Mould and Insect Attack in Collections
I. M. Godfrey and N. King Smith
Although mould and insect attack are treated separately in this section it is most important that pest control is dealt with in a coordinated fashion. Pest management should be a routine part of the operations associated with a collection not just a series of responses to crises.
Strang (1993) recommends conducting a survey to assess the existing situation before any attempt is made to implement a pest management regime. This survey should consider the following factors:
- the building and its location;
- portable fittings and hardware; and
- procedures and people.
These factors should be examined to determine their impact on pests. Is the building well sealed? Do storage and display cabinets restrict the entry of pests? Do staff eat and keep foodstuffs well away from collection areas? Are fresh plants or flowers allowed into the building? These are just some of the questions that should be considered in the initial survey.
Having assessed the existing situation, design and implement pest management activities that will minimise the risks posed to collections by pests (Strang 1993, Strang and Kigawa 2009). These activities should be based on ‘five stages of control’:
- respond; and
This approach to pest control, often called ‘integrated pest management’, involves:
- preventive measures;
- monitoring of collection areas;
- treatment of artefacts;
- treatment of infestations; and
- review and assessment of procedures.
The key points are summarised below with more details provided later in the chapter.
Key pest preventive measures include:
- excluding pests by using screens and sealing cracks and access points;
- ensuring that additions to a collection are not infested by quarantining and monitoring objects before adding them to the collection;
- isolating susceptible objects in pest-proof bags or containers;
- good housekeeping and maintenance in and around the building including removing rubbish, keeping plants away from the building and reducing or removing sources of moisture and food;
- positioning external lights so that they either draw insects away from building openings or at the very least don’t attract insects into the building;
- control of relative humidity, temperature and air circulation to prevent mould formation;
- not allowing food, drink or plant material (fresh or dried) in collection areas;
- thoroughly cleaning all areas associated with a collection (vacuuming with a HEPA filter is recommended). This includes storage areas, display areas, tea rooms, offices and the area immediately outside the collection; and
- applying appropriate pesticides to the external perimeters of buildings and on internal skirting boards (see Appendix 3) .
Monitoring Collection Areas
Collection areas should be monitored by:
- regularly inspecting for dead or live insects, insect frass, cocoons, silk webbings, holes in objects, musty smells, changes in the appearance of objects and any other signs of pest activity;
- using commercially available sticky traps to identify insect activity; and
- using termite baits to monitor activity around a building.
Treatment of Artefacts
Ways to treat artefacts to prevent or control pest infestations include:
- bagging and removing any affected artefacts from the storage and display areas; and
- freezing or fumigating artefacts, using inert gases, low oxygen or chemical techniques.
Treatment of Infestations
If an area of the collection is infested by pests, then:
- after isolating the affected artefacts, thoroughly clean the area (vacuuming with a HEPA filter is recommended); and
- apply pesticides if pest activity continues.
Review and Assessment of Procedures
Management of pest infestations should be assessed against existing procedures to determine their effectiveness. Take the following steps after a pest outbreak:
- document the outbreak. Record details concerning the artefacts affected, the extent of damage incurred, when the infestation occurred and the nature of the infestation (for example, type of insect attack);
- determine the circumstances that led to the infestation; and
- decide what action can be taken to prevent similar problems in the future.
Note that the cycle does not end with the treatment of an object. In fact it begins again. There is no point in eradicating pests if reinfestation is likely to occur. Follow treatment with remedial action and careful monitoring to reduce the chances of pests returning.
If in any doubt, consult a conservator who will be able to provide you with the latest information on control and treatment techniques.
Most people have seen mould growths on old bread, cheese, jam, damp wood and leather. This mould growth, usually appearing as a fine fluffy mass is referred to as the ‘fungal colony’. Mould growth is favoured under warm, dark and moist conditions when air circulation is limited. The presence of dirt, particularly organic detritus also increases the risk of mould growth.
The appearance of the colony changes when conditions are favourable to mould growth, usually when the relative humidity is high. The older parts take on a different colour and fruiting structures appear. These fruiting structures, which contain the individual reproductive bodies called spores, are usually only visible with a microscope. The fruiting structures stand up above the mass of the colony so that the spores may be discharged into the atmosphere and carried by air currents to suitable growth surfaces.
Under such conditions spores absorb water, enlarge rapidly and branch repeatedly, forming a new colony. When the colony is established and large enough, fruiting structures appear, spores are produced and the cycle is repeated.
As long as the environment is suitable mould will form on organic materials (dead plant and animal matter). Provided there is sufficient moisture available, either in the object itself or in the environment, fungi will feed on leather, cotton, wool, bone, paper, wood, bark, fur and rawhide (Figure 1). Fungi are also capable of growing on metal and stone surfaces, particularly those with a coat of dust or organic debris. Some species even grow on creosote-treated wood while others can incorporate poisonous, chlorine-containing compounds into their diets.
Examples of undesirable effects of fungal infestation include the complete destruction of wooden and paper materials, staining of textile and paper-based objects and attack on photographic gelatin.
Some fungi can tolerate temperatures as low as minus 10 °C while others can survive in temperatures as high as 110 °C. Fungi are affected more by relative humidity levels than by temperature. A few species can survive below 60 % relative humidity but the majority require a relative humidity of at least 70 % to survive and reproduce. Fungal bodies normally die if the relative humidity drops below 60 %. The spores released at such adverse times will lie dormant however until suitable conditions for growth are available again.
Preventing Mould Growth
Preventing contamination by fungal spores within any building is practically impossible as spores are everywhere in the air and will be present on all surfaces. It is possible however, to control conditions so that it is very difficult for spores to germinate. This can be done by controlling the relative humidity and air circulation. Methods used to control relative humidity are described elsewhere (see the chapter Preventive Conservation: Agents of Decay).
If the relative humidity is maintained at a sufficiently low level spores cannot germinate. A relative humidity range of 40 to 65 % is considered safe. Too low a value (below 40 %), while stopping mould formation, can damage susceptible materials such as paper, leather and silk.
As mould growth is encouraged under stagnant conditions, it is also important to maintain adequate ventilation in storage and display areas. The use of something as simple as a fan will help to stop the build up of high relative humidity microclimates in susceptible areas such as against exterior walls and in basements.
Although elimination of the sources of fungal spores is not possible, surface contamination of indoor objects can be reduced by:
- sealing windows and doors and using central air-conditioning;
- storing objects in dustproof containers;
- covering objects that have been temporarily removed from containers; and
- vacuum cleaning objects carefully prior to storage or display.
It is more efficient to control collection conditions and prevent mould growth rather than have to treat objects contaminated by active colonies.
Material on Display
Provided the temperature does not vary greatly individual display cases can act as a buffer zone within a collection, helping to maintain relative humidity at a reasonably constant level. High relative humidity periods may occur sporadically (average relative humidity below 65 % but with occasional spells above this level) or endemically (average relative humidity above 65 %). Different methods are needed to control display case humidity under these conditions.
Sporadically High Relative Humidity
The strategy adopted to deal with occasional periods of high relative humidity will be largely determined by the nature of the storage or display equipment and the condition of objects in the collection. High relative humidity can usually be controlled by using a well-sealed display case or cabinet as a buffer against the ambient conditions. If this is not successful it may be necessary to use a dehydrating agent, such as self-indicating silica gel, to absorb the excess moisture. A change in colour from orange-yellow to either dark green, pale yellow or colourless (the colour change is determined by the type of silica gel used) indicates that the silica gel is saturated with moisture and needs to be changed. Note that the formerly recommended blue self-indicating silica gel is now classified as a toxic substance and should not be used. Other alternatives for controlling relative humidity levels are described elsewhere (see the chapter Preventive Conservation: Agents of Decay).
A disadvantage of using silica gel is the large quantity required to achieve and maintain a reduced relative humidity environment. For transporting objects or for short periods of high relative humidity, one kilogram of silica gel is needed for each cubic metre of space. If the problem is endemic then 20 kg/m3 is necessary (Thomson 1986).
Endemically High Relative Humidity
The options for controlling prolonged periods of high relative humidity are to use silica gel (20 kg/m3) in a well-sealed display case or to improve the ventilation so that mould growth is not possible. Although the use of air curtains and fans greatly improve conditions in tropical regions these may have to be combined with the use of silica gel or other desiccants if mould growth continues to be a problem.
Treatment of Mould
Mould is indicated by the presence of a ‘musty’ smell, stains and ‘fluffy’ or powdery growths on susceptible materials. Mould’s presence can be confirmed by either microscopic examination of the colony and/or culturing of the fungus under controlled conditions.
Initial steps in the treatment of mould (Guild and MacDonald 2004) include:
- protection of people;
- isolation of artefacts;
- deactivation of the mould by either air-drying or freezing; and
- cleaning of artefacts.
The first step in the treatment of mould is to protect all people in the vicinity of the outbreak from exposure to fungal spores. Moulds can cause health problems, especially for anyone with pre-existing respiratory ailments. Some moulds are toxins while others are primarily allergens. If the presence of mould is suspected it is important to take appropriate precautions when handling contaminated objects. Wear disposable gloves, a high quality mask or respirator, safety glasses and protective outer clothing (washable or disposable overalls).
Isolate mould-affected artefacts to prevent dispersal of spores. Seal small objects in polyethylene bags or boxes and wrap larger objects in plastic sheeting. If the objects are still damp or wet, minimise the time that objects are sealed in plastic so that further mould growth is not encouraged.
Mould must be deactivated before it is removed from an object. If living mould is physically removed from artefacts there is a danger of spreading spores to other objects, not to mention the associated health risks of such a practice.
Whereas chemical treatments such as paradichlorobenzene and thymol have often been recommended for mould treatment, avoid their use as they are harmful to both people and many materials in the collection. It is preferable to focus on deactivating the active mould using non-chemical methods (see below), cleaning up any affected storage areas and taking steps to eliminate the cause of the outbreak (water leak, air conditioning failure etc).
It is most important to do something about the conditions that led to the development of mould in the first place such as implementing passive environmental controls, effective housekeeping techniques and improved building maintenance. It is important to examine the area from which the object came, to treat this if necessary (see ‘Treatment of previous storage and display areas’ below) and to take steps to ensure that the risk of future outbreaks is minimised.
There are a variety of methods for treating mould-affected objects, including freezing, air drying between 30 and 40 °C and exposure to either ultraviolet radiation, sunlight or gamma radiation. As some of these methods can be damaging to objects, consult a conservator if you are unsure about possible treatment effects. Additional details of two of these methods are presented below.
Air Drying Treatment
Drying a mould-affected object will deactivate growing mould, minimise further mould growth and will also allow the object to be vacuum cleaned to remove fungal hyphae and spores from the surface.
If the object is still damp it should be supported, preferably on an absorbent, clean surface such as blotting paper and spread out as much as possible. If the object will not be damaged by elevated temperatures, raising the temperature to between 30 and 40 °C will deactivate growing mould. If this is not possible then dry the object using natural ventilation, fans or dehumidifiers. Fans used to increase the air circulation should not be aimed directly at the object to reduce the risk of damage to the object and the spread of spores.
Once the object is dry it may be cleaned by careful vacuuming using a HEPA-filtered vacuum cleaner. See the Textiles chapter for details on appropriate vacuum cleaning techniques.
Freezing artefacts is a further example of a non-chemical approach to mould treatment. Freezing has the potential to kill germinating spores and associated hyphae (fine fibrous strands), but may not be effective against dry spores (Florian 1994). Slow freezing to minus 20 °C and slow thawing enhance the effect of freezing on mould and spores (Florian 1997). Controlling environmental storage conditions following freezing treatment is critical to avoid activating dormant spores.
Whereas previously there were many objects for which freezing was considered unsafe (Berkouwer 1994), it is now considered safe to freeze most materials, including composite organic/inorganic items (Carlee 2003, Strang 2008). The low moisture contents of most museum objects, the inclusion of adsorbent, buffering materials with the object to be frozen and sealing in polyethylene bags all help to prevent damage to susceptible objects. Some objects that should not be frozen however include the following:
- desiccated or seriously degraded objects;
- some objects which contain a variety of materials, especially layers; and
- objects that have a high natural water content.
Such objects may not be able to withstand the fluctuations in relative humidity due to temperature changes and subsequent dimensional changes. Examples of such materials include:
- oil or acrylic paintings on canvas or panels, painted textiles and painted wood;
- glass plate negatives, glass colour transparencies, lantern slides and mounted glass slides, daguerreotypes, ambrotypes and tintypes;
- plant specimens or other objects that have high water contents (> 26 %); and
- computer, magnetic and grooved audio media such as tapes, disks, cassettes, discs and cylinders.
Many textiles and wooden artefacts, most paper-based objects including modern photographic prints, acetate film, books, herbarium specimens and artefacts made from materials of plant origin (leaves, seeds etc), leather artefacts, natural history specimens and objects made from materials of animal origin (feathers, fur, horn, bone etc) may be frozen to eradicate mould and insects.
If mould attack has occurred already, then the affected object, its storage area and any materials in contact with the object should be treated following the steps described below (Dodd 1991):
- isolate all affected material to stop further contamination;
- treat affected materials and storage areas;
- determine the cause of the mould outbreak; and
- rectify any problems which may have contributed to mould formation.
Isolation and Preparation for Freezing
The steps involved in the isolation of mould-affected objects are summarised below:
- place affected objects in plastic bags and remove them from the collection;
- if the mould outbreak is due to wetting of an object then allow it to dry out partly (see above), acclimatise to a relative humidity of 50 - 60 % and then wrap it in clean tissue paper, clean white cotton or linen;
- place the object in a polyethylene bag and remove most of the air from the bag. Carry out this procedure in a well-ventilated area and wear appropriate personal protection to reduce the risk of inhaling fungal spores; and
- seal the bag with tape or by heat sealing.
Treatment of Affected Materials
To ensure treatment of mould-affected materials is effective and that the risk to the artefacts is minimised, follow these steps:
- either wrap the objects in absorbent material like clean white cotton or linen or clean tissue paper or add these materials as packing in the bag to provide additional protection against relative humidity changes during freezing;
- seal the objects in polyethylene bags and place them in a freezer for at least four days at minus 20 °C or below ;
- allow objects to thaw slowly (in a refrigerator) before removing them from the refrigerator. Allow them to warm to ambient conditions before unsealing; and
- use a HEPA-filtered vacuum cleaner in a well-ventilated area to remove dead hyphae and spores from objects. Correct vacuum cleaning techniques are described elsewhere (see the chapter Textiles).
Vacuum clean objects in a fume cupboard or similar containment enclosure. If a suitable enclosure is not available then clean the objects outdoors in a dry, well ventilated area away from air-conditioning intakes and other people.
Unless access to industrial freezers is possible, there are limitations on the size of objects that can be treated in this fashion.
Treatment of Previous Storage and Display Areas
If possible, dry storage and display equipment at elevated temperatures. Spores cannot survive three to four hours of temperatures greater than 36 °C. While this is desirable, it may not be possible for large storage racks or indeed may damage the equipment. If the climatic conditions are warm enough, seal contaminated shelving units in black plastic and evacuate as much air as possible. Place the sealed shelving in the sun for the required period. As long as the plastic is well sealed the shelving will not be desiccated as there will be insufficient exchange of moisture from the shelving to the surrounding air.
Unless the storage equipment is likely to be damaged by contact with alcohol, swab the equipment with a 70 % alcohol/water solution as a further precaution against re-contamination. Methylated spirits can be used to make up the alcohol solution. Carry out this treatment in a well-ventilated area. Alternatively use a 0.5 % solution of sodium hypochlorite (household bleach) to wash the equipment. If bleach solution is used, wet the surfaces for 15 to 20 minutes with disinfecting solution to ensure that complete disinfection occurs (Guild and MacDonald 2004). There are also commercially available products containing ethanol, water and ortho-phenylphenol that are suitable for use on non-painted surfaces.
Determining the Cause of the Outbreak
All possible causes of mould growths should be examined and maintenance organised if a leaking roof, overflowing gutter or similar building problem contributed to increased relative humidity levels in the collection area. Implement relative humidity control strategies if the mould outbreak was the result of adverse weather conditions (see the chapter Preventive Conservation: Agents of Decay).
For some mould-affected objects, non-chemical techniques may be neither effective nor appropriate. In such cases it may be necessary to consider chemical fumigation. If there is no alternative to chemical treatment, consult a conservator to obtain the most recent information on chemical techniques and their effects on specific material types. This is important because many chemical treatments are damaging to objects and great care is needed. It is preferable to avoid chemical intervention if possible.
Use commercial pest control agencies to deal with heavy infestations of either fungi or insects. Again, consult a conservator prior to such intervention to minimise risks to artefacts.
When removing dead fungal matter, wear suitable safety gear such as a high quality dust mask or respirator, disposable gloves, protective outer clothing and glasses or goggles. Wear protective gloves and goggles when handling methylated spirits and other potentially dangerous chemicals.
The most common insects encountered in museums or private collections include the:
- case bearing clothes moth;
- webbing clothes moth;
- hide beetle;
- carpet beetle;
- cigarette beetle;
- drugstore beetle;
- furniture beetle (wood borer);
- cockroach; and
Details of the life cycles and identification of some of these pests are provided elsewhere (Appendix 4). The insects mentioned above can contaminate, damage or even completely destroy artefacts contained in ethnographic, natural science, archaeological and historic collections. Unless the attack has been in progress for considerable time, most of the damage is detectable only by close inspection.
Depending on the species, insects will feed on organic materials composed of either proteins or cellulose. Objects made up of materials such as wool, fur, leather, paper, wood, feathers, hair and plant specimens are particularly susceptible to attack (Figure 2). Occasionally some insects will attack non-food materials during certain stages of their life cycle and may even burrow or tunnel through artefact materials to get to a food source.
The procedures involved in a coordinated approach to minimising the risk and dealing with insect attack are detailed below.
The first step in the control of biological pests is the adoption of preventive measures to minimise infestations. Although no building will ever be insect-proof, steps can be taken to reduce potential problems. Non-chemical strategies to minimise insect problems have been reviewed by Pearson (1993). Some recommended strategies include:
- thoroughly inspecting objects to be incorporated into a collection. If necessary, quarantine objects by storing them in polyethylene bags and checking for any signs of activity over a two month period. If in doubt, assume that pests are present;
- keeping garden and household rubbish away from buildings and disposing of it as soon as possible;
- positioning external lights so that they either draw insects away from building openings or at the very least don’t attract insects into the building. Mercury vapour lights are attractive to insects but sodium lighting is less so;
- ensuring food and food preparation areas are away from collection and display areas, food containers are sealed and spills and scraps are cleaned up promptly;
- using clean white paper as a liner on the bottom of storage drawers or shelves so that it is easier to see if there is any insect activity (insect bodies, frass etc);
- keeping doors and windows closed. Fit screens and draft excluders to doors, windows, vents and drains;
- regularly inspecting buildings to highlight and fix any problems such as leaking pipes, drains etc;
- sealing spaces around pipes and ducts with caulking compounds (both interior and exterior). Repair cracks and other possible entry points;
- cleaning drains and gutters regularly;
- thorough housekeeping (regular vacuum cleaning rather than just dusting) is essential as dirt and debris provide food and shelter for pests and create damaging microenvironments; and
- using storage units and display cases as secondary barriers to insect intrusion. These should be well sealed (gaskets) to prevent insect entry to collected objects. Polyethylene and polyester bags can also be used.
It is also important to note that unfortunately it is often the people working with collections who transport insect pests into their collections. Insects may be hidden in the corrugations of cardboard boxes, in work bags and satchels and even in books brought into collection areas. Be vigilant!
Low temperature storage will assist in the control of insect pests. Breeding cycles are interrupted and insect development are reduced below 5 °C, while temperatures between 5 and 10 °C effectively prevent large increases in insect numbers (Pinniger 1989).
Chemical treatments are a useful adjunct to the range of preventive measures described above but should never be seen as the main focus of pest management activities. Applying chemicals to the exterior of a building, skirting boards and display surfaces that are particularly susceptible to insect attack (dioramas for instance) will assist in minimising damage to collections.
In choosing a particular insecticide find a balance between the effects of the chemical on insects and its impact on the environment (e.g. residual nature and toxicity), the artefacts in the collection and the people who may come into contact with it. A summary of commonly used pesticides, their characteristics and known effects on materials is provided elsewhere (Appendix 3).
If correctly applied, pyrethrins, organophosphorous and carbamate-based pesticides can be used on buildings and internal structures with minimum risk to artefacts housed there. Very few adverse reactions have been observed from using pyrethrins near artefacts. Pyrethrins may be either natural or synthetic, persistent or non-persistent. They generally have a low toxicity towards humans and are characterised by their ability to rapidly knock down insects. Synthetic pyrethrins (permethrin for example) have been prepared to overcome the lack of persistence of the naturally occurring compounds. The increased persistence of permethrin however, is accompanied by an increased toxicity to humans. Other synthetic pyrethrins such as bioresmethrin and phenothrin are less toxic than natural pyrethrins and consequently are suitable for space treatments.
Note that spray treatments of rooms and galleries is not an effective way of controlling insect pests. Most pests will be sufficiently hidden in cracks and crevices and inside the covers of books for example so as to be unaffected by aerosol spraying. It is almost a Darwinian way of improving the gene pool of insects by killing the most vulnerable and leaving the damaging ones to continue to breed! This approach, while often making those responsible for collections feel that they are really doing something to care for their valuable objects, is therefore not recommended. Targeted use of residual insecticides on skirting boards and other surfaces is far more effective.
Organophosphorous insecticides, which include chlorpyrifos, dichlorvos, diazinon and malathion are usually highly toxic. They have a moderate residual activity and among other effects on materials, most compounds stain red carpets.
Carbamates, like the organophosphorous insecticides, have a moderate residual activity and effective knockdown ability. They are found often in commercially available household products. Carbamate-based insecticides include carbaryl, bendiocarb and propoxur.
Desiccant dusts, usually made up of diatomaceous earth or silica, are non-poisonous and are extremely effective for use under cupboards, in cracks and in other secluded areas. They remain active for long periods and can be obtained with pyrethrin additives which enhance their knockdown capability.
Monitoring Collection Areas
Inspect collection areas regularly and carefully. Observe objects in the collection, storage and display units and secluded areas (cracks, crevices, under furniture) in which either insects or signs of activity may be present. Use a torch to inspect secluded places. Systematic inspections will substantially reduce the risk of major infestations. As well as inspecting general collection areas, priority should be given to the careful inspection of highly susceptible objects (fauna, furs etc). Indications of potential problems include:
- sightings of insects (adult, pupal or larval forms);
- frass (wood dust and faecal debris), remnants of cocoons, webs, skins, and the like; and
- exit holes in wood, damage to materials in the collection.
Sticky (blunder) traps are commercially available and are an excellent early warning system that provide evidence of the presence and the type of insects in a collection long before they are noticed by visual inspections (Figure 3). Traps are available for both crawling and flying insects. Follow the guidelines below for the most effective use of these traps:
- place traps along the edges of walls, near doors, in corners, under furniture and away from strong lighting;
- use one trap in every corner and every 30 to 50 square metres in small areas or every 100 to 200 square metres in large areas like warehouses;
- keep records of the dates, positions of placement, person responsible and insects detected. Label each trap with a unique number so the findings remain well defined;
- inspect traps regularly (weekly at first and then monthly); and
- replace traps after 3 – 6 months or when they are either full of insects, have become dirty or have lost their stickiness.
Use insect traps and visual inspections of the collections and collection areas in a complementary fashion - the visual sighting of larval casts should be followed up by placing sticky traps in that area and the capture of insects by a sticky trap should be followed by a careful inspection of the collection area and of vulnerable objects.
If it appears that insects are active the next steps are to determine the nature and extent of the infestation. Any material that would aid in the identification of pests should be collected - frass and obviously insects (at any stage of their life cycle) are diagnostic. Place live insects, especially larval forms, in a glass jar containing methylated spirits so their shape and size will be maintained until they can be examined by experts.
If a potential insect outbreak is indicated then it is possible to use sticky traps that incorporate specific pheromones (insect sex attractants) to get an indication of the extent of the likely problem. These traps are effectively ‘baited’ with pheremones that will attract specific insects to them, thus giving an idea of the local population of that particular species. Follow manufacturer’s guidelines to ensure that traps are placed in the most appropriate locations. Do not use pheromone sticky traps in the first instance as they are likely to attract insects into your collection areas. Pheremone traps should be inspected weekly.
To determine if an infestation is new or old, objects (textiles for example) should be isolated and sealed in polyethylene bags or, in the case of wooden objects in particular, placed on a contrasting background. Regular inspections in the following months will allow new frass or other signs of activity to be noted. When placing objects in polyethylene bags, take care to ensure that unfavourable microenvironments do not form within the bag. Monitor the condition of the object as well as looking for signs of insect pests.
At this stage two concurrent activities take place. Treat artefacts that have been infested by insects and determine the extent and source of the infestation. Eradication of the infestation follows. The latter activity may involve treatment of the object, the storage and display area, the building itself or a combination of these. Each of these aspects of pest management will be dealt with in turn.
Treatment of Artefacts
There has been a very strong move away from the use of chemicals to treat insect infestations with much research being directed towards the development of techniques based on the use of either freezing, low oxygen or heated environments. If correctly applied, these approaches have the potential to treat infestations without any risk of damage to artefacts. Despite this trend, there is still a place in an overall management program for chemical treatments whether they are used in treatments of infested objects or purely in a prophylactic mode.
While heating to 55 °C is a very effective means for rapidly killing insects, this method is not described here. Those interested are referred to Strang and Kigawa (2009) for more details.
Of the non-chemical treatments, freezing is recommended because of the quick treatment times, relatively low cost, ease of preparation for the objects, low monitoring requirements and its inert, non-toxic nature.
Freezing techniques may be used for most organic materials but are not suitable for all artefact types. Details of materials for which freezing is not appropriate are outlined in the earlier section on Treatment of Mould.
To ensure treatment of insect-affected objects is effective and that the risk to the artefacts is minimised, wrap the objects in clean absorbent tissue, cotton or linen, remove as much air from the bag as possible, seal the objects in polyethylene bags and place them in a freezer according to one of the following regimes:
- for two weeks at – 20 to – 30 °C;
- for 5 days at - 30 C or below; or
- for seven days at -18 °C to – 20 °C, followed by thawing (still sealed in polyethylene) at room temperature for 2-3 days followed by a further seven days at -18 °C to - 20 °C.
Pack objects in the freezer so that the drop in temperature is as rapid as possible for all objects and allow objects to thaw slowly and warm to ambient conditions before unsealing them. Keeping the objects in their bags will prevent condensation forming on objects as they warm to ambient conditions (allow about 24 hours for this).
The double freezing method (Berkouwer 1994) mentioned above is convenient because the temperature range -18 °C to - 20 °C is that at which most household chest freezers operate. The object is thawed between freezing cycles to allow any eggs to hatch that were not killed during the initial freezing period. The hatched larvae will then be killed during the second cycle.
Freezing is the cheapest, easiest and most convenient form of insect treatment. Check with a conservator if there is doubt about the suitability of an artefact for this type of treatment.
Low Oxygen Atmospheres
Killing insects using oxygen-depleted atmospheres is a useful, non-contaminating alternative for objects that cannot be treated by freezing. Low oxygen atmospheres can be produced by flushing a storage system with either nitrogen or argon or by using an oxygen scavenger to remove oxygen from a closed system.
Note that pigments such as litharge, cinnabar and sienna are not suitable for low oxygen treatments because of colour changes that result from the anoxic environment. Prussian blue and ultramarine experience temporary colour changes but these reverse when the materials are re-exposed to oxygen.
Although the use of carbon dioxide as a fumigant is not a strict application of the use of low oxygen atmospheres, it will be considered later in this section as the principles for its use are similar to those for low oxygen applications and it is an easy and very effective technique for pest eradication.
There have been numerous studies carried out to determine optimum treatment times for particular insect pests but at this time no hard-and-fast rules have been developed as there are many factors to consider. Some general principles do apply however. Shorter treatment times will be effective if higher temperatures, lower relative humidity levels and lower oxygen concentrations are used and if argon is used instead of nitrogen. To be most effective against insect pests the oxygen concentration must be kept in the range 0.1 – 0.3 %. A typical treatment would take approximately 4 weeks if the temperature is 25 °C.
The method of lowering the oxygen concentration will be determined by the size of the object to be treated and by the availability and cost of appropriate equipment and expertise.
Methods which may be used to treat infested objects include:
- flushing a bag with an inert gas (e.g. nitrogen, argon) until the oxygen concentration is less than 0.1 % and then adjusting the gas flow to maintain the low oxygen level;
- flushing a bag as described above and sealing it after an appropriate amount of an oxygen scavenger has been added; and
- adding a calculated number of satchels of oxygen scavenger to a bag and sealing it.
These three methods have been described in detail elsewhere (Daniel et al 1993) with additional information on the properties and use of the oxygen scavenger, Ageless® provided by other sources (Grattan and Gilberg 1994, Brandon and Hanlon 2003). Alternative commercial oxygen scavengers include Atco™, FreshPax™ and RP-System™. While Ageless®, Atco™ and FreshPax™ are iron oxide-based, the RP-System™ is based on unsaturated organic compounds that absorb corrosive gases as well as oxygen. The iron oxide-based products increase the relative humidity environment inside the treatment bag. For prolonged treatment, it would be prudent to include conditioned silica gel or Artsorb in the sealed bag to maintain an appropriate relative humidity. While limited information is available about the RP-System™ scavenger, it is claimed to have a neutral effect on relative humidity levels. More details about these products are available elsewhere (Maekawa and Elert 2003).
In the text below, while Ageless® will continue to be referred to, it is important to note that this is not to be taken as a recommendation for this particular product, but merely as a convenient way of referring to low oxygen scavengers in general.
In all cases the bags into which the artefacts are placed must have a very low gas permeability. Suitable bags for use with oxygen scavengers are laminates based on a number of different polymers including polychlorotrifluoroethylene, polyvinylidene chloride, polyethylene terephthalate and copolymers of ethylenevinyl acetate and ethylenevinyl alcohol. These products are sold under a variety of commercial names. Consult a conservator or conservation supply company to find the name of an appropriate product that is locally available.
The first two oxygen-depletion methods, which are best suited for the treatment of large objects, should be left in the hands of professionals. Specialised equipment is needed and careful control must be exercised over the relative humidity of the gas used to flush the system to avoid desiccation of sensitive objects.
The third method is well suited to the treatment of small objects, in particular those with a volume of less than about 100 litres. The oxygen scavenger Ageless® Z-2000, which is capable of absorbing 2000 millilitres of oxygen from 10 litres of air is recommended for museum objects. An indicator, Ageless-Eye® must be placed in the treatment bag. The indicator is pink when the oxygen level is less than 0.5 % and blue if it is above that level. Note that there are different types of oxygen indicators available, designed to cater for different temperatures and relative humidity levels. These indicators should be stored in the dark and protected from strong light when in use to reduce fading under strong light conditions. Of the available indicators, Ageless-Eye® K is recommended for typical anoxic treatments. Fresh indicators should be used with each treatment.
Steps involved in the treatment of small objects include:
- buy or make bags using a heat sealer and appropriate barrier film (see above). The latter option is recommended as bags can be tailored to particular objects. Unfortunately both bags and barrier film are quite expensive;
- calculate the approximate volume of oxygen in the bag, using the dimensions of the bag, the approximate volume of the object and the concentration of oxygen (O2) in the air.
Volume O2 = (bag volume minus object volume) x 0.2
- place the objects, the oxygen indicator and enough packets of Ageless® Z-2000 in the prepared bag, using about 25 % more Ageless® than calculated. As the Ageless® packets become hot when the scavenger reacts with oxygen they must not come into direct contact with the objects;
- place conditioned silica gel or a similar desiccant in the bag if the bag is not going to be flushed with nitrogen or argon before sealing;
- build an internal protective frame of cardboard for very fragile objects to prevent crushing;
- seal the bag. Heat sealing is necessary to ensure air-tight seals. If possible, flush the bag with nitrogen or argon before sealing as this will extend the life of the Ageless® scavenger significantly and will also counter the increased relative humidity that is produced in the bag by the oxygen scavenger;
- store the bags at 25 °C for about four weeks. If the objects are left at room temperatures of approximately 20 – 22 °C treatments should continue for at least three months; and
- following treatment remove the objects from the bags, discard the Ageless® packets, inspect and carefully vacuum the objects. The bags may be reused.
It is important to maintain the temperature at or slightly above 25 °C to ensure that all insects are killed.
While the above method appears quite straight forward, practice is required to ensure that the correct technique, pressure and temperature are used to seal the bags effectively. If unsure, make a second seal line as insurance against failure of the first. If attempting to seal large sheets of film, use tape or clips to hold the sheets in place before heat sealing. This will minimise wrinkling and leakage.
Carbon dioxide is more effective and generally cheaper than either nitrogen, argon or oxygen scavengers but requires specialised equipment to operate and monitor. Although some have reported concern about the slight possibility of interaction between carbon dioxide, moisture and objects to produce potentially damaging carbonic acids and/or carbonates, this is extremely unlikely and fumigation with carbon dioxide is considered to be safe for all material types.
The usual procedure is to evacuate most of the air from a vapour-proof bag that encloses the object and then to fill it with carbon dioxide. Alternatively the air inside the bag can be displaced by the carbon dioxide that is heavier than air. By using an inlet and outlet system with openings on the upper surface of the bag or enclosure, the lighter nitrogen and oxygen will be forced out by the incoming, denser carbon dioxide.
Maintain carbon dioxide concentrations above 60 % for the treatment to be effective. A special monitor must be used to measure carbon dioxide levels in the bag and to ensure that the gas, particularly if used in large volumes, does not leak into the surroundings. It is also prudent to monitor the relative humidity of the bag environment during treatment. Typically objects need to be exposed to carbon dioxide for 4 weeks at 25 °C. Treatment times can be reduced if higher temperatures are used.
As exposure to carbon dioxide can be damaging to human health, it would be wise to check with local authorities before using this technique for treating large objects.
Factors to be considered when choosing chemical insecticides include:
- toxicity to the insect;
- toxicity to people;
- effects on objects;
- environmental effects; and
The area of chemical control of insects is highly specialised. Insecticides act in a variety of ways, as stomach poisons, by interrupting developmental stages or by contact. They may be applied in many different forms, as dusts, emulsions, oil concentrates, gases and impregnated resin strips. The active ingredients of pesticides also vary.
Insect growth regulators are persistent chemicals which may be used in storage and display areas. They do not rapidly knock down insects, rather they act by upsetting the balance of hormones needed to control growth and development of insects. In this way breeding cycles are interrupted and control is exerted over developing insect populations.
While there is merit in the application of residual pesticides to a building and its internal structures, direct application of pesticides to artefacts is strongly discouraged. As many chemicals react with artefact materials, carefully choose insecticides for use near collections. Information about particular chemical treatments is provided elsewhere (Appendix 3).
Review and Assessment of Procedures
It is important not just to ‘clean up’ after a pest problem. Assess the situation so that similar problems do not occur in the future. Prepare an action plan outlining the steps to be taken to minimise future risks. Re-examine all factors which affect pest management to identify and take remedial action against any deficiencies in the control mechanisms.
Berkouwer, M., 1994, Freezing to eradicate insect pests in textiles at Brodsworth Hall, The Conservator, No. 18, pp. 15-22.
Brandon, J. and Hanlon, G., 2003, A low tech method for insect eradication using AgelessTM, American Institute for Conservation, 2003 Wooden Artifacts Group Postprints, Arlington, Virginia, (http://aic.stanford.edu/sg/wag/2003/brandon_hanlon_03.pdf)
Carrlee, E., (2003), Does low temperature pest management cause damage? Literature review and observational study of ethnographic objects, Journal of the American Institute for Conservation, vol. 42, pp. 141-166.
Daniel, V., Maekawa, S., Preusser, F.D. and Hanlon, G., 1993, Nitrogen fumigation: a viable alternative, Preprints, ICOM Committee for Conservation 10th Triennial Meeting, Washington DC, pp. 863-867.
Dodd, W., 1991, Freezing as part of an integrated pest management system: Review after seven years maintaining a textile collection, AICCM National Newsletter, pp. 12-13.
Florian, M.-L. E., 1986, ‘The freezing process – effects on insects and artefact materials’, Leather Conservation News Volume 3 (Number 1), Materials Conservation Laboratory of Texas Memorial Museum, Austin, Texas, pp 1-17.
Florian, M.-L. E., 1994, Conidial fungi (mould, mildew) biology: A basis for logical prevention, eradication and treatment for museum and archival collections, Leather Conservation News, vol. 10, pp. 1-29.
Florian, M.-L, 1997, Heritage eaters, insects and fungi in heritage collections, James and James, London, pp 129-130.
Grattan, D.W. and Gilberg, M., 1994, Ageless oxygen absorber: chemical and physical properties, Studies in Conservation, vol. 39, pp. 210-214.
Guild, S. and MacDonald, M., 2004, Mould prevention and collection recovery: Guidelines for heritage collections, CCI Technical Bulletin No. 26, Canadian Conservation Institute, Ottawa, Canada.
Maekawa, S and Elert, K., (2003), The use of oxygen-free environments in the control of museum pests, Getty Publications, Los Angeles, California, pp. 157.
Pearson, C., 1993, Building out pests, AICCM Bulletin, vol. 19, pp. 41-55.
Pinniger, D., 1989, Insect Pests in Museums, Institute of Archaeology Publications, London.
Strang, T., 1993, Museum Pest Management, Student Course Notes, Copyright 1992, Canadian Conservation Institute, Ottawa, Canada.
Strang, T., 1997, “Controlling Insect Pests with Low Temperature” Canadian Conservation Institute Note 3/3, 1997, updated 2008 (http://www.cci-icc.gc.ca/crc/notes/pdf-documents/3-3_e.aspx).
Strang, T. and Kigawa, R., 2009, Combatting pests of cultural property, CCI Technical Bulletin No. 29, Canadian Conservation Institute, Ottawa, Canada.
Thomson, G., 1986, The Museum Environment, 2nd Edition, Butterworths, London.