Insect Information Service
Psocids (Book-lice, Dust-lice)
Order: Psocoptera
Domestic psocids are tiny, soft-bodied, pale grey or brown
insects, less than 3mm long, often seen running over walls
and furniture in houses. They may also be found in large
numbers in packaged food such as cereals and flour, or in
dusty situations on floors and among piles of books and
papers. As a result, they are often termed Dust- or Booklice, though they must not be confused with the true lice
which are blood-sucking parasites.
Psocids are relatively harmless insects which feed on
mildews and moulds. Large numbers can be found in new
houses before plasterwork and timbers have dried out and
also in older property which may be subject to damp. In
such places, mildews and moulds which are scarcely visible
to the human eye may be present, not only on plaster, but
also on paper, packaged foods, wickerwork, and upholstery padding.
The presence of psocids is therefore an indication of dampness. They do not attack
materials upon which moulds grow but may cause incidental damage by their feeding
action, and contaminate foodstuffs with their waste products. They are active by day and
when disturbed, disperse quickly by running or flying, though they are weak flyers.
The vast majority of British species, most of which have wings, live out of doors on the
bark and leaves of trees where they feed on lichen, algae and fungi. Indoor species are
usually wingless, the most common in this country being Liposcelis bostrychophilus (pale
brown), Lepinotus patruelis and inquilinus (dark brown), Trogium pulsatorimn (creamy
yellow with a spotted abdomen) and Psyllipsocus ramburii (grey/brown) - only the last of
these has winged forms.
Biology
Psocid eggs hatch into nymphs, resembling adults in general appearance.
At a temperature of 25°C and a relative humidity of 70-75 per cent, development from egg
to adult may take 25 to 50 days, depending upon the species. In drier conditions, the time
taken is increased.
Damp places with poor ventilation such as cellars, basements, and kitchens provide ideal
conditions for these insects to breed rapidly. It is possible to hear sounds produced by
psocids - they are said to resemble the ticking of a watch.
This leaflet has been produced by the Insect Information Service of the Natural History Museum. For details of our
Identification and Advisory services, please contact: The Enquiries Manager, The Angela Marmont Centre for UK
Biodiversity, The Natural History Museum, Cromwell Road, London, SW7 5BD. Telephone 0207 942 5045; Email:
ias1@nhm.ac.uk or ias2@nhm.ac.uk
© Insect Information Services, Natural History Museum 2010
Eradication
There is no simple way to eradicate psocids although their numbers can be kept in check
with the use of household liquid and powder insecticides with a pyrethrum or derris base.
Care should be taken not to allow such substances to come into contact with foodstuffs.
Infested larders and kitchen cupboards should be emptied, vacuum cleaned, scrubbed
with a household bleach solution and thoroughly dried before the contents are replaced,
ensuring that infested foodstuffs are discarded.
The only certain way of ridding a house of psocids is to ensure that it is so dry that the
moulds and mildews on which they feed cannot grow. If the house cannot be kept free of
damp, then materials such as wicker furniture and paper under floor coverings should be
avoided. Regular vacuum cleaning, especially with a crevice tool around skirting boards
and under furniture, is very helpful. A de-humidifier is likely to be helpful also. Effective
fungicides are included in most modern vinyl paints and wall covering adhesives, but
domestic heating should be adequate to dry out the house, particularly in autumn.
Structural defects such as lack of damp courses, porous brickwork, damp window frames
and leaky roofs and gutters may have to be treated by a builder.
Additional information
What do psocids look like?
Size:

Less than 1 mm - 10 mm in length, but most are shorter than 5 mm.
Body:

Ovoid or abdomen swollen making it look like a lopsided dumb-bell;

Some flattened as if pressed from above;

Front of head bulges like a big round nose;

Appears soft and fragile.
Antennae:

Thread-like, usually longer than half their body length.
Eyes:

Small to large, prominent and well separated.
Mouthparts:

For chewing or munching; held downwards at rest.
Wings:

Two (males only) pairs if present.

Both pairs membranous and clear,

Forewing usually wider and longer than hindwing.

Forewing venation simple, few cross-veins forming inconsistent shaped cells, with 'Sshaped' veins.

Wings usually extend beyond abdomen but some species have shortened wings.

At rest, wings held tent-like over body; hindwing hidden.
Limbs:

Six legs, short and stocky.
Abdomen tip:

Cerci (tails) absent.
This leaflet has been produced by the Insect Information Service of the Natural History Museum. For details of our
Identification and Advisory services, please contact: The Enquiries Manager, The Angela Marmont Centre for UK
Biodiversity, The Natural History Museum, Cromwell Road, London, SW7 5BD. Telephone 0207 942 5045; Email:
ias1@nhm.ac.uk or ias2@nhm.ac.uk
© Insect Information Services, Natural History Museum 2010