What Are Willow Borers? (And How To Prevent Them)

Last updated on October 23rd, 2023 at 08:28 pm

While digging in our garden we came across this specimen. But who or what is it?

This worm, or also known as willow borer is becoming a large moth, which is really very rare to see. The worm (caterpillar) however, is seen more often especially near willow trees. This is also the case at our home. Since the caterpillar is very large and really hard to miss, it is often seen from August to May. Very true August to May!!! The moth, on the other hand, you get to see between May and August 🙂 .

Appearance and the short life of the willow borer.

What Are Willow Borers? (And How To Prevent Them)

Both the caterpillar and the moth have very specific characteristics. The caterpillar can reach a length of 10 cm which is really big.
The appearance of the caterpillar I personally feel is not so “pretty”, thick and scary. The caterpillar has a red back, on which white hairs can be seen. On the underside it is yellow, the head to the nape is black.
The adult moths can reach a wingspan of 8 cm. The females are larger than their males. The physique of both is rather bulky, so really not to be overlooked. The wings of the moths are dark gray to brown and resemble tree bark.
The willow borer lives only a very short time, due to its bent proboscis it can not take food and therefore lives max. only 3 months.

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The caterpillar needs almost 4 years to complete its development, only then it can pupate.

The caterpillar is here 🙁.
The red and yellow caterpillars of the willow borer are considered a pest and eat through all the wood. In our home it was the willow what it actually prefers, but it does not stop at birch, poplar and fruit trees.
The female prefers to lay her offspring in diseased or dead trees. The female can lay up to 700 eggs.
The caterpillars live and feed in the bark and moult several times. During their molt they penetrate even deeper into the bark and leave behind tunnels that can be up to 2 cm wide. If there is one caterpillar, the infestation on one tree is not really dramatic. But if there are several caterpillars attacking the tree, it can happen that the tree dies.

The infestation

The infestation can be recognized by the smell of vinegar in the immediate vicinity of trees. Many also say that it smells like goats there. If this is the case, this smell attracts even more females to lay eggs. …and then it really starts.

You can find caterpillars in different stages of development and they sometimes migrate to neighboring trees. This small “ugly” caterpillar travels very long distances for a caterpillar just to eat. If you cut the branches of an infested tree, you will often find fungal infections in the feeding tunnels. This is completely normal and typical for the willow borer. Because to be able to digest the wood better, the caterpillars trigger this infection.

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Prevention ? Good question…

In an area where willow borers are common, trees must be inspected for infestation from June to July. It is important to examine the bark of the tree. If one sees feeding traces, then it is there, the caterpillar !!!

The control is really difficult. Because in most cases you can not reach the caterpillars in the wide long corridors of the tree. Only the very young caterpillars on the bark can be easily fought, here suitable insecticides help.
It is difficult to control the caterpillars in the tunnels. There is a method, which I personally do not approve. Pierce with a wire through the feeding tunnels, the caterpillars are impaled and killed. However, one does not get all caterpillars to grasp.

But I keep my fingers crossed that you will be spared from the willow borer… 😉


  • James Jones

    Meet James Jones, a passionate gardening writer whose words bloom with the wisdom of an experienced horticulturist. With a deep-rooted love for all things green, James has dedicated his life to sharing the art and science of gardening with the world. James's words have found their way into countless publications, and his gardening insights have inspired a new generation of green thumbs. His commitment to sustainability and environmental stewardship shines through in every article he crafts.

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