10 Pests and mold
In nature’s life cycle, abiotic and biotic processes very quickly cause irreversible changes, degradation and transformation in organic materials and substances. Artworks and cultural artifacts made of such materials are no exception.
Objects in museums are attacked most frequently by parasites, mold and fungal pests. As it often happens in hidden places, an infestation can begin slowly and imperceptibly. If suitable conditions exist, however, an outbreak can spread very rapidly and cause enormous damage. The staff concerned must therefore always be vigilant and carry out routine monitoring. Any infestations can then be spotted quickly and action can be taken to eliminate them. Staff in museums, libraries and archives must therefore have a basic knowledge of the biology of museum-relevant pests and be aware of the latest findings on how best to prevent and deal with such infestations. An essential first step for efficient long-term preventive protection and treatment measures is the correct identification of the pest concerned.
What insects attack what materials (paper in German)?
What mold attacks whar materials (paper in German)?
I. Materials at risk
Wood
Wood is a natural composite material. In terms of its chemical composition, wood comprises chiefly polysaccharides (~70%) and lignin (~25%). At around 50%, cellulose makes up the greatest proportion of the polysaccharides.
Pests usually target individual components of the wood. For instance, many types of insect have specialized in substances that are only present in small quantities in many woods, such as proteins or starches.
Insects can attack and digest these substances more easily than the matrix-like structure of cellulose or indigestible lignin containing phenols. However, proteins also age more rapidly, that is to say they can be converted more quickly, so these insects tend not to attack these types of wood, or at least less frequently. Environmentally friendly treatment methods are based on this. Fungi can biodegrade both the main components of wood (brown, white and soft rot) as well as live off its byproducts (mold and wood-discoloring fungi). Biological infestations by insects, fungi and bacteria are greatly influenced by the moisture content of the wood.
Paper
Paper consists predominantly of cellulose. The amounts of other important components present, such as lignin, hemicellulose, pectins, waxes, tannins, proteins and minerals, depend on the material used for production. The cellulose obtained from wood by various pulping methods is mixed with water to make a paste, then formed and dried. High-grade papers are produced by adding barium and calcium sulfate as well as kaolinite and rosin soaps (colophony). Low-grade papers contain up to 90% groundwood, the high lignin content of which causes paper to yellow over time.
The higher the proportion of impurities in paper, the more susceptible it is to infestation by organisms. In addition, organic paper additives, such as animal and vegetable glues, offer a good source of nutrients for harmful organisms. Since paper tends to rapidly absorb moisture, it is particularly susceptible to fungal attack, as can be seen by the foxing marks caused by mold.
Textiles
The natural fibers used to manufacture textiles can be of animal, vegetable or mineral origin. Plant fibers include cotton, flax, hemp, jute and sisal. The resistance of these fibers to biological attack is determined by their cellulose content, the length of the cellulose chains and their crystalline structure. Lignin and waxes increase biostability in the order jute, hemp, cotton and flax.
Animal fibers come in the form of wool, fine and coarse hair/fur and silk. Since they consist of proteins, they can serve as a source of nutrition for organisms that live off the organic materials of other creatures (heterotrophic organisms), by biodegrading the proteins with the aid of special enzymes.
Wool is generally sheep’s wool, which consists primarily of keratins (fibrous proteins or scleroproteins). The finer types of animal hair include angora wool, camel hair and mohair. Coarse animal hair includes horsehair and goat hair. Silk is a natural protein fiber produced by silkworms in cocoons. Silk fiber comprises 70% to 80% fibroin, a highly crystalline scleroprotein, and 19% to 28% sericin, a water-soluble protein. Removing the latter increases the biological resistance of the silk. Textiles made of protein fibers are particularly vulnerable if they are very dirty and are exposed to damp and warm climatic conditions. Generally speaking, the tighter the weave of the silk, the less vulnerable it will be.
In the case of natural fibers, bioresistance can be increased by applying a suitable finish. Although chemical fibers are far less susceptible, they can also be attacked by insects and microorganisms.
Parchment, leather and other materials of animal origin
Parchment consists of collagens and keratins (scleroprotein) as well as albumins and globulins (globular or spheroproteins). It is obtained from the untanned, scraped and oiled hides of donkeys, pigs and calves. The stability of the collagen in the parchment depends on the temperature and moisture, the pH value and the level of UV radiation. If parchment is stored at a temperature above 22 °C and at a humidity of 65%, the collagen can degrade enzymatically.
Leather is likewise composed of collagens and is obtained from animal hides by tanning and other processes. It has a fibrous porous air-permeable structure, as a result of which it can absorb up to 28% of its mass in water vapor and desorb the moisture again. This capacity to store water makes it highly susceptible to mold attacks.
The proteins found in parchment, leather and in other skins, furs and hides serve as a source of nutrition for a variety of insect species, for example carpet beetles and cockroaches. If environmental conditions are unfavorable, stuffed animals and mummies can very quickly be attacked by mold and bacteria, which can also produce allergens and toxins that are hazardous to humans.
Glues made of skins, bones, leather, parchment waste and the swim bladders of fish also quickly go moldy and are attacked by many species of insect. By denaturing the proteins (making them unpleasant to consume) or adding biocides, infestations by such pests can be greatly reduced. Owing to their carbohydrate content, herbaria and ethnographic objects made of plant matter are particularly susceptible to destruction by insects (e.g. silverfish).
Plastics
The resistance of plastics to biological attack depends on their chemical composition, their structure, the form of the macromolecules (chained, branched or networked) and their arrangement (completely unordered, straight or partially crystalline structure). The hardness and surface properties (smooth, rough, porous) also play a role, as do additives and contaminants. Over time plastics that were originally biostable can become less stable due to their chain length shortening. In comparison with fully synthesized materials, plastics produced using natural substances are generally less stable.
Bioresistance of plastics and synthetic resins (paper in German)
II. Pests
Animal pests
From the point of view of museums, libraries and archives, the most significant animal pests are insects. Problems with mice, rats and birds can be considered an exception and will therefore not be discussed further here.
The insects that constitute the main threat to materials and collections in cultural institutions are beetles (Coleoptera), moths (Tineida) and silverfish (Zygentoma). Further common pests include ants, cockroaches, dust mites, lice, fleas and ticks. Occasionally termites and other insects native to the tropics or subtropics can be brought in, for example along with ethnological artifacts.
In addition to a plentiful supply of nutrients, the right environmental conditions (temperature, humidity and moisture content of materials) are the key factors for the growth of insects, especially at the larval stage. As far as temperature is concerned, a range between 20 °C and 30 °C is most favorable. In addition, it is possible to specify minimum and maximum temperatures for individual insect species, below and above which their metabolism is arrested and activity ceases. However, the temperatures required to actually kill them off are much lower and higher than that, e.g. below -20 °C and above +45 °C. As the temperature rises, the activity of the insects increases up to an optimum. Along with the growth rate of larvae, the swarming of adult insects also depends on the ambient temperature, among other things.
Insects are relatively tolerant with respect to humidity and the moisture content of materials. Some pests such as silverfish prefer a high humidity of 80% to 90%; but 60% to 80% humidity is generally sufficient for most species. Powderpost beetles (Lyctidae), a woodboring insect, require a wood moisture content of only 7% to 8%, which can occur if the air has a relative humidity of around 30%.
Pests tend to inhabit – during their flying stage – windows and windowsills, as well as dark corners of rooms, cracks in the plaster or in the floor. In order to identify them, all suspicious dead or alive insects must be picked up with tweezers and stored in a specimen jar sealed by a plastic stopper with small holes for ventilation. Due to the widespread risk of cannibalism, living pests should always be kept separately. Note that some beetles can “play dead”! With the assistance of a magnifying glass or reflected light microscope, identification guides (e.g. Weidner 1993) and samples for comparison, non-experts can narrow down the suspects to certain insect species. A trained entomologist will be able to remove any remaining doubt.
Pheromone traps (Binker 1996, see references in the knowledge base) are very helpful for finding insect pests. Pheromone traps are available for some woodboring beetles (Anobiidae), for example the common furniture beetle (Anobium punctatum), the drugstore beetle (Stegobium paniceum) and the cigarette beetle (Lasioderma serricorne). They are only effective during the flying stage of the beetles and have a maximum range of 5 m. Traps for flightless and flying moths, with a range of up to 2 m and 10 m respectively, as well as for flightless cockroaches (range approx. 1.5 m), are also available. Finally, the presence of certain species of carpet beetle can be determined in this way. One disadvantage is that pheromone traps are effective for only 2 to 12 weeks at most. However, clever positioning of the traps enables the main sites of infestation in a room to be localized.
When eating and moving around, in particular the larvae of woodboring beetles and termites make a noise that can be detected on the object in question and amplified by means of a structure-borne sound measuring device known as an acoustic emission device (AED). Drywood termites can be clearly identified in this way. X-ray images before and after treatment can be used to measure success.
There are various ways of combating pest infestations.
From a toxicological and ecological point of view, physical methods are preferable to liquid chemical treatments sprayed or painted on the objects. In contrast to these agents, some of the components of which will remain on the treated items in the collection for a long time and often cause unwanted changes, physical methods leave no residues and there is no need to determine the compatibility of the conservation agent and the original substance. However, unlike many liquid chemical pest treatments, they do not provide any preventive protection. Treatments can also result in material changes to a greater or lesser degree. Before using any cold or heat treatments, microwaves or gamma rays, it is essential to check the thermal tolerances of the various materials as well as the precise lethal temperatures for the various species of insect at their respective stage of growth.
Chemical preparations should only be used if all other treatment options have been ruled out. Also make sure that the pest treatment chemicals contain natural biocides that biodegrade as rapidly as possible.
Fungi
Within this category of pests, mold poses the most significant threat to collections. The term “mold” is not clearly defined. It refers primarily to lower forms of fungi (Ascomycota, Deuteromycota). This group also includes wood-discoloring fungi. Genuses such as Aspergillus, Penicillium, Cladosporium and Fusarium are frequently encountered. Wood-decaying brown, white and soft rot, as well as blue stain fungi which often attack structural timber in buildings, can also spread to collection objects stored there.
Fungal growth is greatly dependent on the environmental conditions and the availability of nutrients. In particular, relative humidity (RH) plays a crucial role. During extremely dry periods, the ubiquitous spores can remain dormant for many years, but they will begin germinating and growing again if the relative humidity rises. Germinating spores pose a threat to collections above an RH of 65%. Mold infestation is unlikely only when the RH is below 55%. The mycelia (invisible fungal filaments) of some species of fungus are able to draw up water over great distances, for example dry rot (Serpula lacrymans). The presence of water as a solvent and means of transport is a prerequisite for the enzymatic biodegradation of nutrition substrates.
As well as relative humidity, fungal growth is greatly influenced by temperature. Most species of fungi thrive at temperatures between 5 °C and 40 °C, with temperatures between 15 °C and 40 °C being most favorable.
Light appears to be unimportant for the vegetative development of fungi. The mycelium can grow in the dark. Only the fruiting body (sporocarp) requires light to grow.
It is often assumed that a white or gray bloom or pustules on objects must be mold. However, this can also be produced by efflorescent biocides (e.g. DDT), alkaline earth metal and lead salts of higher fatty acids (soaps) as well as hygroscopic salts. This can be ascertained by a microscopic examination and simple chemical tests.
A range of detection methods are available for detecting mold activity. In most cases it is not enough simply to determine the genus of the mold, as the individual species within the genus can differ widely with respect to the damage they cause to objects and the health hazard they pose to humans. It is also important to differentiate between living and dead cells and to determine whether the living cells are capable of germination.
Although fungi are aerobic organisms and require oxygen from the air to breath, they can withstand long periods without oxygen. It is therefore virtually impossible to control such outbreaks by means of modified or controlled atmospheres. Spores are finally killed off only after a lengthy period at temperatures above 80 °C.
Gamma rays can be used to destroy both the hyphae (the filamentous cells of a fungus) and mycelium of the fungi as well as fungal spores and bacteria. This method is suitable for books, for smaller objects made of wood or leather, as well as for mummies with a major mold or bacterial infestation. The disadvantage is that the objects need to be taken to the radiation equipment and the treatment does not provide any preventive protection.
Objects with mold bloom can be cleaned and disinfected with 70% ethanol or isopropyl alcohol. Before resorting to antimicrobial agents, it is essential to check that they are “compatible” with the materials to be treated and that the latter will not be dissolved by the alcohol in the agents. It must also remain possible to subsequently restore the original substance. In general, fungicides and bactericides should only be used if all other methods have been ruled out or would not work in the specific circumstances.
Preventive measures should be taken to avoid insect and mold infestations. These include both structural and conservation measures, regular monitoring, and cleaning the objects and rooms (cf. the recommended actions in the Pests questionnaire for more information).
Wibke Unger, Katrin Schöne and Bill Landsberger