Silverfish and firebrats belong to a group of primitive insects called thysanura (thī΄sә΄ nūr ә). They are wingless and have slender, carrot-shaped bodies that are covered with scales. Both insects have two long slender antennae attached to their heads and three long tail-like appendages at the hind end. Each appendage is almost as long as the body. Adults are about 1/3 to 1/2 inch long (8 to 13 mm). Silverfish are shiny and silver or pearl-gray. Firebrats are mottled gray or brown.
Silverfish and firebrats are common in homes and can be found in places with high humidity and little airflow. They are active at night and hide during the day. When objects under which they hide are moved, they dart about seeking a new hiding place. Large numbers may be found in new buildings in which the newly plastered walls are still damp. Silverfish and firebrats may cause damage in the home by eating foods or other materials that are high in protein, sugar or starch. They eat cereals, moist wheat flour, paper on which there is glue or paste, sizing in paper and book bindings, starch in clothing, and rayon fabrics. In apartment buildings, the insects follow pipelines to rooms in search of food. They may be found in bookcases, around closet shelves, behind baseboards, or around windows or door frames.
Silverfish live and develop in damp, cool places. Firebrats live and develop in hot, dark places, such as around furnaces and fireplaces and in insulation around hot water or steam pipes.
Under normal house conditions, silverfish and firebrats develop slowly and have few young. Females lay eggs year-round in secluded places, such as behind books or on closet shelves; however, occasionally they lay eggs in the open.
Silverfish lay only a few eggs at a time but may lay several batches over a period of weeks. The adult female can live for two to five years depending on the species, and can lay up to 100 eggs. The eggs are whitish, oval and about 1/32 inch long (0.8 mm). They hatch in two to eight weeks.
Firebrats lay about 50 eggs, one at a time, and will lay several batches. The adult female can live for about two years and can lay thousands of eggs. The eggs are soft, white, and opaque when they are laid. They later have a yellowish tinge. Firebrat eggs hatch in about two weeks.
After hatching, the young silverfish and firebrats look like the adults except they are smaller. Both insects reach maturity in three to 24 months. Their rate of growth depends on temperature and humidity.
Sanitation can help prevent infestations but will not eliminate current infestations. Seal or remove hiding places. Sealing up cracks and crevices around plumbing, wall molding, and windowsills will help eliminate harborage (places for them to hide). Removing cardboard boxes and old newspapers eliminates food sources and harborage. Vacuuming can physically remove both silverfish and firebrats.
If insecticides are necessary, sprays should be applied to floors and wall moldings, behind drawers, under furniture, in cracks and crevices, and the floor and ceiling of attics. Outside, treat eaves, mulched flower beds, and storage sheds. Control may not be immediate because hiding insects must come out and contact spray residue. Dusts of the recommended materials may be used for treating walls, voids, crawl spaces, and attics. Ten days to two weeks may be required to determine whether or not control has been achieved. Space sprays of pyrethrins are useful for controlling exposed insects.