Extension News

Silverfish: They are Silver, But They're Not Fish

Note to media editors: This is the Garden Column for use during the week beginning March 20.

3/16/2009

By Laura Jesse
Plant and Insect Diagnostic Clinic
Iowa State University Extension

Silverfish are one of the insects that really made an impression on me as a kid.  I think maybe it is because they didn't even seem like insects, but rather some alien creature with incredible powers of speed.  Silverfish always seemed to appear as if from nowhere right in the middle of the wall as if taunting us to see if we could catch them.  We always practiced catch and release and I can attest that trying to get a glass clamped over a silverfish on the run is not an easy task.

Silverfish and firebrats are about a half-inch long, wingless, flat insects with two long, slender antennae on the front and three long, slender "bristles" at the rear of a tapered, carrot-shaped body.  As a kid those three bristles held at right angles always seemed to add to my presumption that there were perhaps an alien listening device and not insects at all.  Most silverfish and firebrats are covered with scales (similar to those on butterfly wings) that have a silvery sheen in the case of silverfish.

Silverfish may be found almost anywhere in the house, but are most commonly found in moist, warm locations (such as around sinks and other plumbing fixtures). They are covered with shiny silver scales that give the body a metallic sheen. They are frequently found in sinks or bathtubs because they fall in seeking moisture and then cannot climb out. Silverfish are most active at night and run very swiftly with a wiggling motion that resembles the swimming action of a fish.

Firebrats are similar to silverfish in overall appearance but lack the silvery sheen. The body color is gray or brown, usually with numerous dark markings that give a mottled appearance. Unlike the silverfish that may be found in any part of a house, firebrats prefer areas of high temperature (90° F and above) and high humidity. Therefore, firebrats are more common in attics and around furnaces, ovens, and water heaters.

Silverfish and firebrats are pests primarily because they are a nuisance and an annoyance. They may eat or stain foods, fabric, paper, books, or wallpaper. Damage to these items is significant, however, only in cases of very large infestations present over long periods of time.

Control of silverfish and firebrats may not be necessary if only a few are present and no damage is noticed. For silverfish, eliminating moisture problems where the pests can develop may be of some benefit. Household residual insecticides applied as sprays, dusts, or baits can be used if conditions warrant. Treat cracks, crevices, wall voids and other likely hiding spots in the areas where the pests are noticed. Sprays need only be applied lightly and in localized areas of infestation. Boric acid dust can be used for firebrats and silverfish. Boric acid has the advantages of being very low in toxicity and very long lasting if applied in dry voids where it will remain undisturbed. However, dusts are more difficult to apply in a light, even coating, especially into cracks and crevices.

Sticky traps can be placed in areas where you suspect silverfish and firebrats are living as another good control option.  Place traps on the floor near walls.  However, I always warn people that they may be best off not knowing what is crawling about their house at night.  Ignorance can sometimes be bliss

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Contacts :

Laura Jesse, Plant and Insect Diagnostic Clinic, (515) 294-5374,
insects@iastate.edu

Del Marks, Extension Communications, (515) 294-9807, delmarks@iastate.edu