How to recognize & fight the pest

the essentials in brief

• The feeding activity of the Willow Borer Caterpillar can easily be confused with the corridors of other wood-dwelling species. However, it is Willow Borer 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 boreholes 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. As soon as 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 bores' feeding sites are similar to those of other pests

The caterpillars of the willow drill can significantly impair the stability of the infested tree. They can destroy entire tree populations 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.

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 borer from those of other reportable wood pests.

How to identify damage caused by Willow Borers

The willow borer is considered a pest because its caterpillars destroy already weakened woody plants and trees. At first glance, the damage can not be clearly traced back to this species. There are numerous wood pests that leave holes in the wood. Therefore, watch out for small traces that reveal the cause.

Damage image

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 out 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 feces and drill dust are disposed of. At the base of an infested tree you will often find reddish-colored drill chips and droppings. 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 the inside of the tree to the outside. Occasionally the caterpillars move clearly visible on the Tree bark away.

This is what caterpillars look like

Older caterpillars are mostly reddish brown

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 hibernating 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.

causes

The Willow Borer is a butterfly that is widespread and nocturnal. It mainly inhabits moist tree populations in which there are willows. Therefore, the moths can often 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 promote 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 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 from the willow borer, it can be very painful. A bite does not pose any health risks and the moths are not considered to be toxic to humans.

Digression

Wood drill bits and their food value

First aid by pruning

Affected parts of the plant should be generously removed and disposed of or burned

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.

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 in the upper third of the trunk and is clearly thickened, as a processing rice was grafted onto a typical willow species here. If you cut off this point, your harlequin willow will no longer sprout but the actual rootstock.

Fight Willow Borers

There is usually no need to fight the Willow Borer. Since it is an important part of a functioning ecosystem and is native to Europe, it is not necessarily considered 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. Removal 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.

Quassia

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 preventative measure. Note that quassin is not beneficial to beneficial insects.

Making 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 use 250 grams Soft soap(€ 38.88 at Amazon *) Dissolve in the boiling stock. It ensures that the liquid adheres better to the bark. After two to three days, the residues on the tree should be rinsed off with clean water.

Chrysanthemum extract

The beautiful Chrysanthemums Willow bores - and other beneficial insects - can cost their lives

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. Through the continuous poking, the caterpillar is to be impaled lengthways. However, the success of this measure is not very great. The feeding tunnels sometimes extend up to a meter deep 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 will never get all of them.

prevention

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.

Way of life and development

Willow borer belong to the wood borer family (Cossidae). Males have a wingspan of 80 millimeters. Females reach a size of 100 millimeters, which is why the species is 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 willow and poplar are preferred for laying eggs. The eggs are protected from drying out by a sticky secretion.

development

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, as wood fibers have hardly any nutrients. The young moths hatch from their pupae in summer.

Pupation

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:

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

frequently asked Questions

Is the Willow Borer Useful?

Willow borers are, in principle, very useful animals

This species, which prefers to settle on 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 boring tunnels in the wood that can run through the entire tree. The fungi that subsequently settle in the drill holes also ensure that wood decomposes more quickly. This moth tidies 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, her back is covered by a dark to wine red band and the sides appear yellow.

The body glows strongly and is occasionally covered with white hair. The black head and the partially 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 reddish yellow in color. 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 toxic substances, 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.