Booklice: The Cutest of Pests

Humans have a tendency to think of 'nature' and the 'environment' as something distinct from our own society. Environments unmodified by humans are seen as 'natural' whereas structures created by human activity, such as buildings, are not 'natural' and thought to be somehow outside the 'environment'. As such, people often react strongly to the idea of things associated with the 'environment', such as non-human wildlife, encroaching on their homes. But of course, human houses are as much an environment of their own as any other of the world's habitats, and many animals find them to be places where they can thrive. Among the animals that most regularly share our houses with us are booklice of the genus Liposcelis.

Liposcelis bostrychophila, copyright Andreas Eichler.

Representatives of Liposcelis can be found almost anywhere in the world except in the coldest of regions. About 130 species have been described in the genus to date (Yoshizawa & Lienhard 2010) with doubtless more yet to be discovered (by comparison, Broadhead's review of the genus in 1950 recognised only 22 species, with a six-fold increase since then). The family Liposcelididae, to which Liposcelis belongs, differ from other free-living members of the Psocodea (or 'Psocoptera') in their flattened body form, as well as being smaller than most other examples (Liposcelis grow little more than a millimetre in length). In the flattened habitus, they resemble the parasitic true lice of the Phthiraptera, and recent studies have agreed that the liposcelidids represent the closest relatives of true lice (Yoshizawa & Lienhard 2010). Liposcelis species are readily distinguished from other liposcelidids by the shape of the hind legs: an obtuse tubercle on the outer margin of the hind femur gives it a distinctly broad appearance* (indeed, the genus name Liposcelis translates into English as 'fat thigh'). Liposcelis are also distinctive in being invariably wingless; other liposcelidid species typically come in both winged and wingless forms. Though the genus as a whole is easily recognised, distinguishing individual species is often a far more challenging prospect requiring microscopic examination of fine features of the chaetotaxy (arrangement of bristles on the body) and cuticular sculpture. Authors have divided Liposcelis species between a number of diagnostic sections and subgroups based on these and other features but the monophyly or otherwise of these subdivisions is largely unstudied.

*This feature is also shared with a cave-dwelling species from Ascension Island currently placed in its own genus, Troglotroctes ashmoleorum, but it seems more than likely that this species is itself a derived offshoot of Liposcelis.

Liposcelis species can feed on a wide range of organic matter but, like other 'Psocoptera', their primary source of food is probably yeasts and fungal spores (their vernacular name has been attributed to their feeding on yeasts growing on the glue binding books, though I would note that they are also probably more likely to be seen crawling on the light background of a book's page than in other, less closely examined corners of the house). Turner (1994) provided a detailed review of the natural history of one of the most widespread domestic pest species in the genus, L. bostrychophila, and reports that he was able to maintain cultures on "'Weetabix'™, 'Shreddies'™, baby rice, soya granules, sage and onion stuffing mix, skimmed milk powder, 'Oat Krunchies'™, red lentils, and yellow split peas". Other stored foods from which complaints had been received of booklice included "sugar, bread, salt, bay leaves, gelatine powder, poppadoms, custard powder, dried yeast, instant potato, nuts, dried fruit, baby food, sauce mix, dried mushrooms, pasta, coconut, cocoa, milk powder, spices, glace cherries, garlic, baking powder, icecream mix, dried soup, cracked wheat, carob powder, maize meal, wheat germ, jellied sweets and bread crumbs". They have also been found on cured meat and may damage curated insect specimens. As well as obtaining moisture from their food, Liposcelis are also able to extract water directly from the atmosphere owing to the hygroscopic properties of their saliva. A booklouse will hold a drop of saliva inside its mouth, then swallow it when the ball has absorved enough water from the air.

Liposcelis sp. (possibly L. meridionalis?) from southern France, copyright Jessica Joachim.

Female Liposcelis bostrychophila generally reach maturity and begin producing eggs about two weeks after hatching and may produce two or three eggs a day. As each egg is about one-third the size of the adult, this means that a female at peak fecundity is producing her own body mass in eggs in a single day. Most Liposcelis species reproduce sexually but some are parthenogenetic. Domestic L. bostrychophila, for instance, seem to be entirely parthenogenetic with males of the species only known from isolated collections in Hawaii, Arizona and Senegal (Georgiev et al. 2020). Studies on an unnamed species of Liposcelis from Arizona found that sex determination seemed to be facultative, determined by the mother, with no evidence for differentiated sex chromosomes (Hodson et al. 2017). Females seemed to produce more males early in life and more females later. The same studies also established the occurrence of paternal genome elimination in this species, where chromosomes inherited from the father were inactivated in the offspring and not passed on to their own progeny (which raises the question that, if males are effectively a genetic dead end, why would a female produce male offspring at all?) Paternal genome elimination has also been found in the human louse Pediculus humanus, and may be characteristic of the broader clade encompassing these species, but other species remain unstudied. Liposcelis genomes are also remarkable in the occurrence of fragmentation of the mitochondrial genome. Whereas some Liposcelis species have only a single mitochondrial chromosome, as is standard for most other animals, some species have the mitochondrial genome divided between two, three, five or seven chromosomes (Feng et al. 2019). The functional significance, if any, of this feature remains unknown.

Though booklice may be found in houses and stores on the regular, they are mostly only minor pests, only causing distress when reaching large numbers (an exceptional case quoted by Turner, 1994, involved a house in New Jersey at the beginning of the 1900s that became so infested "'that a pinpoint could not have been put down without touching one or more of these bugs"). They are not believed to transmit pathogens, except perhaps incidentally by carrying microbes from one store to another. For the most part, these little beasties are just another part of the wildlife that shares our homes with us, whether we are aware of them or not.

REFERENCES

Feng, S., H. Li, F. Song, Y. Wang, V. Stejskal, W. Cai & Z. Li. 2019. A novel mitochondrial genome fragmentation pattern in Liposcelis brunnea, the type species of the genus Liposcelis (Psocodea: Liposcelididae). International Journal of Biological Macromolecules 132: 1296–1303.

Georgiev, D., A. Ostrovsky & C. Lienhard. 2020. A new species of Liposcelis (Insecta: Psocoptera: Liposcelididae) from Belarus. Ecologica Montenegrina 29: 41–46.

Hodson, C. N., P. T. Hamilton, D. Dilworth, C. J. Nelson, C. I. Curtis & S. J. Perlman. 2017. Paternal genome elimination in Liposcelis booklice (Insecta: Psocodea). Genetics 206: 1091–1100.

Turner, B. D. 1994. Liposcelis bostrychophila (Psocoptera: Liposcelididae), a stored food pest in the UK. International Journal of Pest Management 40 (2): 179–190.

Yoshizawa, K., & C. Lienhard. 2010. In search of the sister group of the true lice: a systematic review of booklice and their relatives, with an updated checklist of Liposcelididae (Insecta: Psocodea). Arthropod Systematics and Phylogeny 68 (2): 181–195.