Psocids or "Book Lice": A Warning of Dampness – Canadian Conservation Institute (CCI) Notes 3/4

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CCI Note 3/4 is part of CCI Notes Series 3 (The Museum Environment: Biological Factors)

Image of a Psocid or Book Louse
Book lice

Book lice, called psocids, are members of the insect order Psocoptera, which has about 340 identified species in North America (Mockford, ). These can be roughly divided into two types: winged and wingless. Winged psocids usually live out-doors, primarily on bark and leaves, although they will occasionally come indoors; wingless types are more commonly found indoors. The most common and most studied psocids are Liposcelis species. These insects are 1 mm long, pale to translucent in colour, and have a dry weight of 27 µg. To put them in perspective, a dry, single sheet of 21.6-cm by 28-cm (8.5-in. by 11-in.) paper weighs about 4 g, which is equivalent to 150,000 psocids.

Psocids are typically seen in moist conditions, although they can be found in dry areas. The bodies of Liposcelis species contain about 66% water at 73% relative humidity (RH), but only 22% water at 33% RH. It is this ability to utilize their own water reserves that allows them to survive in dry atmosphere for up to three weeks, after which they will rehydrate quickly if returned to a damp area but will die if the exposure to dry conditions is continued. A dry atmosphere for psocids can be defined as RH under 58% for indoor types and under 70% for outdoor types. Above this critical RH, psocids utilize food energy to transport water vapour into their bodies, enabling them to replace transpired water vapour and maintain body mass even in the absence of food that contains water (Knülle and Spadafora, ). In high humidity, their life spans range from six months to a year (Broadhead and Hobby, ).

Psocids are omnivorous. Various authors cite moulds, yeasts, whole grain, starches, and, to a lesser extent, pollen, algae, lichen, feathers, hair, and insects as food sources. Psocids thrive in humidities that are conducive to extensive mould growth, and moulds are often cited as their major food. Yeasts have been found to improve egg-laying capacity more so than other foods (Broadhead and Hobby, ). Feeding activity of these insects is restricted to surface grazing; they do not eat holes in objects. In turn, psocids are prey for insectivorous insects and small parasitic wasps (New, ).

Many psocid species are parthenogenetic, that is, they have the ability to reproduce without mating. Maximum oviposition observed in Liposcelis is two eggs per day. However, egg laying can be stopped by decreasing the surrounding humidity (Knülle and Spadafora, ). Indoor-dwelling psocids do not lay their eggs in clusters; they will, however, breed continuously under suitable conditions and live up to a year, which explains the large numbers that can develop in humid conditions.

Indoor psocids, the most common type seen in collections, are rarely implicated in damage unless found in large numbers. Seeing a few psocids on an object is not likely to constitute a risk to the object unless the object is small, such as a tiny insect specimen. Large numbers, however, have been reported. In one instance, thousands streamed out of a damp straw mattress and covered household surfaces. The mattress probably contained millions of psocids. In this type of case, eliminating the prime breeding area would be the key to treatment. Lesser numbers can be readily controlled by airing and drying the objects on which the insects are found (Back, ).

The greatest problem caused by psocids has not been physical damage, but stalled house sales, lawsuits, and occupant unease (New, ). For cultural collections, the presence of psocids sounds an alarm indicating unacceptably damp conditions — conditions that promote mould growth on objects and in buildings, which in turn damages collections and human health. The most reliable way to eliminate psocids and moulds is to dehumidify the building and affected objects.


  1. Back, E.A. "Book-lice or Psocids, Annoying Household Pests." Farmers' Bulletin 1104. Washington, DC: United States Department of Agriculture, .

  2. Broadhead, E., and B.M Hobby. "Studies on a species of Liposcelis (Corrodentin, Liposcelidae) occurring in stored products in Britain, I, II." Entomologist's Monthly Magazine, 80 (), pp. 45-59, 163-173.

  3. Knülle, W., and R.R. Spadafora. "Water vapour sorption and humidity relationships in Liposcelis (Insecta: Psocoptera)." Journal of Stored Products Research, 5 (), pp. 49-55.

  4. Mockford, E.L. "Order Psocoptera: Psocids." In, An Introduction to the Study of Insects. 6th edition. D.J. Borror, C.A. Triplehorn and N.F. Johnson, eds. New York: Harcourt Brace College Publishers, .

  5. New, T.R. "An introduction to the natural history of the British Psocoptera." The Entomologist, ( to ), pp. 59-97.

Written by Tom Strang

Copies are also available in French.
Texte également publié en version française.

© Minister of Public Works and Government Services,
Cat. Nº NM95-57/3-4-1998E
ISSN 0714-6221

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