Integrated Pest Management Manual
IDENTIFICATION AND BIOLOGY OF SILVERFISH
The term silverfish is used for the Thysanura and for any of the species within the order. Thysanurans have a distinct carrot shaped body, short legs, long slender antennae and three tail-like appendages (anal cerci) at the end of the body. Silverfish are wingless with scale covered bodies which are about 1/2" long. Nymphs resemble adults.
There are thirteen species of silverfish (Thysanura) in the United States. Mallis (1982) lists six species which may be pestiferous: they are the firebrat, Thermobia domestica (Packard); T. campbelli (Barhart) (no common name), the silverfish, Lepisma sacchrina L.; the fourlined silverfish, Ctenolepisma quadriseriata (Luccas); the gray silverfish (also called the giant silverfish), C. longicaudata Escherich (C. urbana in some texts); and Acrotelsa collaris (Fab.) (no common name). See page 45 of U.S. Department of Health, Education, and Welfare (1967) for pictorial keys to common species. See also Mallis and Caur (1982) for detailed descriptions of pest species. Brief descriptions of the habitats and life cycles of the six pest species are given below.
Firebrat. This insect is found throughout the world in warm climates. Found indoors in the United States in heated buildings, it is occasionally seen outdoors in the West and Southwest. Firebrats occur around ovens, bakeries, and other warm areas, as they prefer temperatures above 90
The females deposit eggs in crevices. The number of eggs per batch ranges from 1 to nearly 200, with an average of 50, but stressed females lay fewer eggs. Eggs will hatch in 12-13 days under optimum conditions. Newly hatched nymphs are 1/8" long, white, and scaleless. Development is rapid, with only 1 day spent in the first instar and more time passing between successive instars, although the later instars may last up to two weeks. A firebrat may pass through 45 to 60 instars during its lifetime. The nymphs resemble adults. Females produce one batch of eggs per instar beginning at about the 12th week but can begin to oviposit at six weeks at temperatures of 90-106
Firebrats can be killed when exposed to temperatures above 120
T. campbelli: This species is found indoors in libraries. Little is known about its habits, but its life cycle resembles that of the firebrat.
Silverfish. This pest is common indoors on the East Coast, and is also found indoors in the Midwest and Pacific Coast. It is found indoors in warm, humid areas such as basements.
The eggs are deposited in crevices or under objects singly or in groups of 2 or 3. The female deposits 1-3 eggs per day or at irregular intervals of up to several weeks depending upon availability of food. Eggs hatch in 43 days at 72
Fourlined silverfish. This species is common on the East and West Coasts and in the Midwest. It lives indoors, often infesting attics, particularly if the roof is made of wooden shingles. It may be found outdoors in summer. Its life cycle is similar to that of the silverfish but not as limited by temperature and moisture.
Gray silverfish. This species occurs indoors in the South, Midwest, and southern California. It prefers drier areas than common silverfish, such as crawl spaces and attics, but may occur around water pipes in bathrooms. It deposits its eggs in cracks in groups of 2-20. They hatch in about 60 days at room temperature. The nymphs are scaleless when hatched; scales appear in the fourth instar, and sexual maturity is reached in 2-3 years. This species may live up to 5 years.
A. collaris: This species was recently introduced into Florida, probably from the tropics. Little is known of its life cycle but it may resemble that of silverfish.
Feeding habits of silverfish species are very similar. Once a source of food is located, silverfish remain in the vicinity. Silverfish feed on human foods, especially those containing starch or flour, as well as on paper, especially glaze-coated paper. They eat sizing on paper, as well as glue and paste. They may feed on wallpaper or the paste behind it, causing the wallpaper to become detached from the wall.
Materials such as writing paper, tissue, onion skin paper, and cellophane are preferred, as well as cotton, rayon, and other vegetable textiles. Newsprint, brown wrapping paper, and most cardboards are seldom eaten. Silverfish feed on bound volumes for the paper, the starch and sizing in the bindings, and the linen in some bookcovers. Stored papers, books, and other printed materials are especially susceptible. Sizing and glue are main attractants, especially if humidity is high. Silverfish seldom feed on wool and other animal based textiles. Cereals may become infested due to the insects' preference for starches and flour. Enzymes and cellulose-digesting bacteria in the gut break down cellulose in paper or other wood products. Silverfish can live for nearly a year without feeding.
Temperature is the most important factor influencing the thysanurans. Low temperatures result in high mortalities, especially among nymphs. Mallis (1982) reports that temperatures below freezing or above 112
Thysanurans are primarily important as archival pests although they may infest foodstuffs. Individuals are long-lived and reproduction rate is moderate, so populations grow slowly. Large populations can cause severe damage to paper and paper products.
MONITORING AND THRESHOLDS
Monitoring is best performed by detecting damage caused by silverfish (Mallis 1982). Book bindings will show minute scrapings. The sizing of paper will be removed in irregular fashion, and the edges of paper will appear notched. In cases of high populations irregular holes will be eaten directly through paper. Other signs of feeding include feces, scales, and small yellow stains.
Active infestations can be detected by observing the small, dark feces, which are visible to the eye as well as scales, which are visible through a hand lens (Mallis and Caur 1982). In addition the feeding activities of silverfish can be observed by coating a piece of paper with a thin layer of flour paste and placing it in an area suspected of harboring silverfish. If silverfish are present, the paper will show small feeding marks.
Two kinds of traps have been used to confirm the presence of silverfish. The first uses a small jar coated with flour on the inside and tape on the outside to provide a climbing surface. Jars should be placed in areas of suspected silverfish infestations and regularly inspected for silverfish, which will climb in the jar and become trapped (Mallis and Caur 1982). Conventional sticky traps such as those used for monitoring cockroach populations can also be used for detecting silverfish (Olkowski et al. 1991).
Due to their small size and reclusive nature, silverfish are seldom seen. However, their damage can be significant if populations are high. The decision to intervene should be based on the amount of damage associated with the silverfish and the confirmation of an active infestation by the methods described above.
NON-CHEMICAL CONTROL OF SILVERFISH
Warm temperatures and high relative humidities favor most silverfish species. Controlling or eliminating moisture in areas infested with silverfish can reduce populations. Air conditioners or dehumidifiers placed in rooms where documents and books are stored can help to reduce humidity and temperature. Lower temperatures may also slow population growth by reducing rates of development and reproduction in silverfish.
Silverfish found in books and documents can be killed by exposure to microwave radiation. Olkowski et al. (1991) recommend placing books in a microwave oven for a period of 30 to 60 seconds to kill silverfish. Caution should be used with books or documents containing color plates or in fragile condition.
Sealing cracks and crevices where silverfish hide and breed also reduces populations by reducing suitable habitat. If sealing or caulking is not possible, then cracks and crevices (particularly around bookcases) should be regularly vacuumed to remove silverfish. Good sanitation practices should be followed. All valuable paper products, books, and documents should be placed in tightly sealed containers and cabinets. If this is not possible, access to potential food sources should be limited by removal of food and harborage such as empty cardboard boxes and other waste paper.
There is no information in the literature on natural enemies of the Thysanura.
CHEMICAL CONTROL OF SILVERFISH
Several pesticides are recommended for use on silverfish in Park Service areas. Pesticides for silverfish should be applied in the same manner and with the same thoroughness as for cockroaches. Boric acid should be spread thinly in areas where silverfish are active. Dusts and powder formulations should be applied only in cracks, crevices, attics, and other storage areas where park visitors will not regularly come into contact with the pesticide (Olkowski et al. 1991). Consult your regional Integrated Pest Management coordinator to determine which chemicals and application techniques are best suited for your area.
Regularly monitor high risk areas by looking for damage, droppings, scales, or insects or by placing flour paste cards or trap jars. Reduce harborage by enclosing vulnerable materials in insect-proof containers. Reduce relative humidity and raise or lower temperatures to make environmental conditions unfavorable for silverfish.
If a problem arises, evaluate the magnitude of the problem before initiating intervention tactics. After other management options have been considered and implemented, if appropriate, select approved chemical controls that are least disruptive to the environment.
1. Ebeling, W. 1975. Urban Entomology. Univ. of Cal. Press, Los Angeles.
2. Mallis, A., and R.V. Caur. 1982. Silverfish. In: Mallis, A. (ed.). Handbook of Pest Control. Franzek and Foster, Cleveland, OH.
3. Olkowski, W., S. Daar, and H. Olkowski. 1991. Common-Sense Pest Control. Taunton Press, Newtown, CT.
4. Schwartz, P.H. 1982. Guidelines for the control of insect and mite pests of food, fibers, feeds, ornamentals, livestock, and households. USDA-ARS Handbook 584. U.S. Department of Agriculture, Agricultural Research Service. Washington, D.C.
5. U.S. Department of Health, Education, and Welfare. 1967. Pictorial keys to arthropods, reptiles, birds, and mammals of public health significance. U.S. Department of Health Education, and Welfare. Washington, D.C.