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Insect Attack

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);
  • silverfish;
  • cockroach; and
  • termite.

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 remains of a collection of butterflies after an insect attack. Only the outlines of the butterfly remain.

Figure 2: The remains of a collection of butterflies after an insect attack. Only the outlines remain.

The procedures involved in a coordinated approach to minimising the risk and dealing with insect attack are detailed below.

Preventive Measures

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.
A baited sticky trap with two bugs caught within it.

Figure 3: A baited sticky trap.

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

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.

Chemical Treatments

Factors to be considered when choosing chemical insecticides include:

  • toxicity to the insect;
  • toxicity to people;
  • effects on objects;
  • environmental effects; and
  • cost.

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.