If items on your bookshelf have chewed-on pages and bindings, suspect the look–alike household pests silverfish and firebrats. Both insects have enzymes in their gut that digest cellulose, and they choose book–cases, closets, and places where books, clothing, starch, or dry foods are available.
Silverfish and firebrats are nocturnal and hide during the day. If the object they are hiding beneath is moved, they will dart toward another secluded place. They come out at night to seek food and water. Both insects prefer dry food such as cereals, flour, pasta, and pet food; paper with glue or paste; sizing in paper including wallpaper; book bindings; and starch in clothing. Household dust and debris, dead insects, and certain fungi also are important sources of food. However, they can live for several months without nourishment.
Large numbers of these insects can invade new homes from surrounding wild areas, especially as these areas dry out during the summer. They also can come in on lumber, wallboard, and similar products. Freshly laid concrete and green lumber supply humidity, while wallpaper paste provides food.
The common silverfish, Lepisma saccharina, is shiny, silver or pearl gray, and about 1/2 inch long, although it can grow as long as 3/4 inch. The common firebrat, Thermobia domestica is shiny, a mottled gray or brown, and about 1/2 inch long. Adults of both species are slender, wingless, soft-bodied insects with 2 long, slender antennae. Their bodies taper gradually from front to rear to 3 long, thin, taillike appendages.
Although small nymphs (those that are less than 1/8 inch long) lack scales, both large nymphs and adults have them. If you see scales around or beneath damaged items, it is a good indication that these pests are the culprits. The scales are delicate, dustlike, and slightly incandescent in the light, and they stick to most surfaces.
Eggs of both species are about 1/25 of an inch long. The females lay the eggs in crevices, on cloth, or buried in food or dust. The average clutch contains 50 eggs, but this can vary from 1 to 200. Firebrat eggs hatch in about 14 days and silverfish eggs in about 19 to 32 days. In colder environments eggs can remain dormant for up to 6 weeks, hatching as soon as the temperature rises.
Newly hatched nymphs are about 1/16 of an inch long, plump, white, and have no scales. They acquire their scales about 12 days after their fourth molt; at that point the firebrat and silverfish look like smaller versions of the adults and gradually grow in size until they reach maturity. Unlike other insects, firebrats and silverfish molt continuously throughout their life, passing through 45 to 60 instars or development stages. Firebrats live about 2 years and silverfish about 3 years.
When the temperature is below 50°F, firebrats can take as long as 300 days and silverfish as long as 500 days to develop into adults. When it is above 75°F, it can take firebrats as few as 40 days and silverfish as little as 60 days to develop into adults.
Adults have an interesting courtship in which males attract females with a mating dance that includes antennae contact and head wagging, after which the male deposits a protected capsule containing sperm for the female to pick up. To keep the capsule from drying out, males often will lay it on a moist surface, so it is common to see adult insects clustered around damp areas when mating. Firebrats and silverfish tend to crowd together in a central hiding place during the day to rest, and it is here where the females often lay their eggs. Both sexes produce pheromones, or chemicals, that initiate these aggregations, and when these insects crowd together, it helps regulate the temperature and humidity around the eggs and young nymphs.
Silverfish live and develop in damp, warm places, preferring areas such as laundry rooms that are 71° to 90°F. At higher temperatures, the relative humidity must be above 75%. As their common names implies, firebrats thrive best in very warm, dry places with a relative humidity as low as 30% and temperatures above 90°F, with the optimum being 98° to 102°F. However, they can survive at temperatures ranging from freezing to well above 100°F. You can find them around ovens, heating units, fireplaces, hot water pipes, attics during the summer, and the furnace during winter. In apartments and homes, this insect crawls along pipelines and through openings in walls or floors from one level to another. Sometimes you’ll see these pests in your bathtub or sink. Even though they can’t crawl up through the drain, if they fall in they can’t climb up the slippery sides to escape.
Firebrats and silverfish have very weak mandibles, or jaws, which causes them to scrape, instead of bite, the surface of paper. Page discoloration often occurs because of the fungi associated with their feeding damage. On paper and fabrics feeding usually begins in one area and spreads as unconnected, irregularly shaped holes.