Willow borers have a bad reputation because they bore into fresh wood and destroy ornamental trees. The butterfly lays its eggs on certain trees for a reason. Its caterpillars are specialized creatures that pose no danger in he althy trees.
Is the willow borer a pest?
The willow borer is a butterfly that attacks diseased shoots of willow and poplar. The caterpillars leave feeding passages about two centimeters thick and one meter long and can thus affect the stability of the tree. Cutting back into he althy wood prevents infestation.
Are willow borers subject to reporting?
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 affected tree. They can destroy entire stands of trees within a few months. If avenue trees are infected, there is an increased risk of wind breakage. The species is widespread in Europe and prefers to nest in old willow trees. It can also appear in the home garden.
Nevertheless, it is not necessary to report willow borers. There is no obligation to report. If you discover feeding burrows of a butterfly caterpillar, you should accurately identify the species. The burrows of willow borers often cannot be clearly distinguished from those of other reportable wood pests.
| Shape of the feeding ducts | Special feature | Larvae | required to report | |
|---|---|---|---|---|
| Willow Borer | circular | Drill dust is carried outside with the feces | dark red back, yellow sides, black head capsule | no |
| Asian longhorned beetle | oval | coarse drilling chips remain in the passages | legless, with brown chitin plate on the forechest | yes |
| Chestnut Borer | round | only in branches with a diameter of less than 10 cm | first pink, later light yellow with black dots | no |
| Poplar Bock | oval | limited to poplars and willows | spherical, front plate grained | no |
How to recognize willow borer damage
The willow borer is considered a pest because its caterpillars destroy already weakened bushes and trees. At first glance, the damage cannot be clearly traced back to this type. There are numerous wood pests that leave holes in the wood. Therefore, pay attention to small traces that reveal the culprit.
malicious image
Willow borers prefer to attack older and already diseased trees, to which they can be dangerous. Their irregular boreholes usually lead downstream and can extend to a meter in length. The tunnels reach a diameter of up to two centimeters.
Affected trees suffer from a disrupted nutrient and water supply, so that branches and leaves dry out. The heavily hollowed wood can easily break in winds. Rot fungi often spread in the boreholes, which further weaken 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 feces and drilling dust are disposed of. You will often find reddish drill shavings and feces at the base of an infected tree. You can see sap flow in older holes.
Activities of a willow borer caterpillar can be recognized by the typical vinegar smell that surrounds the tree. If there is 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.
The 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 usually 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 back in the later stages of development. The fact that the caterpillars are actually yellow in color becomes clear when 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 features:
- usually 60 to 100 millimeters long
- Head and parts of the neck plate black all year round
- very shiny body
- Warts covered with short white hairs
- strongly developed mouthparts
Why are trees attacked
Willow borers are among the wood borers whose caterpillar development takes place in fresh wood. The adult moths have vestigial proboscis and cannot take in food. Their only reason for existence is to reproduce and thus maintain their species.
Causes
The willow borer is a butterfly that is widespread and nocturnal. It mainly inhabits moist stands of trees in which there are pastures. The moths are therefore often found along flowing water, but also in parks or meadows. The species are occasionally found in mixed forests or in house and orchards with old trees.
Females seek out sick 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 smell that older larvae secrete in the wood.
Raupe des Weidenbohrer (Cossus cossus) am 21.09.2013
Commonly infected trees
Willow borers prefer deciduous trees and occur predominantly on various native willow species or ornamental shrubs such as harlequin willow. When trees 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: Salweide
- 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 if it feels threatened. Children, dogs and cats should keep their distance from the caterpillars. If the willow borer caterpillar bites, it can be very painful. A bite poses no he alth risks and the butterflies are not considered poisonous to humans.
Excursus
Wood drills and their food value
Numerous wood borer caterpillars have been considered food insects since ancient times. Greeks and Romans enjoyed the caterpillars as a delicacy. To obtain a protein-rich treat, the caterpillars were fattened with flour. In Mexico, the caterpillars of a wood borer, which prefers to live in agave plants, have been considered edible since 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 rootworm or longhorned beetle caterpillars.
First aid by pruning
Affected plant parts should be generously removed and disposed of or burned
Cutting measures are the most successful method of eliminating 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 the affected tree back until he althy, unpunctured wood is visible. When there are no more caterpillars living in the wood, the tree can recover and sprout again.
Tip
Burn the cuttings, otherwise the willow borer caterpillars will continue to develop in the wood and can trigger a new infestation.
Refined varieties
If a refined harlequin willow is infected by the willow borer, pruning becomes more difficult. You must ensure that the finishing point remains intact. This is located in the upper third of the trunk and is significantly thickened because a breeding rice was grafted onto a typical willow species. If you cut this area off, your harlequin willow will no longer sprout, but rather the actual rootstock.
Fighting willow borers
It is usually not necessary to control the willow borer. Since it represents 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 combat willow borer. However, the species is not recognized 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 contained in some bitterwood species such as the Brazilian quassia tree (Quassia amara) and is available as a powder in pharmacies. An extract of the bitter substances helps against willow borer caterpillars if they are sprayed directly. You can also spray the solution on the trees from spring to autumn as a preventive measure. Please note that Quassin is not friendly to beneficial insects.
Preparation of Quassia Solution:
- Pour two liters of water over 150 grams of bitterwood or quassia powder
- Let it steep overnight and then bring to the boil
- Let the stock simmer for about an hour
- Strain 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 bitterness value of 13.000,000. After dilution 13 million times, the solution still tastes bitter. You can also dissolve 250 grams of soft soap in the boiling broth. 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 clear water.
Chrysanthemum extract
The beautiful chrysanthemums can cost the lives of willow borers - and other beneficial insects
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 insects, 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.
Does wire method make sense?
It is often recommended to push a sturdy wire into the drill holes. The continuous poking should impale the caterpillar lengthwise. However, the success of this measure is not very great. The feeding passages sometimes extend up to a meter deep into the wood, so you cannot catch the caterpillar living there with a wire that is too short. Severely damaged trees can be inhabited by several caterpillars, so you will never catch all of them.
Prevention
Willow borers require deciduous trees whose bark is moist, rough and furrowed to lay their eggs. The caterpillars bore into wood that has been damaged by drought or is 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 pruning the tree, any cuts should be sealed immediately with a wound dressing. Open wounds offer the caterpillar an ideal entry point.
Tip
To prevent egg laying, you should lime the trunk regularly. Beforehand, the bark is removed with a trunk scraper or a brush, which makes 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 considered the largest small butterfly in Central Europe. As a nocturnal butterfly, Cossus cossus is colored inconspicuously brown. There are black transverse lines on the forewings. The main flight season is from June to July, although the butterfly can be observed from late May to early August.
Egg laying
Females produce around 700 eggs after mating. They lay several egg packets, each containing 20 to 50 eggs, in the grooves of rough tree trunks. Weakened deciduous trees such as willows and poplars are preferred for egg laying. 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 undergoes several molts. 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 because wood fibers contain hardly any nutrients. The young butterflies emerge from their pupae in summer.
Pupation
Pupation normally occurs after the third overwintering in the wood. The caterpillars produce a solid cocoon that is about six centimeters large. This consists of web threads and is covered by drilling chips when it is in the feeding passage. Pupae in the substrate are often interspersed with soil particles. There are three ways in which willow borer caterpillars can pupate:
- Variant 1: burrow into the substrate to overwinter in autumn and pupate in spring
- Variant 2: overwinter as a caterpillar in the tree and pupate in the ground litter layer in spring
- Variant 3: pupate in a chip cocoon behind an opening blocked with drilling chips
Frequently asked questions
Is the willow borer useful?
Willow borers are basically very useful animals
This species, which prefers to settle in pastures, is an important part of a functioning ecosystem. It is one of the wood borers that targets fresh, living wood from trees that are already sick or weakened.
By its caterpillars drilling tunnels into the wood that can run through the entire tree, the willow borer accelerates the death of diseased trees. The fungi that then settle in the drill holes also ensure faster wood decomposition. This moth cleans up nature and makes room for fresh seedlings.
How many willow borer caterpillars live in a tunnel?
As soon as the young caterpillars hatch from their eggs, they look for an entry gate into the wood. They live gregariously under the bark for the first year. This so-called space eating ends in the second year of development. The caterpillars are distributed in individual tunnel systems in the wood that are separated from each other. Their feeding activity usually occurs downstream of 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. Looking for a place to overwinter or pupate, the caterpillars can be observed in autumn on the bark of infected trees or on the ground. At this time their back is covered in a dark to burgundy band and the sides appear yellow.
The body shines brightly and is occasionally covered with white hairs. The black head and the partially black neck area create a signal pattern. The scent of wood vinegar, which is somewhat reminiscent of the smell of goats, is typical. The pupae are about six centimeters 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 striking red color on their backs in the late development stage has evolutionary reasons. It often happens that the caterpillars can be found on the bark or crawling over the ground in search of a place to pupate. In these cases they are easy prey.
Using the signal colors black and red, they try to tell potential predators that their bodies are 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 are hardly recognizable when they sit 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, creating optimal camouflage. The legs also fit into this camouflage pattern because they are ringed in black and white. Females are slightly fatter than males. They reach a wingspan between 65 and 100 millimeters.
