia wood, cut, is also often available. It is important to use it early, ideally in June / July. By repeatedly spraying the trunk, you can destroy the butterfly when it hatches or when it is laying eggs. Hatching caterpillars that are still on the outside of the trunk are also recorded.

Quassia wood, for example, is soaked in water and boiled. It is absorbed in significantly more water. There is also soft soap. The solution is sprayed in a targeted manner. The solution can also be dripped directly into the ducts or injected with the help of a syringe. The broth can be kept from spring to autumn. The bitterwood itself can be dried again and used about two or three times.

The effect on beneficial organisms is minimal. Neither predatory mites nor bees are killed. Side effects on beneficial insects such as ladybirds, lacewings, bed bugs, bees and predatory mites could not be found in experiments.

The willow borer mainly attacks willows. Every now and then, however, other trees are also affected. Things get annoying when fruit trees are attacked. If the infestation is recognized in good time, targeted control is still possible. Things get more complicated later. Initially, you can still use sprays against the newly hatched caterpillars. If these penetrate deeper into the wood, fighting becomes increasingly difficult. In the case of light infestation, an attempt should be made to cut off affected branches. Of course, if the trunk itself is affected, that doesn’t help. If the infestation is severe, it is usually better to fell and burn the tree, with all the caterpillars and pupae in it. In the wild, almost all willows are infested with the willow borer. If there are only a few pests, that is usually not a problem. In the case of severe infestation, entire branches often break away and in the worst case the trunk simply kinks. This can be dangerous with large trees in residential areas and on roads and paths.

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