Booklouse (Psocid)

Liposcelis bostrychophila is the most common indoor species of booklouse — a 1 mm pale insect of order Psocodea (psocids) that has spread worldwide through the global book and grain trade. The 'booklouse' name is misleading: the species is NOT a louse (true lice are also Psocodea but in a different suborder, and are blood-feeding parasites of vertebrates), and she does not eat books directly. Booklice feed on the microscopic molds and fungi that grow on book bindings, paper, photographs, dried plant material, stored grain, and other organic substrates in humid environments. The species reproduces parthenogenetically — no males needed, all females, all clonal — meaning a single founding female can establish a population of thousands within weeks. Booklice prefer humid conditions (above 60% RH); populations crash at humidity below 50%. They are a major pest of libraries, archives, museums, and grain storage worldwide because of the damage to mold-affected materials and because the dead bodies trigger allergic responses similar to those caused by dust mites. They do not bite, do not transmit disease, and are otherwise harmless. Booklice are also one of the few insect species whose female-only parthenogenesis is maintained by an obligate Wolbachia bacterial endosymbiont — male embryos are converted to female through Wolbachia-mediated cytoplasmic incompatibility.