Willow borers have a bad reputation for boring into fresh wood and destroying ornamental trees. The moth lays its eggs on certain trees for a reason. Its caterpillars are specialized creatures that pose no threat to a healthy stand of trees.

The willow borer prefers to attack diseased and weak pastures

Table of Contents

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  1. the essentials in brief
  2. Willow borers - obligation to report?
  3. Willow borer damage
  4. causes of an infestation
  5. Willow borer poisonous?
  6. First aid by pruning
  7. combat
  8. lifestyle and development
  9. frequently asked Questions

    10. the essentials in brief

    • The feeding activity of the willow borer caterpillars can easily be confused with the burrows of other wood-dwelling species. However, the willow borer is not notifiable. The native species infests willows and poplars in moist tree stands.
    • Caterpillars eat into the wood of ailing trees and leave burrows up to two centimeters thick and one meter long. Typical are boreholes in the bark, drill dust and crumbs of droppings 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 means of controlling the caterpillars. Once they have eaten their way into the wood, fighting them is almost impossible.
    • Females lay their eggs on rough bark, allowing the caterpillars to burrow into the wood. They take several years to develop and pupate either in the wood or in the substrate.

    Are willow borers notifiable?

    The feeding sites of willow borers are similar to those of other pests

    The caterpillars of the willow borer can significantly affect the stability of the infested tree. They can destroy entire stands of trees within a few months. If avenue trees are infested, there is an increased risk of wind damage. The species is widespread in Europe and prefers to nest 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 discover feeding passages of a caterpillar, you should identify the species exactly. Often the burrows of the willow borer cannot be clearly distinguished from those of other notifiable wood pests.

    shape of the burrows special feature larvae notifiable
    willow borer circular Bore dust is transported outside with the droppings dark red back, sides yellow, head capsule black no
    Asian longhorned beetle oval coarse drilling chips remain in the aisles legless, with a brown chitinous plate on the fore chest Yes
    chestnut borer around only in branches less than 10 cm in diameter first pink, later light yellow with black spots no
    poplar goat oval restricted to poplars and willows nodular, face plate grained no

    How to recognize damage caused by willow borers

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

    damage picture

    Willow borers prefer to attack older and already diseased trees, which they can become dangerous for. Their irregular burrows 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 disrupted supply of nutrients and water, so that branches and leaves dry up. The heavily hollowed wood can easily break in winds. Rot fungi often spread in the boreholes, which further weakens the tree. If the infestation is severe, there is a risk that the entire tree will die.

    detect infestation

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

    Activities of a willow borer caterpillar can be recognized by the typical smell of vinegar that envelops the tree. In the case of a severe 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.

    Boreholes of the willow borer caterpillars are irregularly shaped, up to two centimeters in diameter and usually lead from top to bottom.

    This is what caterpillars look like

    Older caterpillars are mostly reddish brown

    While the damage 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 overwintering individuals. If you find a caterpillar in the top layer of substrate 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 are black all year round
    • very shiny body
    • Warts covered with short white hairs
    • powerfully developed mouthparts

    Why are trees attacked

    Willow borers belong to the wood borers whose larvae develop in fresh wood. The adult moths have stunted proboscises and cannot take in food. Their only reason for existence is reproduction and thus the maintenance of their species.

    causes

    The willow borer is a widespread and nocturnal butterfly. It inhabits mainly moist tree populations in which there are willows. Therefore, the moths are often found on flowing waters but also in parks or meadows. The species are occasionally found in mixed forests or in house gardens and orchards with old trees.

    Females look for ailing and older trees to lay their eggs, so that in nature they initiate and promote the decomposition of the old wood. They are probably attracted to the acetic acid-like odor that older larvae emit in the wood.

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    Commonly infested trees

    Willow borers prefer deciduous trees and are mainly found 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 are found in maple, beech, ash, poplar, oak, walnut, basswood, and elm.

    Preferred tree species:

    • Salix: willow
    • Betula: silver birch
    • alnus: black alder
    • Pyrus: Pear
    • penalty: Apple

    Is the willow borer caterpillar poisonous?

    The willow borer caterpillar is aggressive and can use its mandibles if it feels threatened. Children, dogs and cats should keep their 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 poisonous to humans.

    digression

    Wood borers and their food value

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

    First aid by pruning

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

    Pruning is the most successful way to get rid of the willow borer at an early stage of infestation. The longer the caterpillars live in the wood, the more they expand. The burrows of the moth caterpillars can be up to one meter long. Cut back the infested tree until healthy and unperforated wood is visible. When no more caterpillars live in the wood, the wood can recover and sprout again.

    tips

    Burn the prunings, otherwise the willow borer caterpillars will continue to develop in the wood and can trigger another infestation.

    Refined Varieties

    If a grafted harlequin willow is affected by the willow borer, pruning is more difficult. You must be careful to keep the graft site intact. This is in the upper third of the trunk and is significantly thicker, since grafted rice was grafted onto a typical type of willow. If you cut off this spot, your harlequin willow will no longer sprout, but the actual rootstock.

    Fight willow borers

    Usually there is no need to control 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 approved to control the willow borer. Nevertheless, the species does not find recognition in the home garden. Removal measures make sense for ornamental trees.

    Early control necessary

    If the caterpillars have eaten into the wood, it is almost impossible to fight them. 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 contained in some types of bitter wood 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 they are sprayed directly. You can also spray the solution on the trees as a preventive measure from spring to autumn. Note that quassin is not beneficial.

    Preparation of the Quassia solution:

    1. Pour two liters of water over 150 grams of bitter wood or quassia powder
    2. leave overnight and then boil
    3. Simmer for about an hour
    4. Strain the liquid and dilute with ten liters of water

    If you use bitterwood, you can dry and reuse the pieces after boiling them up. Quassin has a bitterness 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 (€44.90) in the boiling liquid. 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 clear water.

    Chrysanthemum Extract

    The beautiful chrysanthemums can kill willow borers - and other beneficial insects

    The Dalmatian insect flower produces the natural toxin 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, outdoor use should be carried out 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 should be impaled lengthwise. 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 capture the caterpillars living in them with a wire that is too short. Severely damaged trees can be inhabited by several caterpillars, so you will never catch all of the specimens.

    prevention

    Willow borers need deciduous trees whose bark is damp, rough and grooved to lay their eggs. The caterpillars burrow into wood that has been affected by drought damage or has been weakened by diseases and fungi. A regular water supply and potash fertilization are the most important measures to prevent an infestation, because they support the vitality of the tree. After subjecting the tree to a pruning, pruning wounds should be sealed immediately with a wound dressing. Open wounds offer the caterpillar an ideal entry point.

    tips

    Lime the trunk regularly to prevent egg laying. Before that, the bark is removed with a trunk scraper or a brush, making the trunk unattractive to the butterflies.

    lifestyle and development

    Willow borers 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 moth, Cossus cossus is inconspicuously colored brown. There are black transverse lines on the forewings. The main flight season extends from June to July, with the butterfly being observed from late May to early August.

    egg laying

    Females produce about 700 eggs after mating. They lay several egg packets, 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.

    development

    After the larva hatches, it penetrates the bark. It feeds on tree sap and wood fibers and goes through several moults. In the second year, the caterpillars eat 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, since wood fibers have hardly any nutrients. The young butterflies hatch from their pupae in summer.

    pupation

    Normally, pupation occurs after the third hibernation in the wood. The caterpillars produce a solid cocoon about six centimeters in size. This consists of web threads and is covered by drill shavings when it is in the feeding tunnel. Pupae in the substrate are often interspersed with soil particles. There are three ways that willow borer caterpillars can pupate:

    • version 1: burrow into the substrate for overwintering in autumn and pupate in spring
    • Variant 2: overwinter as caterpillars in the tree and pupate in spring in the soil litter layer
    • Variant 3: pupate in a cocoon of shavings behind an opening clogged with drill shavings

    frequently asked Questions

    Is the willow borer useful?

    In principle, willow borers are very useful animals

    This species, which prefers to settle on pastures, is an important part of a functioning ecosystem. It is one of those wood borers that target fresh and living wood from already diseased or weakened trees.

    The willow borer accelerates the death of diseased trees by boring tunnels into the wood, which can go through the entire tree. The fungi that subsequently settle in the boreholes also ensure faster wood decomposition. This moth tidies up nature and makes room for fresh seedlings.

    How many willow borer caterpillars live in a tunnel?

    As soon as the young caterpillars have hatched from their eggs, they look for an entry point into the wood. They live sociable under the bark for the first year. This so-called space eating ends in the second year of development. The caterpillars are distributed in the wood over individual tunnel systems that are separated from each other. Their feeding activity usually takes place down the trunk, since 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. Looking for a place to hibernate or pupate, the caterpillars can be observed in autumn on the bark of infested trees or on the ground. At this time their back is covered by a dark to burgundy band and the sides appear yellow.

    The body shines brightly and is occasionally covered with white hair. The black head and the partially black neck area create a signal pattern. Typical is the scent of wood vinegar, which is somewhat reminiscent of the smell of goats. The pupae are about six inches long and reddish yellow in color. The individual abdomen segments are provided with dark rows of thorns.

    Why do wintering caterpillars look different than summer caterpillars?

    The fact that the caterpillars in the late development stage have a conspicuous red coloration on their backs has evolutionary reasons. It often happens that the caterpillars can be found on the bark or crawl over the ground in search of a pupating place.In these cases they are easy prey.

    With the signal colors black and red, they try to tell potential predators that their body is inedible or poisonous. In fact, the caterpillars do not have any toxic ingredients, so the red color is an important protection against voracious birds and other predators. During the winter months they lose this coloring and appear completely yellow.

    What do willow borers look like?

    The moths can hardly be seen when they are perched on the bark of deciduous trees. Their plump bodies have light gray colored wings that are marbled with dark gray. Parts of the wings appear brownish. The wings imitate the coloring of tree bark, which creates 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.

Category:

Animals in the garden