How to Identify Biological Damage in Library Materials

Walk into any old library, archive, or record room and you will eventually meet the quiet enemies of paper. Long before a book crumbles in your hands, biological agents leave behind clues that something is wrong. Fungi, insects, and rodents each attack library materials in their own way, and each leaves a recognisable signature on paper, cloth, leather, and bindings. Learning to read these signs is the first real step in conservation. You cannot treat damage you cannot identify. This post breaks down the tell-tale marks of fungal, insect, and rodent attack so you can spot trouble early and act before a collection is lost.

Why identifying biological damage matters 🔗

Library materials are made largely of organic matter: cellulose in paper, protein in leather and parchment, and starch or animal glue in bindings. To a microorganism or a hungry insect, a book is simply food. The warm, humid climate found across much of the subcontinent makes the problem worse, which is why the National Mission for Manuscripts reports that a large share of the country’s manuscripts arrive at conservation centres already insect-ridden, fungus-infected, or brittle. Identification is not just an academic exercise. Different agents need completely different responses, and misreading the damage can let it spread. The aim is to look at a damaged item and correctly name the culprit from the pattern it left behind.

Fungal damage: the silent spread 🔗

Fungal growth is often the hardest damage to catch early, because it begins invisibly. Fungi thrive in high humidity and poor air circulation. Research on libraries shows that mould grows actively when relative humidity climbs above roughly 65 percent and temperatures sit in the warm range, conditions that describe a poorly ventilated store room during the monsoon almost perfectly. A study on airborne fungal spores in libraries found that genera such as Aspergillus were among the most active in breaking down cellulose, making them a serious biodeterioration risk.

Discoloration and foxing 🔗

One of the clearest visual signs of fungal activity is discoloration. You will notice patches that range from white and grey to green, brown, or black, often spreading in irregular blooms across pages or covers. A more subtle form is foxing, the scattered reddish-brown or yellowish spots seen on aged paper. Foxing has two main causes working together: traces of metals like iron and copper left in the paper during manufacture, and the growth of microorganisms such as fungi. As explained by paper conservators, fungal spores constantly float in library air and settle on paper, where they can feed and grow for years if conditions allow. When materials are new there may be no visible stain, but the fungal products slowly create a chemical environment that produces colour over time.

Musty smells and surface texture 🔗

Sometimes your nose detects fungi before your eyes do. A musty, earthy odour is one of the earliest warnings. According to a review of fungi found in libraries, this smell comes from volatile organic compounds released during the biodeterioration process, and it is frequently the first reason staff suspect mould at all. Alongside the smell, affected surfaces may feel fuzzy, powdery, or slightly furry to the touch. Spores can also appear as a fine dusty layer on the tops of shelved books.

Weakening of paper and bindings 🔗

Beyond looks and smell, fungi physically degrade the material. They digest cellulose, leaving paper weak, brittle, and prone to crumbling at the edges. Pages may become stuck together as fungal growth and moisture interact. Leather and cloth bindings lose strength and may show staining or rot. The same review notes that the worst mould outbreaks often follow water events, such as a leak or a flood, where lingering dampness gives spores everything they need. If you find soft, stained, easily torn paper combined with discoloration and odour, fungal damage is the likely answer.

Insect damage: surface feeders and burrowers 🔗

Insects are among the most destructive pests in any collection, and their damage is usually easier to see than fungal attack. It helps to divide the offenders into two groups based on how they feed: surface feeders that graze across paper and bindings, and burrowing insects that tunnel deep into the material.

Surface feeders: cockroaches and silverfish 🔗

Silverfish are small, wingless, silvery insects that love dark and damp spaces. They are drawn to the starches, sugars, and cellulose in paper, and they also feed on the glue that holds bindings together. Their grazing leaves irregular holes, frayed or notched edges, and a thin, scraped appearance on the page surface, sometimes with yellowish staining and a fine powdery residue. The conservators at Chetham’s Library point out that insect damage can usually be recognised by its sharply defined edges and curving shapes, which separates it from random tearing.

Cockroaches, especially larger species, also feed on the starch in cloth bindings and on paper itself. Their damage can resemble silverfish grazing, producing ragged edges and light patches where the surface layer has been eaten away, occasionally down to the binding thread. The clearest cockroach signature, however, is contamination: dark fecal specks, brown smears, and oily stains on pages and covers. These insects prefer paper already soiled by food, body oils, or other insects, so dirty storage invites them in.

Burrowing insects: termites and bookworms 🔗

Termites are a severe threat in warm, humid regions because they eat cellulose, the very substance of paper and wooden shelving. They tend to work hidden from view, hollowing out material from the inside. Recognisable signs include mud tubes, which are protective earthen tunnels termites build to travel, along with hollowed or caked sections of books and shelves and clusters of droppings. Because they react sharply to moisture and heat, termites can ruin organic materials within a few years if neglected, a danger that accounts of palm-leaf manuscript preservation describe vividly in the tropical context.

The word bookworm does not refer to a single creature. It describes the larvae of several beetles, as well as other insects, that bore into books. The larvae tunnel through pages seeking food or a sheltered place to develop, and when the adult beetle finally emerges it leaves a small, clean, round exit hole. These neat circular holes punched through many pages are a classic burrowing signature. The damage is more extensive than surface grazing because it cuts deep grooves and channels right through the text block, and in severe cases an entire volume can be tunnelled into uselessness. A useful identification tip recorded in museum pest guidance from the National Park Service is to look for the debris insects leave behind, called frass, which appears as a soft powder, granular pellets, or stains depending on the species, along with shed skins and pupae.

Rodent damage: tearing, staining, and droppings 🔗

Rodents are sometimes overlooked, but mice and rats can be even more destructive than insects. They gnaw on almost anything, including books, papers, and bindings, partly to feed and partly because they use materials to build nests and wear down their constantly growing teeth. Their damage has a distinct character that sets it apart from insect work.

The first sign is irregular tearing and gnawing. Unlike the sharp, curved edges or neat round holes left by insects, rodent damage looks ragged and chewed, with large chunks missing from corners and covers. You may find shredded paper gathered together where a nest is being built. The second sign is staining and contamination. Rodent urine leaves dark stains and a strong smell on paper, while their droppings appear as scattered dark pellets on shelves and inside books. As the Chetham’s Library conservators note, mice both chew materials and foul them with urine and droppings, so the combination of coarse tearing plus pellet-shaped feces is the most reliable way to confirm a rodent rather than an insect or fungus. Greasy rub marks along edges and shelves, left by oily fur as rodents follow regular paths, add further confirmation.

Reading the clues together 🔗

In practice, more than one agent is often at work, since the damp conditions that invite fungi also attract insects and rodents. The skill lies in matching the pattern to the pest. Discoloration, musty odour, and crumbling paper point to fungi. Sharp-edged holes, frayed surfaces, neat round exit holes, mud tubes, and frass point to insects. Coarse tearing, shredded nesting material, urine stains, and pellet droppings point to rodents. Careful, regular inspection of storage areas, especially corners, low shelves, and humid spots, lets you catch these signatures while the damage is still small. Documenting what you find, as conservation manuals recommend, helps track whether an infestation is active or old.

What do you think? Looking at a library or personal collection you know, which biological signs would be easiest to miss until the damage was already serious? If you discovered both fungal staining and insect holes on the same shelf, which threat would you address first, and why?

References

1. https://www.namami.gov.in/conservation
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11508409/
3. https://www.ipm.org/show/amomentofscience/2021-02-09/foxing
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10672585/
5. https://library.chethams.com/blog/pesky-pests/
6. https://thebetterindia.com/culture/india-palm-leaf-manuscripts-preservation-techniques-11876650
7. https://www.nps.gov/subjects/museums/upload/MHI_Ch5_BiologicalInfestations.pdf