Bookworm is a general name for any insect that is said to bore through books.
The damage to books that is commonly attributed to "bookworms" is, in truth, not caused by any species of worm. Often, the larvae of various types of insects including beetles, moths and cockroaches, which may bore or chew through books seeking food, are responsible. Some such larvae exhibit a superficial resemblance to worms and are the likely inspiration for the term, though they are not true worms. In other cases, termites, carpenter ants, and woodboring beetles will first infest wooden bookshelves and later feed on books placed upon the shelves, attracted by the wood-pulp paper used in most commercial book production.
True book-borers are uncommon. The primary food sources for many "bookworms" are the leather or cloth bindings of a book, the glue used in the binding process, or molds and fungi that grow on or inside books. When the pages themselves are attacked, a gradual encroachment across the surface of one page or a small number of pages is typical, rather than the boring of holes through the entire book.
The term has come to have a second, idiomatic use, indicative of a person who reads a great deal or to perceived excess: someone who devours books metaphorically.
Main article: Psocoptera
The booklouse, also known as a paperlouse, is a soft-bodied, wingless insect in the order Psocoptera (usually Trogium pulsatorium), typically 1 mm or less in length. Booklice feed on microscopic molds and other organic matter found in or on aging items that have been stored in places that lack the climate control necessary to inhibit organic growth. Areas of archives, libraries, and museums that are cool, damp, dark, and generally undisturbed are common sites for such growth, generating a food source which subsequently attracts booklice. Booklice will also attack bindings, glue, and paper.
Despite their name, booklice are not considered to be true lice, as they do not feed on a living host.
By the 20th century, bookbinding materials had developed a high resistance against damage by various types of book-boring insects. Many museums and archives in possession of materials vulnerable to booklouse damage employ pest control methods to manage existing infestations and make use of climate control to prevent the growth of potential booklouse food sources.
Of the quarter million species of beetles, some adults damage books by eating paper and binding materials themselves. However, their larvae do the most damage. Typically eggs are laid on the book's edges and spine. Upon hatching, they bore into, and sometimes even through, the book.
These beetles have been known to feed on leather bindings.
Termites are the most devastating type of book eating pest. They will eat almost every part of a book including paper, cloth, and cardboard, not to mention the damage that can be done to shelves. Termites can make entire collections unusable before the infestation is even noticed.
Some species of ants can damage books in a way that is similar to termites.
Moths that feed on cloth will also feed on bookbindings, decaying organic material (which includes paper), and mold.
Bookdamaging cockroach species chew away at the starch in cloth bindings and paper. Their droppings can also harm books.
These insects consume portions of books that contain polysaccharides. Paper that is slightly ragged at the edges is usually the work of silverfish.
Pesticides can be used to protect books from these insects, but they are often made with harsh chemicals that make them an unattractive option. Museums and universities that want to keep their archives bookworm free without using pesticides often turn towards temperature control. Books can be stored at low temperatures that keep eggs from hatching, or placed in a deep-freezer to kill larvae and adults. The idea was taken from commercial food storage practices, as they are often dealing with the same pests.  Pseudoscorpions such as Chelifer cancroides may live in books and feed on book-eating insects, controlling their numbers.
The term is also used idiomatically to describe an avid or voracious reader, or a bibliophile. In its earliest iterations, it had a negative connotation, referring to someone who would rather read than participate in the world around them. Over the years its meaning has drifted in a more positive direction.