People are from time to time finding that their food cupboards and especially dry goods have become infested with tiny grey or brown insects. This leaflet explains what they are and how to prevent them.

Psocids or booklice are tiny (about 1mm – 1.5mm long) smaller than a pinhead. The most common psocid is Liposscellis bostrycholphila, which is a tropical species and cannot live outdoors in the UK. They prefer to live in dark, warm, humid places, such as the folds of food packaging in cupboards. They dislike light or disturbance. They can live for about six months and in that time the female will lay up to 200 eggs. Although they are found in food areas they are perfectly harmless, feeding on mould and bacteria. They will usually be found in dry food packets but this is not the source of them. They can also be seen on drying plaster and in some starch based glues used in books, hence the name.

Psocids prefer areas with high humidity, but can tolerate dry conditions for some days. They are always associated with damp and are therefore usually found in kitchens and bathrooms. Inadequate ventilation or a leaky pipe and even plaster drying out can attract them. Some food products such as flour naturally contain moisture and it is here that psocids can rapidly increase in number. This is most likely to happen during the summer months when temperatures are higher, leading to their discovery in the autumn.

It is best to keep all dry foods such as flour, powdered milk, cereals in cool, well ventilated cupboards. If this is not possible, make sure that your cupboards are always free from condensation and damp.

Packets of food that have been opened should be used up quickly and not pushed to the back of the cupboard. This could be ideal conditions for psocids to increase and then move out and infest other, newer packages. Always make sure that you check the "best before date".

Regular cleaning of cupboards can remove them, but always make sure that it is thoroughly dry before replacing the food. The use of a vacuum cleaner is preferred, providing that the contents are disposed of immediately in an outside bin.

Make sure that the room is well ventilated. When cooking, boiling the kettle or even just using a washing machine or tumble dryer make sure that windows are open and extractor fans are used.

If these prevention measures fail and you do discover psocids in your food, do not use an insecticide, as there is danger of contaminating your food. If ideal conditions are still there, the psocids will remain after treatment and can still re-infest.

Dispose of all visibly infested packages in an outside waste bin and thoroughly clean the cupboards, preferably using a vacuum cleaner. Pay particular attention to the crevices.

It is important to make sure that when foodstuffs are replaced in cupboards that have been cleaned they do not reintroduce the psocids. Dry goods should be treated by placing them in a deep freeze for 24 hours (enclose products in a plastic bag). This will kill any eggs or juveniles that might be hidden in the folds of packaging. Labels should be removed from tinned food, bottles and jars to ensure that they are not in the labels. Food can then be placed in clean, dry containers with sealed lids and clearly labelled.

Check for sources of humidity and take appropriate measures to increase ventilation. Make sure windows/doors are opened when cooking, and do not place a kettle under a cupboard. In some cases, the use of a dehumidifier may sufficiently dry out a property to prevent further cases.