What are museum pests?

The Prince Philip Maritime Collections Centre is on the front line when it comes to keeping Royal Museums Greenwich's collection safe from moths, insects and other damaging pests.

With thousands of objects to care for, one of the conservation team's key jobs is to protect the collection from these unwelcome guests.

Pests have no prejudice when it comes to what they destroy. Whether it's an 18th century naval uniform or your favourite woolly jumper, the risks they face and the ways we can look after them are the same.

Here, we talk to three conservation experts to find out how they identify, prevent and treat the damage caused by pests.

Along the way, we ask for their top tips for keeping our own clothes safe from moths and other tiny household dangers.

What are museum pests – and how can you tell the difference between an insect and pest?

Insects are considered a museum pest (or insect pest) when they have the ability to eat materials on Museum collection objects – for example wool, silk, fur, feathers, wood and paper.

Royal Museums Greenwich has such a vast collection of objects made from a wide range of materials, and therefore identification of the specific insect pests that can find these materials attractive is crucial.

There are three main groups of insects that are considered pests for Museum collections: protein-eaters (moths and beetles consuming animal based materials such as wool, feathers, fur), starch-eaters (beetle consuming materials such as wood, some food based objects) and relative humidity indicating insects (silverfish and booklice consuming paper based materials).

Insects such as spiders, ladybirds and flies regularly wonder into museum buildings, but these are not classified as pests because they do not harm the collection.

All members of staff who look at insect traps are trained in insect pest identification. This training is specific to the types of insects found in museum collections and which could damage (eat) the collection materials. Through this training, we are taught the characteristics of each pest and how to correctly identify them – for example, we look for if they have spots on specific parts of their bodies, the colour and shape of the wings, the antenna shape, etc.

To help the identification process, we have reference images and samples of the insects collected by our team of conservators. All of the insect pests found in museums are less than 10mm (1cm) in size and therefore we use high-magnification microscopes in our conservation laboratory to aid the identification of insects found on traps.

What damage can pests cause in museums?

The materials with the largest threat from insect pests are animal-based textiles, hair, fur or animal skins, feathers and horn.

These protein-eaters create irregular shaped holes in materials and leave behind frass (insect poo). If moths are present webbing or cocoons can also be evidence of activity. If left undisturbed for a period of time, the damage can be extensive – resulting in large holes, bald patches of fur or complete destruction of hair and feathers.

The most common protein-eating insect pests are moths (case-bearing clothes moth and webbing clothes moth) and carpet beetles (Guernsey, Varied, Brown and two-spot carpet beetles).

Some insect pests like to eat starch-based materials, for example Furniture beetles or Biscuit beetles (starch-eaters). Furniture beetles can consume wood, particularly soft woods and other objects like paper made from wood pulp. This means that in a historical house they can feed on floorboards, furniture, picture frames, sculptures, and even books. Fortunately for museums and historical houses, furniture beetles need a very high humidity to thrive and therefore are unusual under normal museum conditions.

When active, Furniture beetle larvae bore into wood and live undiscovered for thee to five years inside the material, creating a secret network of tunnels. The main visible signs of furniture beetle activity, are the ‘flight holes’ created by adults emerging and the subsequent wood powder and frass that comes out of these holes.

Another starch-eater, the Biscuit beetle likes to eat hard, dried vegetable material including baskets, biscuits, nuts and objects made with dried plants (particularly in our ethnographic collection). The damage caused by these beetles small holes and tunnels, with material and frass deposits.

Relative humidity indicating insects are pests which only thrive in very moist conditions (higher than 75% relative humidity), such as silverfish and booklice. Silverfish and booklice feed on paper-based collections, causing damage to manuscripts, books, wallpaper, inks on paper, photographs, prints and drawings.

Although it is unusual for museums to experience the moist conditions required for these insect pests to thrive, cold materials such as tiles, concrete and metal racking can create micro-climates (or localised moist conditions) on which the insects can thrive – many people will be familiar with silverfish found in their tiled bathrooms.

Due to their small size, both silverfish (1-15mm) and booklice (1mm) feed by scraping or eroding the materials. Damage is very rare and usually only discovered if large numbers are feeding on single items – resulting in a grazed appearance, thinning and ragged holes.

Moths and beetles (protein and starch-eaters) start their life cycle when females lay eggs on food sources or attractive materials. These eggs hatch to produce larvae.

It is during this larvae stage that these insect pests cause the most damage. They are highly mobile and if undisturbed can continue feeding on materials for several months throughout their larvae stage until the temperature is high enough to complete the cycle.

When conditions are correct, the larvae will develop through the pupal stage to become an adult moth or beetle. This is why we often see moths and beetles flying around during summer months, more than any other time of the year. The adults however do not cause any damage to materials – their only purpose to mate and reproduce.

How to control pests in museums

Insect pests are a persistent threat to the collection, as they are small, highly mobile and can thrive in museum environmental conditions. As a result it is not usually practical within a museum or heritage setting to completely exclude all insect pests from a building. 

The focus of the Integrated Pest Management programme is therefore placed on denying insect pests a suitable environment where they can feed, breed and grow in population to become an active infestation.

The programme has two main preventive activities: monitoring and cleaning.

Monitoring is done by placing insect traps in locations across our galleries and stores. The best place to put them is on floors, near to doors, in undisturbed areas, in corners of rooms and behind or under furniture. In addition, we place traps in spaces where we are exhibiting objects that we know are made from materials that are highly attractive to insect pests.

After three months, these traps are collected for identification: recording the type of insect, the number of each type of insect pest, and the life cycle stage of the insect. When the number of insect pests is high in a trap it means that we have a higher risk to the collection. The data is collated and compared to previous seasons, so that we can take further actions if a pest is becoming more than a problem.

The other activity we do to prevent damage by insect pests is cleaning regularly. This is very important, as insect pests like undisturbed, dark and cosy places with a food source. The food source could be a wool carpet, curtain, tapestry, wardrobes, sofas or any food leftover – however the most attractive food to protein-eating insect pests is dust!

Dust is an accumulation of a range of minute particles including skin, hair and textile fibres, which are attractive materials for protein-eaters. If cleaning is not done regularly, it is easier for insects to complete their lifecycle, reproducing and thriving in our own buildings.

How do you treat any damage to the collection?

If damage has already happened we might look at supporting the holes from behind with a new, similar fabric that has been dyed to blend in with the original. This might be a patch for smaller holes or a full support for a large damaged area.

The fabrics are held in place and further supported with conservation stitching, called laid and couched stitching. Smaller holes may have a dyed nylon net overlay stitched in place to prevent further losses, as can be seen on the linings of the pocket flaps, before and after conservation.