Freezing infested material

Freezing is the standard treatment for insect infestation in dry organic collections. It leaves no residue, needs no specialist chemicals, and when done properly kills every life stage including eggs, which is the requirement that most improvised treatments fail.

What has to be true for it to work

Low enough temperature. Insects survive mild cold. The treatment relies on temperatures well below ordinary domestic freezer settings, and the colder the temperature the shorter the required hold. Around minus 30 °C is the usual target where equipment allows, with longer holds required at warmer temperatures such as minus 20 °C.

Fast enough cooling. This is the requirement most often missed. Cooled slowly, many insects can produce cryoprotectants and survive temperatures that would otherwise kill them. Getting the material cold quickly matters as much as how cold it gets, which is why loading a full freezer with warm boxes can compromise a treatment.

Long enough hold. The hold time is counted from when the object core reaches temperature, not from when the door closed. A dense, well-insulated package can take many hours to reach temperature throughout, and that lag is invisible unless something is measuring it.

Correct packaging. The object goes into a sealed polythene bag with as much air excluded as practical, before it enters the freezer. This is what prevents condensation, and it is the single most important handling step.

The condensation problem

When a cold object is brought into a warm room, moisture condenses on the coldest available surface. If the object is bagged and sealed, that surface is the inside of the bag. If it is unbagged, it is the object.

The rule that follows: seal the bag before freezing, and do not open it until the package has returned fully to room temperature. Full return takes considerably longer than people expect for a dense object, and opening early undoes the whole precaution. When in doubt, leave it another day.

A working procedure

1. Inspect and photograph the material. Document the infestation before treatment.
2. Support fragile items so they cannot shift inside the package.
3. Bag and seal, excluding as much air as reasonable. Double bagging protects against a puncture.
4. Put a small logger inside the package, next to the densest part of the object. This is what turns an assumption into a record.
5. Load the freezer without overfilling it, so cold air can circulate and the cooling rate is not compromised.
6. Hold for the required period, counted from when the internal logger shows the target temperature has been reached.
7. Remove and allow to return fully to room temperature while still sealed.
8. Open, inspect, vacuum away dead insects, frass and debris, and rehouse in a clean enclosure.
9. Increase trap checking in the area and find the source.

What not to freeze

Freezing is not universal. Material where it is inadvisable or needs specialist advice includes wet or damp objects; painted, varnished, gilded or lacquered surfaces where layers may respond differently; composite objects combining materials with very different responses; panel paintings; some plastics that become brittle; glass, ceramic and stone with existing cracks; and anything with an applied decoration that is loosely attached.

For these, anoxic treatment is usually the alternative, being slower but gentler.

Afterwards

Treatment kills what is in the object. It does nothing about where the infestation came from. If the material goes back into the same store without the source being found, the same thing will happen again, usually within a year.

The follow-up is therefore part of the treatment: inspect the surrounding area, look for the nest, the debris, the forgotten box or the dead bird, improve the enclosure the object goes back into, and watch the traps in that area more closely for the next season.