Conservation Advice - Museum Pests
"Pests come into conflict with man because they complete with us for resources. Ever since man first started to store food or make clothing and other artefacts he has had this battle" (David Pinniger, 2001).
There are a wide variety of commom museum pests including beetles, moths, silverfish, lice, rodents and other mammals. The insects to be worried about are generally quite small but other larger insects can be an indicator of other environmental problems. In order for pests to flourish they need a food source, harbourage and warm and damp conditions. In a lot of cases it is the larvae, rather than the adult of the insects do the most damage to historic objects. There are however exceptions to this rule, such as silverfish.
Integrated Pest Management (IPM) is a term used to describe the process of pest monitoring, isolated and appropriate treatment and creating a suitable environment to discourage pest activity.
Identification is the first step in dealing with a pest outbreak. The Pest Identification Poster produced by Historic England is an excellent place to start. There are also some very helpful pest fact sheets on the Collections Link website.
Wood Boring Insects
There are two main types of wood boring insects that occur in the museum environment. They generally prefer warm and damp conditions and will flourish in environments above 60% RH.
Death Watch Beetles enter the museum environment through open crevices and doors and windows. The adult beetle is 6-9 mm long and lays eggs in cracks and crevices of wood. The larvae like to feed on hard wood (generally oak and elm) as well as soft wood, which can cause extreme structural instability of wooden objects and buildings. Once the larvae have hatched they tunnel through the wood leaving behind frass consisting of small round pellets. Once the larvae have matured into an adult beetle they emerge from the wood leaving 3mm flight holes. The death watch beetle got its name from the knocking sound that the adult beetles make in the spring.
Indications of a recent infestation are small plies of light wood coloured frass and fresh, bright looking flight holes. An object may already have old flight holes from a previous infestation but new ones will be a similar colour to freshly cut wood.
The adult Furniture Beetle is 3-5 mm long and the larvae, although being smaller than Death Watch beetle, can cause the same amount of damage. The only different is that they do not tend to feed on solid heart wood. Again, as the larvae tunnel through the wood they leave being frass, which is smaller and wheat grain shaped. Furniture beetle has also been found in damp books and wood pulp. When the adults emerge they leave flight holes that are 1.5-2cm in diameter.
Adult moths can fly into the museum environment through open doors and windows and can also be prevalent in bird’s nests. There are many types of household moth that exits in historic environments. Moths such as the White Shouldered Moths and Brown House Moths rarely attack cleaning, dry textiles. The two main which will attack historic collections are the webbing Clothes Moth (Tineola bisselliella) and the Case Bearing Clothes Moth (Tineola pellionella).
The adult moth Webbing Clothes Moths measures 6-10 mm long and is a shiny beige colour with no spots on the wings. They will lay eggs on a suitable food source for their larvae such as woollen textiles and animal specimens. The larvae spin a web of silk over the surface of the food source, under which they can freely move over the surface, eating as they go.
The larvae of the Case Bearing Clothes Moth will spin a cocoon of silk around itself for protection, leaving one end open so they can eat and move around. It then eats across the material taking the cocoon with it. The cocoons are often made from the material the larvae are eating and so can be varying colour and texture. The adult moth is 6-10 mm long and is a shiny greyish-beige colour with two spots on each wing.
There are various kinds of carpet beetle which can cause damage to collections. These include Spotted Carpet beetle, Vodka beetle, Varied Carpet beetle and Guernsey Carpet Beetle. Again it is the larvae that cause most of the damage, but they are easily recognisable. Knows as “wooly bears” they can be up to 5mm long, have brown stripes and are hairy and shed their skins as they grow. The adult beetles are roughly 2-3 mm long and vary in colour depending on the type of beetle. The varied carpet beetle is a mottled cream, light and dark brown, the Guernsey beetle is mottled grey and dark brown and the Vodka beetle is plain brown. The spotted carpet beetle has 2 spots on its back.
The wool bears tend to feed on textile such as wool and silk, as well as fur, feathers and skins. They prefer to be in warm, dark places so can often be found it rolled carpets and dark areas of cabinets and boxes.
Adult beetle tend to take flight in warm weather and can often be found on windowsills. Their natural homes are birds’ nests and animal burrows, so birds nesting in attics and chimneys could be a cause of infestation.
The Australian Spider Beetle and the Golden Spider Beetle, do superficially look like spiders with a bulbous body, 6 legs and a 2 long antenna at the front which could look like legs. The adults are generally 3-5 mm long and will eat vegetable and animal debris. Insect and plant collections can be particular affected by spider beetles but the larvae will also eat wood, paper and textiles.
As the name suggest the larvae of the Biscuit Beetle and Cigarette beetle feed on dried food, plant material and freeze-dried animals. The Biscuit beetles had also been known to feed on papier mache objects. In the same way the wood worm larvae do, they burrow into hard materials and once they have grown into adults they leave neat flight holes. Cigarette beetles are generally only found in warm climates or heated buildings.
Silverfish are different to beetles and moths as they do not have larvae, they just have smaller versions of the adults which are called nymphs. These nymphs are significantly smaller and transparent compared to the adults. Both the Nymph and adult will eat the microscopic mould found on the surface of paper, books and textiles. The damage they cause can be seen as either holds with irregular edges or patches where the surface has been grazed. Silverfish are a good indicator of high RH as they like to live in dark and damp conditions. They are also attracted to certain types of dyes so they attack these areas first. This can sometime be seen in wallpaper where one colour has been eaten away and others haven’t.
Booklice are troublesome insects as the adults are very tiny (less than 1mm). Like silverfish they feed off the microscopic mould on paper and cardboard, causing a grazing effect on the surface and have been know at attack insect collections. The squashed bodies of booklice can also cause staining to the material as well as being a food source for mould.
Rodents and Other Mammals
Rodents can do an immense amount of damage to collections, particularly paper and textiles which can be chewed and ripped up for nesting materials. Prevention of a rodent problem is easier than trying to remove it. Hygiene and good housekeeping are important to remove any possible food sources for rodents. Removing hiding places such as over grown under growth outside the museum and proofing the building against infestation ie blocking up holes, protecting the bottom of doors with metal plates and covering opening with mesh will also reduce the likelihood.
Bats can often be found roosting in the roof spaces of museum and old buildings. Their urine is alkaline and cause damage to some objects, especially metal items. If using pest traps in an area where bats live or feed then they should be “bat proof” by either taping the side of you blunder trap or using the flat, plastic traps to ensure the bats do not get stuck on them, should they decided to try and eat the insects caught on the trap.
Make sure the temperature of a space is not consistently above 20C. Beware of direct sunlight as this can cause hot spots. Avoid areas of high humidity as warm and damp spaces are excellent breeding grounds for pests. Make sure all windows / doors are sealed properly and there is no space for pests to crawl in through gaps in windows and under doors. The use of brush strips on the bottom of doors is particularly helpful. Always check any plants/cut flowers coming into display spaces as they can bring pest in with them. Be aware of any “dead” spaces such as chimneys or under floorboards. These are good places for birds and rodent nests which can also harbour pest insects.
One of the major food sources for pests are skin cells, fibres, organic material etc so good housekeeping is a very important part of pest prevention. Avoid food and drink in gallery and storage areas as any small crumbs or droplets can be a food source. This is especially important in areas when cleaning is not undertaken so regularly like storage areas. Make sure that regular inspections are integrated as part of regular cleaning routines. Make sure that all storage spaces including attics, cellars, and cupboards are kept clean and tidy. Any warm, dark spaces are good homes for pest so try and keep all areas free of cutter.
Cleaned and check storage areas as regularly as possible. Used well sealed boxes and display cases to make it harder for pest to get (or out!). Make sure that suitable packing materials are used packing material that might be infested (i.e. an old cardboard box from someone’s basement) will cause an outbreak in your store. Mark boxes which contain vulnerable material and check those boxes more often for signs of infestation.
Quarantining new objects coming into the museum helps prevent fresh infestations of pest. Make sure this is done in a designated area away from display or storage areas. Think about where the objects might have come from and check the object for sign of infestation. If in any doubt, seal in a clear plastic or box and monitor over a spring / summer season to see if any adult insect emerge. Be especially careful of what the objects come in, especially if it is an old cardboard box from someone garage.
When using pest traps make sure you have the correct trap for the correct pest. The most frequently used are blunder traps which are sticky traps and catch pests as they walk across it. Some blunder traps also use pheromone lure to attract specific pests such as moths. Ensure you use the correct type of lure for the correct pest. Make sure the traps are in suitable locations such as entry points into the museum space (doors, windows, chimneys etc).
If you have bats in your storage or displays ensure you are using bat proof traps so they do not become trapped on them. Regular monitoring will help to discover an outbreak quickly
Inspection of traps should take place quarterly and the results recorded in a simple way so that it is easy to see patterns and trends. Sticky blunder traps and lure should last a year but should be replaced if they get overcrowded, very dusty or loose their stickiness. To help keep a track of them make sure you date your traps. Make a plan of where the traps are and try to have one or a couple of people who have responsibility for checking.
Cleaning / removal
If an infestation is detected it is very important to clean the surface of the object. This removes any debris left by the insects and any source of food for any other insects and helps prevent re-infestation. The object should be checked and re-cleaned at intervals as necessary. If more invasive cleaning is required a specialist conservator should be consulted. Where possible remove the infested item away from display and storage areas and contain in its own box or plastic bag.
Freezing is used widely for killing pests and their eggs in textiles and natural history objects. Care should be taken with fragile, composite or unstable materials as they should not be exposed to extreme temperatures. When freezing objects, make sure that:
- You are using a freezer capable of reducing temp of object with 24 hours, do not use self defrost freezers
- Seal objects in plastic bag – remove as much air as possible – absorbent material can be added to stabilise the RH
- Store at - 30C for 7 days, -25 C for 7 days, -18C for 2 weeks
- Do not unwrap until the objects have returned to room temperature