Woodworm, understanding it to fight it

Sun Tzu's famous quote may seem a bit out of place to you, but we assure you it is not. Learning about woodworms will enable you to understand how to avoid their colonisation. In the event of an attack in progress, you will know how to deal with it correctly and be able to assess what measures to take for effective pest control.

 

What you will find in this article is a detailed description of woodworms: what they are, how to recognise them, how they spread. You will learn these and many other important facts, so that you will understand how to deal with - and in the future avoid - the attack of these small woodboring animals.

Usually the word 'woodworm' is used to identify an insect that eats wood. This, however, is not entirely correct and it is good to shed some light on the subject.  Insects that feed on wood are called xylophages (from the Greek words 'xýlon/wood' and 'phagèin/eating') and belong to various taxonomic Orders. Those that cause the most damage to working wood belong to two Orders: the Coleoptera and the Isoptera.

 

Let us go into more detail.

 The woodboring Coleoptera that are grouped under the common name of 'woodworms' belong, for the most part, to the families Anobidae, Cerambycidae and Lyctidae.
The Isoptera, also more commonly called 'termites', are instead subdivided into six families; those present in Italy are mainly the families of the Rhinotermitidae and the Kalotermitidae.

We will, however, talk about termites in more detail in a future article. Now let us turn our attention to woodworms!

Woodworms are insects and consist of an external skeleton - called an exoskeleton - that acts as armour for their soft body.
The body is divided into three parts: the head, the thorax and the abdomen. On the head they have the antennae, which are true sensory organs. The thorax, besides bearing a double pair of wings, consists of three pairs of legs. The abdomen, on the other hand, is the seat of the reproductive apparatus.

 

Woodboring beetles share certain characteristics:

• a thick, hard exoskeleton;
• stiff outer wings, called elytra, which are intended to protect the second pair of wings underneath, suitable for flight. The functional wings are only visible when the woodworms fly.

The size of these woodboring insects varies depending on the species:

• 2-9 mm for Coleoptera Anobidae;
• 12-25 mm for Coleoptera Cerambicidae;
• 3-5 mm for Lyctid Coleoptera.

 

Unfortunately, it is not easy to see an adult insect, and it is even more difficult to observe a woodworm larva, as it lives inside the wood. If you continue reading this article, you will find out how you can tell if the wood is infested by this type of insect. Now let us see together what the biological cycle of a woodworm is.

So far we have seen what woodworms are and what they look like, but we still do not know how they are formed.

 

Coleoptera reproduce by means of eggs from which, after a certain incubation period, the larvae hatch. Once they have reached maturity, they transform into pupae: at this stage the larvae undergo profound changes, which will lead the pupa to become an adult insect. This is referred to as a true metamorphosis.

 

We can define the life cycle as the time from egg laying to the formation of the adult insect. Let us look at it in more detail.

 

The life cycle of woodworms

 

The female woodworm generally lays her eggs in cracks into the wood, in old flicker holes, on rough surfaces or directly in wood vessels. Depending on the species, the egg may vary in size between 0.5 and 2 mm. After an incubation period of one or more weeks, larvae hatch from the eggs.

 

The larva has the appearance of a whitish worm and is cyrtosomatic, i.e. it has a squat, curved body with poorly developed legs. To grow, the larvae dig tunnels in the wood, feeding on substances such as cellulose and hemicelluloses - which make up the cell wall of plant cells - and on substances contained in the wood's reserve tissues, such as starch, simple sugars, vitamins, mineral salts and protein substances. When they eat, woodworm larvae deposit what is called rosume (saw dust), a residue consisting of wood fibres and excrement.

The larval period lasts one or more years, depending on the species. The speed at which a larva grows depends on various factors:

 

• the ambient temperature;
• the nutritional value of wood;
• the moisture level of the wood.

 

A mature larva can vary in size from 5-6 mm up to 22-25 mm, depending on the species. All woodworms can do damage, but Cerambicidae are the most dangerous, as the larvae can create tunnels up to a centimetre wide.
Once the larvae have finished their activity, they carve out a space near the surface of the wood called the pupal cell: this will be the place where they will transform into pupae.

During the pupa stage, the insect undergoes major changes in both form and structure. This is a non-operational stage, in which the pupa is dormant and does not feed. It surrounds itself with a thin covering (tegument) and becomes similar to the adult woodworm: from an initial yellowish colouration it darkens over time. The stage lasts a few weeks, at the end of which the tegument opens and the adult insect, also called the perfect bug, emerges.

 

The adult woodworm does not leave the wood immediately: it stays inside the wood for a few days, the time needed for the tegument to harden. Once it is ready, the beetle makes a hole on the surface of the wood to come out and flicker. This forms what is called a flicker hole, which can vary in shape and size depending on the species.

 

The adult woodworm is short-lived, as its only function is to reproduce. Once it has emerged it no longer feeds and lives only a few weeks: the time needed for the male to fertilise the female.

We have seen previously that woodworms fly and this allows them to mate and lay eggs even far from the place where they have flickered.

The laying of fertile eggs and their subsequent hatching will give rise to a new generation of woodworms, which will follow the biological cycle just described.

How long does a woodworm live?

 

The duration of a complete life cycle varies from species to species. Let us look together at the three main families of woodworm:

 

• the cycle of an Anobid woodworm generally lasts between 1 and 3 years depending on the species, with the exception of Xestobium rufovillosum whose cycle can last up to 10 years;
• For a Cerambicide woodworm, the biological cycle can range from 1 to 10-11 years and even reach peaks of 17 years in the case of Hylotrupes bajulus;
• For the Lyctid woodworm, the life cycle lasts about a year, but in the case of very favourable environmental conditions - such as heated environments, for example - two or three generations of insects can originate and develop within a year.

We have just seen the biological cycle of a woodworm and so now you know how it develops. Now you are probably thinking 'yes, but how did it get there?

 

Unfortunately, there is no precise answer to this question, as every situation is different.

For example, colonisation could already have been in progress when the parquet or skirting board or wooden roof was laid. Trivially, woodworm could have entered your home when someone gave you a beautiful wicker basket full of delicious products for Christmas. Even a piece of furniture bought at a flea market or antique shop, if not properly treated, could be a carrier of woodworm insects.

 In short, the cases are numerous and when you detect the presence of woodworms you have to carefully assess what the source may be and what artefacts may have been attacked, since we remind you that woodworms fly and can therefore move from one object to another. We will see later in this article how to tell if the wood is infested.

 

Now let us look in detail at the conditions under which woodboring insects develop and spread.

We have said that the speed at which a woodworm develops depends on a few factors:

• the moisture level of the wood;
• the nutritional value of wood;
• the ambient temperature.

With regard to the moisture content of wood, in this article we only consider xylophagous insects that attack seasoned wood, i.e. wood with a moisture content of approximately 8 to 15%.

The presence of Coleoptera is closely linked to wooden species and their lignivorous action continues through successive generations, as long as environmental conditions and the source of food remain favourable to them.

It should be remembered that wood has a natural durability, i.e. its own ability to resist attacks by biodeteriogenic agents (xylophagous insects, fungi, etc.). This characteristic varies not only from species to species but also between mature wood (heartwood) and younger wood (sapwood) of the same species.

 

Natural durability is influenced by the presence of substances called 'extractives', which condition the damage that xylophagous insects can produce. It should be remembered that the types and quantities contained in wood vary according to species.

While the extractives of the heartwood - tannins, terpenoids, polyphenols - are usually toxic to xylophagous organisms, the extractives of the sapwood - starch, sugars and amino acids - are necessary for their development. Lyctids, for example, feed mainly on starch, and the amount required for their growth must be more than 1.5%. If the starch decreases or degrades, the wood is no longer attractive to Lyctid larvae. The same applies to the degradation of amino acids, which are useful for the development of the Old-house borer (Hylotrupes bajulus).

 

Temperature is another very important element, because it conditions the development of the various woodworm species and the speed with which they decay. Each organism, in fact, has:

 

• a minimum growth temperature below which it does not develop;
• an optimal temperature at which its activity is at its highest;
• a maximum temperature above which the organism dies.

 

Woodworms are able to survive for a long time in a dormant state when the temperature is below the minimum, while they die rather quickly when the temperature exceeds the maximum. Microwave and hot air woodworm treatments are based on this very principle.

 

 

Xylophagous insects	Minimum temperature	Optimum temperature	Maximum temperature
Anobium punctatum	13-14 °C	22-23 °C	30 °C
Hylotrupes bajulus	10 °C	28-30 °C	38 °C

 

Development temperatures of some xylophagous insects

 

 

Let us look together at some differences to better understand how temperature can influence the development of a woodworm.

The eggs of Anobium punctatum ('Furniture beetle') take 59 days to hatch if the ambient temperature is 15 °C, while they take only 15 days if the temperature is 28 °C. The eggs of Hylotrupes bajulus ('Old-house borer'), on the other hand, take 48 days to hatch at 16 °C, while at the optimum temperature of 31 °C they take less than a week.

In the table below you can see the egg hatching times of some of the most common xylophagous beetles in more detail.

 

	Temperature	Days
Eggs of Anobium punctatum	15 °C	59
	20 °C	20
	28 °C	15
Eggs of Hylotrupes bajulus	16 °C	48
	31 °C	6
Eggs of Xestobium rufovillosum	15 °C	50
	20 °C	24
	25 °C	15

 

Hatching times of eggs in relation to ambient temperature of Anobium punctatum ('Furniture beetle'), Hylotrupes bajulus (‘Old-house borer') and Xestobium rufovillosum ('Deathwatch beetle')

 

Under optimal conditions, the woodworm grows and once adult it fulfils its final mission: to reproduce.

The flickering insects are sexually mature: female Anobides, for example, usually release a pheromone to attract the attention of nearby males.

A special case in point is Xestobium rufovillosum: its typical sexual call has led to it being given the common name 'Deathwatch beetle', as both females and males rub their foreheads against the walls of the galleries when they need to communicate that they are sexually ready. In this way they produce a noise at cadenced intervals, similar to the ticking of a clock.

 

Once fertilisation has taken place, the female lays eggs, which, depending on the species, can range from 20 to over a hundred. This is why one speaks of a true colonisation. Once the continuity of the species is assured, the woodworm dies.

The simplest answer would be 'wood'. Going into a little more detail, we can say that not all wood species and not all parts of a single wood species are liked by woodworms.

The larvae, as we explained earlier, feed on cellulose and hemicelluloses - substances contained in the cell walls of the wood - and starch, sugars, mineral salts and protein substances - present in the reserve tissues in which the sapwood, the youngest part of the wood, is rich.

 

Precisely because of its composition, sapwood is more susceptible to degradation by wood pests, which can, however, also draw nourishment from the heartwood, especially if the latter has previously been attacked by fungi: by degrading cellulose and lignin, they allow insects to develop.

It must also be remembered that there are wood species defined as 'undifferentiated heartwood': in these, the heartwood has the same durability as the sapwood.

 

There are woodworms that have a very selective diet, as in the case of Lyctids, whose larvae feed mainly on starch.

Others mainly attack freshly felled wood and the presence of bark favours their establishment: examples are the Cerambicidae.

Depending, therefore, on the type of wood and the environment in which the material is located, woodworms can attack furniture (both old and new), parquet flooring, roof beams and all those wooden objects that can serve as food for them.

Anobium punctatum has been given the common name of 'furniture beetle': this already gives us an indication of which object it prefers, although it should be remembered that we can find it in many different artefacts.

In the table below you will find an indication of the woods and objects attacked by the most common xylophagous beetles.

 

Xylophagous beetles	Attached timber		Attached objects
	Woody species	Affected part
Anobium punctatum	-   Hardwoods and conifers -   Hard and soft timbers	Sapwood; sometimes heartwood	Wooden structures, floors, skirting boards, furniture, plywood, wicker
Xestobium rufovillosum	-   Hardwood; rarely coniferous -   Hardwoods	Sapwood or heartwood	Wooden structures, fixtures
Ptilinus pectinicornis	-   Hardwoods -   Hardwoods	Sapwood	Furniture
Lyctus brunneus	-   Hardwoods -   Hardwood (sapwood only) and soft tropical timber	Starch-rich sapwood	Furniture, parquet, skirting boards, artefacts, panels
Hylotrupes bajulus	-   Conifers; sometimes broad-leaved -   Hard and soft timbers	Sapwood	Wooden structures, beams

 

Types of wood and objects attacked by the most common woodworm species (from "Gli intrusi" by L. Süss)

Let us start with a very important concept: insects are not always alive when wood decay is observed, as they may have been killed by a woodworm treatment done earlier or have died due to a decreased nutritional value of the wood.

 

In some cases, it is possible to see traces of damage caused by insects that have infested the wood prior to its installation and that can complete their life cycle in the wood being installed. These xylophagous insects are not, however, able to reinfest it. We mention some of the most widespread in Italy:

 

• Xyloterus lineatus and Xyleborus dispar (family Scolitidae)
• Platypus cylindricus (family Platyptidae)
• Plagionotus arcuatus, Phymatodes testaceus, Callidium violaceum (family Cerambycidae)
• Urocerus gigas (family Siricidae, belonging to the Order Hymenoptera)

 

Having said that, there are several clues that can tell us if there is an infestation going on:

 

• Presence of rosume
• Presence of new holes in wood
• Noise from wood
• Presence of insects in the environment

 

Let us start with the rosume, which we have said is a residue consisting of wood fibres and excrement. In essence, it is composed of what the larva does not assimilate during its meal and consequently excretes.

 

Cerambycids and Lyctids usually deposit the rosume in the galleries, which therefore appear full. Most Anobids, on the other hand, push the residues outwards and in this case the galleries appear empty.

The habit of keeping tunnels clear, which is more typical of Anoptera beetles, is a very useful indicator of whether an infestation is taking place: the rosume that comes out of the flickering holes generally settles on the surfaces below, forming small, light-coloured, floury mounds.

If the rosume remains inside the galleries, however, it becomes difficult to tell whether our wood has been attacked by xylophagous insects.

One must be careful not to confuse fresh rosume with old rosume: the former is lighter, while the latter is darker, due to oxidation of the wooden residues. Why is it important to note this difference? The light-coloured rosume is attributable to an attack in progress and can be linked either to an adult insect that has recently emerged from the wood or to the activity of larvae that are shedding dust through old flicker holes.

Dark rosume is usually linked to mechanical stresses in the wooden object: if the material has been attacked by woodworm in the past but the holes have not been perfectly cleaned and closed, then one must expect dust to be likely to escape at a later date.

 

Therefore, the presence of rosume may not always indicate an attack in progress.

If you have already had a woodworm treatment and subsequently find rosacea, it does not necessarily mean that the treatment was ineffective, precisely for the reason we explained earlier.

 

The reasoning given above for the two types of rosume can also be applied to woodworm holes: if there are light-coloured holes with a sharp edge, an active attack can be assumed, whereas if the holes are dark, it means that the insect has been flickering for a long time.

Thus, the presence of flicker holes on an artefact indicates that it has suffered an attack, but not necessarily that it is still in place.

 

The amount of holes in the wood may indicate the degree of colonisation intensity, but this reasoning does not apply to all xylophagous insects. Some, such as the 'Old-house borer', have a very long life cycle: the larva digs a dense network of tunnels before the adult woodworm hatches - and therefore before you notice the hole. Other woodworms can also reproduce inside the wood, thus avoiding escaping outside and creating flicker holes.

 

Now let us move on to the third clue that might allow you to assess an attack in progress and start with a question: do woodworms make noise? Know that in many cases it is very difficult, if not impossible, to hear the presence of these woodworm pests.

 

Sometimes it is possible to hear the sound of the Cerambicidae larvae when they are closer to the surface of the wood and there is silence in the room; most of the time, however, they are deeper down and, bearing in mind that the infestation generally occurs in the rafters of attics, the distance from our ear prevents us from hearing and locating them.

 

Another case in which the presence of these insects can be heard is in the case of an attack by Xestobium rufovilossum ('Deathwatch beetle'), which emits a cadenced sound during the period of sexual maturity, as we have already described above.

 

The last useful clue as to whether there is an attack in progress is to notice the presence of woodworm insects in the environment (home, office, etc.). In most cases woodworms flicker in spring and summer, but since they live for a very limited period of time, it is not very easy to notice them around. Adhesive traps that can be attached to light sources can help: during the flickering period, these woodworms move towards the light and can be caught in this way.

We can say with certainty that the woodworm is not dangerous to humans. There are, however, woodworm parasites that can also attack humans. This is the real danger of woodworm!

Let's look at the most common ones together:

• Scleroderma domesticum
• Scleroderma brevicorne
• Pyemotes ventricosus
• Pyemotes herfsi

Scleroderma domesticum and Scleroderma brevicorne are small ant-like Hymenoptera that feed on woodworm larvae. They attack Anobids, Cerambici and Lyctids, preferring Anobium punctatum and Hylotrupes bajulus.
The females of these parasites are equipped with a sting attached to a venom gland that enables them to paralyse xylophagous larvae in order to deposit their eggs on them. Sometimes these insects belonging to the Betilidae family can sting people in a particularly painful way: allergic reactions can occur without being able to ascertain the cause quickly, as Scleroderma are small in size and move quickly.

Scleroderma bites can sometimes be confused with those of Pyemotes ventricosus, a mite that predates woodworm larvae. The two parasitic organisms have similar habits and in some cases may be present in the same environment. The bites of Pyemotes ventricosus, also known as the 'woodworm mite', are not felt immediately: itching appears several hours after contact.
Pyemotes herfsi has been traced more rarely in Italy and hardly ever reaches homes.

 

How do you eliminate the presence of these annoying pests? Eliminating woodworms eliminates their source of nutrition and colonisation, so in one action you eliminate two problems.

 

 

 

With this article we have tried to clarify your ideas about the world of woodworm. Knowing which woodworm has attacked your wood allows you to understand what damage your object is going through and what are the best measures you can take both to eliminate the infestation and to prevent further attacks.

In a future article we will look in more detail at the various species of xylophagous insects. If you want to keep up to date with our content, subscribe to our newsletter!

Bibliography:

• Gli intrusi, L. Süss, Edizioni Agricole
• Funghi e insetti nel legno, A. Gambetta, Nardini Editore
• Gli insetti e i danni del legno, G. Liotta, Nardini Editore

 

 

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