Biological Pests

Insects

Insect life cycles

Most insect pests have similar life cycles: going from egg, to larval and adult stages. In all pests except silverfish, larvae do most of the damage to artefacts. The adults are generally most active in selecting the site for egg-laying.

The silverfish has no larval stage and develops straight from the egg to a miniature adult form, termed the nymph stage. The insect develops to the adult stage by a series of intermediate nymph stages. At the end of each of these stages, it sheds its skin—moults—and continues, slightly bigger, towards its next skin-shedding.

The other museum pests go through the usual egg-larval-adult process of most common insects. This process occurs generally as follows. The adult lays an egg or batch of eggs on a suitable material in a suitable site. Miniature larvae hatch from the eggs, which then either burrow into—or browse across—the material on which they have been laid. As the larvae feed they grow, usually to several times the size of the adults before pupating, during which time the larvae change to the adult form. When they are fully developed, they fly off to mate.

Insect control without damage to collections

In recent years, many new, low and non-toxic methods suitable for use on heritage collections have been developed. These methods are described below.

Use of low temperatures: fumigation by freezing

Exposure to low temperatures is lethal to insects. The technique has been used at the Australian Museum and other institutions for many years. It provides an efficient and inexpensive alternative to other fumigation methods.

A large variety of materials can be disinfested by exposure to low temperatures for varying periods of time. The mortality rate depends upon the temperature used, the type of insect and the thermal conductivity of the material being treated.

The possibility of damage to some artefacts caused by exposure to sub-zero temperatures is a common concern. Any material which may become brittle and crack when frozen should be treated by one of the many alternative methods now available. Seek advice from a conservator to be sure freezing is appropriate.

The object to be treated is placed inside a plastic bag. The bag is necessary to prevent condensation forming on the object after freezing.

As much air as possible is evacuated. This reduces the amount of moisture which would otherwise be absorbed by, or condense on, the artefact. A buffer, such as silica gel or cotton wool, can be added to absorb excess moisture. This should not be necessary with organic materials which do not contain excessive amounts of moisture.

The bag is sealed using a heat-sealer or waterproof tape.

The bagged and sealed object is then placed in the freezer for 48 hours at -20⁰C. Times can vary, so please refer to the notes below for further information.

After the appropriate length of time, the object is removed from the freezer. Allow it to thaw before taking it out of the bag.

Freezing times will need to be extended if:

• the freezer being used cannot hold temperatures of -20⁰C or lower. If the temperature of the freezer is around -15⁰C, the time required for 100 per cent mortality may be up to 14 days. The time needed varies according to the material and insect being treated; and

• fumigating large artefacts, for example, carved trees, or objects where the diameter exceeds 35 centimetres.

Wood, leather, feathers, fibre, books and textiles can all be treated this way. Freezing kills insects at all stages of their life cycles.

Low-oxygen atmospheres

Another method of disinfesting material is by exposing it for varying lengths of time to an atmosphere which lacks oxygen. Low-oxygen atmospheres can be achieved using an oxygen scavenger.

AGELESSTM is a scavenger which is used extensively in the food industry to remove any oxygen present within a sealed bag or container, thus preserving the product and preventing deterioration from mould and insect attack. The chemical oxygen absorber present in AGELESSTM is prepared from powdered iron oxide, which rapidly absorbs atmospheric oxygen.

At some major museums, AGELESSTM is used routinely as part of the fumigation program. Artefacts requiring treatment are placed in a bag manufactured from a material which has a low-oxygen permeability. The amount of AGELESSTM needed to absorb the oxygen present is calculated and then added, along with the indicator tablet AGELESS Eye.

The time needed for disinfestation depends on the temperature at which the bag is then stored. For example, at a constant temperature of 30⁰C it takes, three weeks to achieve 100 per cent mortality—based on the more tolerant insects tested. But if the bag is stored at 15⁰C the time taken to achieve 100 per cent mortality could be as high as 24 weeks.

Fragile artefacts or those which are of an odd shape or size may require large amounts of AGELESSTM.

Insect traps

Blunder traps are non-specific traps which assist in identifying any insects present within the collection.

Although food is the attractant in most traps, the insect's capture is due largely to the location and placement of traps where insects are common.

Most traps incorporate a piece of cardboard, one or both sides of which are sticky; but many other types are available.

Many traps now incorporate a pheromone. Pheromones are chemical messengers similar to the hormones within our bodies. Insects produce them to communicate messages. These chemicals, when passed from one insect of the same species to another, cause a certain response, either behavioural or physiological. Some examples of these are:

• aggregation pheromones, which may attract both males and females, for example, to a food source;

• trail-marking pheromones, such as those used by termites and ants; and

• sex pheromones, which cue for mating.

Many traps use pheromones as an attractant to lure insects. Any insects within a certain distance of the trap, home in on the odour and become trapped.

By checking these traps on a regular basis, it is possible to get an indication of the presence of a specific insect within the monitored area.

Pheromone traps are insect-specific, that is, a clothes moth trap will attract only clothes moths.

An example of a commercially available insect trap. Photograph courtesy of Artlab Australia

Making blunder traps

You will need corrugated cardboard, another non-corrugated piece for use as a backing board, and a roll of double-sided sticky tape, the stickier the better.

Cut the backing board and the corrugated cardboard pieces into rectangles about 15cm x 7cm.

Cut the corrugated board so that the holes formed by the corrugations are along the edge.

Cut a second, smaller rectangle out of the centre of the corrugated cardboard.

Cover one side of the backing board with the sticky tape, and then stick the corrugated cardboard to the backing board.

Another piece of backing board can be placed on top to prevent dust entering if required.

Traps can be baited with wheatgerm oil, or something else which will attract insects.

Check the traps regularly and identify any insects found in the traps. Change traps when they are no longer sticky or when they are full.