Every library holds something fragile inside its quiet shelves. Books, manuscripts, palm-leaf records, and bound journals are made from organic materials like paper, leather, cloth, and natural adhesives. To us these are carriers of knowledge. To a whole range of living organisms, they are simply food. Fungi, bacteria, insects, and rodents constantly threaten library collections, and in warm, humid climates the risk is even higher. Understanding which biological pests attack books, how they behave, and what conditions allow them to thrive is the first real step toward protecting a collection for the long term.
Table of Contents
Why library materials attract pests
The materials that make up books are exactly the things many organisms want to consume. Paper is mostly cellulose. Bindings often use animal-based glues, and covers may be made of leather, vellum, or parchment. Even the starch-based sizing added to paper during manufacture is a nutrient source. According to the Northeast Document Conservation Center, the starches, cellulose, and proteins found in collections provide food for many types of pests, including termites, bookworms, cockroaches, silverfish, booklice, and rodents.
Two environmental factors decide whether these organisms can settle in: temperature and relative humidity. Warm, damp conditions speed up the life cycles of insects and encourage microbial growth. This is a serious concern across much of the subcontinent, where high summer temperatures combine with monsoon humidity. A study of the Kodaikanal Solar Observatory library noted that high humidity creates favourable conditions for mould, fungi, and pests like silverfish and booklice that damage paper-based materials.
Micro-organisms: the silent destroyers
Micro-organisms are the hardest pests to notice because the damage often begins invisibly. By the time stains or a musty smell appear, the deterioration is already advanced. The two main groups are fungi and bacteria.
Fungi and mould
Fungi are widely regarded as the most persistent biological threat to paper-based heritage. They release enzymes such as cellulase, amylase, and pectinase that break down the cellulose fibres in paper, weakening its structure. Research on a 14th-century manuscript published by the National Center for Biotechnology Information found that fungal strains of Penicillium and Aspergillus produced high levels of these hydrolytic enzymes, which played a key role in degrading both the paper and the leather binding.
The common culprits include Aspergillus, Penicillium, Alternaria, Cladosporium, and Rhizopus. A frequent and recognisable sign of fungal attack is foxing, the appearance of small reddish-brown or yellow spots on paper. Studies on the causes of foxing have linked these stains to colonisation by xerophilic (dryness-tolerant) fungi from the Aspergillus, Cladosporium, Penicillium, and Eurotium genera. Beyond staining, fungi also produce pigments and organic acids that cause discolouration and accelerate the chemical breakdown of paper.
Fungi need moisture to grow. The International Federation of Library Associations explains that fungi increase the rate of paper deterioration by breaking down cellulose chains and speeding up acid formation. Mould generally begins to develop when the temperature rises above about 21 °C and relative humidity climbs above 55 percent, which is exactly why poorly ventilated stores and basements are high-risk areas.
Bacteria
Bacteria are less commonly discussed but still play a role, especially on materials that have been wet or are very damp. They tend to attack proteins, which makes leather, parchment, and animal-glue bindings vulnerable. Surveys of deteriorating library materials have recovered bacteria such as Staphylococcus aureus and Lactobacillus species from foxed and discoloured documents, alongside the dominant fungal isolates. Bacterial activity can leave behind unpleasant smells, soften and weaken leather, and contribute to staining. Because both fungi and bacteria flourish in the same damp conditions, controlling moisture tackles both problems at once.
Insects: the most active enemies of books
Insects cause some of the most visible and severe damage to library collections. The signs include holes bored through pages, surface grazing, chewed bindings, and frass (insect droppings) left behind. It helps to know the main offenders and how each one behaves.
Silverfish
Silverfish (Lepisma saccharina) are small, wingless, silvery-grey insects that are active at night and prefer dark, damp corners. They feed on the starches and cellulose in paper, and are especially drawn to the starch-based sizing, glue, and gelatin used in book manufacture. They graze the surface of paper, leaving irregular holes and a thinned, eroded appearance on pages, photographs, catalogue cards, and cardboard. The Archives and Records Association notes that silverfish breed rapidly and cause serious damage in conditions above 70 percent relative humidity, which makes humidity control the single most effective defence against them.
Booklice
Booklice, or psocids, are tiny insects that feed mainly on the microscopic moulds growing on damp paper and on the starch paste in old bindings. They are among the most common pests found in collections. The Royal Botanic Gardens at Kew points out that some booklice reproduce through parthenogenesis, meaning a single female can produce offspring without a mate and start an entire population on her own. Their numbers rise quickly in warm conditions above 25 °C combined with high humidity, so their presence is often an early warning sign of a damp storage problem.
Cockroaches
Cockroaches thrive in warm climates and are a familiar problem in many storage rooms. They feed on paper, glue, and the gummed parts of documents, gnawing away bindings and sizing. They leave behind irregular holes, and their droppings stain pages and can permanently mark covers. Because they move between drains, food waste, and shelving, they also spread contamination across a collection.
Termites
Termites, also called white ants, are among the most destructive enemies of books in tropical regions. They consume cellulose voraciously and can eat through paper, wooden boards, files, and even catalogue cards, often working unseen from inside a shelf or wall. A study describing preservation management in libraries notes that once termites begin destroying documents it becomes very difficult to stop them, and that material left undisturbed for long periods is especially at risk. Because they also attack the building structure itself, termites threaten both the collection and its shelving.
Bookworms and wood-boring beetles
“Bookworm” is a popular name rather than a single species. As the Metropolitan Museum of Art explains, the term covers the larvae of several beetles and moths, including the furniture beetle, the powderpost beetle, and the cigarette beetle. These larvae tunnel directly through the pages and bindings of books, creating networks of small round holes and leaving frass in their tracks. Many of these insects are not after the paper itself but are trying to reach the adhesives and organic matter, yet the damage to the text and structure is serious all the same.
Rodents: destruction and contamination
Rodents such as mice and rats bring a different kind of threat. Their front teeth grow continuously, so they gnaw constantly to keep them worn down. Books, paper, and bindings make convenient targets, and rodents also shred paper to build nests. The result is torn pages, missing corners and spines, and entire volumes reduced to nesting material.
The damage goes beyond gnawing. Rodents leave droppings and urine across shelves, which stain materials, produce strong unpleasant odours, and create a genuine health hazard for staff and readers. Pest guidance from museum and library pest specialists also warns that rodents can chew through electrical wiring, adding a fire risk to the contamination and physical destruction they cause. Their presence often signals gaps in the building envelope and poor housekeeping, both of which need to be addressed directly.
How these pests are kept under control
The good news is that almost every pest discussed here depends on the same favourable conditions, so a few sound practices address most of them at once. The modern approach used in libraries and archives worldwide is Integrated Pest Management, which prioritises prevention and monitoring over heavy chemical use.
The core measures are straightforward. Climate control comes first: keeping temperature moderate, generally around 18-20 °C, and relative humidity stable in a moderate range starves fungi, silverfish, and booklice of the damp conditions they need. The conservation team at Chetham’s Library recommends keeping relative humidity stable to discourage infestations, since both excessive damp and large fluctuations cause trouble. Good housekeeping is equally important, because dust, food crumbs, and dead insects all provide food that draws pests in. Regular inspection and monitoring using simple traps helps catch problems early, while sealing the building against entry points keeps rodents and insects out in the first place.
When an infestation is already present, treatments are matched to the pest and the material. Non-chemical methods are preferred for delicate items. Controlled freezing, for example, is widely used to kill insect eggs and larvae in infested books, and techniques such as controlled-atmosphere fumigation are reserved for more serious cases. The guiding principle is that the cure should never damage the collection more than the pest would.
What do you think? Looking at the books and records around you, which conditions in your own library or home are most likely to invite these pests? And if you could only fix one factor, temperature, humidity, or cleanliness, which do you think would protect a collection the most?
References
- https://www.nedcc.org/free-resources/preservation-leaflets/3.-emergency-management/3.10-integrated-pest-management
- https://www.degruyterbrill.com/document/doi/10.1515/res-2024-0033/html
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9699582/
- https://www.sciencedirect.com/science/article/abs/pii/S0964830517313264
- https://www.ifla.org/treating-fungal-and-mould-infestation/
- https://www.archives.org.uk/collections-care-toolkit/integrated-pest-management-ipm-for-paper-archives
- https://www.kew.org/read-and-watch/battle-of-the-booklice
- https://rsisinternational.org/journals/ijrsi/uploads/vol13-iss2-pg406-412-202602_pdf.pdf
- https://www.metmuseum.org/blogs/in-circulation/2016/book-lovers
- https://www.terminix.com/blog/commercial/pests-of-museums-libraries-historic-buildings/
- https://library.chethams.com/blog/pesky-pests/
Leave a Reply