Long-tailed silverfish (Ctenolepisma longicaudatum) have become established in Norway in recent years and there has been a marked increase in the number of eradications. They are often found in modern buildings. Resembling silverfish, they are slightly larger, hairier and have longer tail filaments. Long-tailed silverfish may be bothersome but cause little damage in homes. Long-tailed silverfish kan be controlled by toxic bait.
Long-tailed silverfish are wingless insects with dark grey, slightly variegated scales (Figures 1 and 2) which makes them less shiny than silverfish. Typical adult body length is 10-14 mm without antennae and filaments but they can reach 18 mm (1;2). The body shape is similar to regular silverfish. There are three long filaments at the end of the flattened, elongated and tapering rear body. The central one points straight back and is approximately as long as the body, while the side filaments often point to the side. The head has two long antennae and two shorter maxillary palps. Nymphs are 2 mm when they leave the egg (Figure 3). After 14 stages of development, they grow and become more like the adults. The youngest individuals do not have scales and are therefore lighter in colour (Figure 4).
Life cycle, habitat and behaviour
In Norway, long-tailed silverfish are only found indoors and can be observed all year round. They are found in buildings all over the country, often in modern apartments (3). They are omnivorous, and even small amounts of breadcrumbs, flour, leftovers, dead insects, dry leaves or other organic material are enough to support a population. Adult individuals can survive for a long time without food (4) and can eat simple material such as paper or wallpaper (5). However, full development from nymph to adult requires a more varied diet (unpublished data). Development time depends on the temperature and can range from 18 months to 3 years (Figure 5). Once mature, adults can live for a long time and produce about 50 eggs per year. Long-tailed silverfish are nocturnal and are often only spotted when lights are turned on. They thrive best in high humidity, but they also cope with drier environments and can be found in rooms without running water or drains (2;3). Both adults and nymphs are sensitive to low and high temperatures. Development and activity are severely hindered at temperatures below 16 °C and stop completely at 11 °C (4). Temperatures above 26 °C limit their lifetime and they will only survive for a few hours at temperatures above 40 °C (4).
Under normal conditions, long-tailed silverfish cause minimal material damage. However, they are considered to be pests because many find them bothersome (3). Museum objects, stamp collections, fabrics made of cotton, linen or other plant materials, and old books can be destroyed if long-tailed silverfish are allowed to develop freely (6). Therefore, museums, historical collections and libraries should be extra vigilant. Long-tailed silverfish can be difficult to eradicate. In many cases, a small number of long-tailed silverfish may have to be tolerated for a period of time, just like brown carpet beetles or silverfish. However, the number of individuals should be kept in check using toxic bait and other measures or they will multiply if nothing is done.
How to resolve long-tailed silverfish problems?
Prevention: Long-tailed silverfish are not found in Norwegian nature and enter buildings on objects. Constant vigilance and inspection of all objects taken into buildings is needed which makes prevention difficult. Storage rental, parcel terminals and customs warehouses for mail and freight, retail storage, second-hand shops and flea markets probably play a role as distribution routes since they operate with large commodity flow, intermediate storage and distribution to stores or customers. Long-tailed silverfish hitch a lift in pallets, boxes and furniture and establish themselves in new places, before reproducing and then spreading further. Key players in the retail trade should take social responsibility and carry out regular inspections and eradication with toxic bait if necessary. Long-tailed silverfish can also hide in handbags and bags in workplaces, schools, kindergartens or private homes but as they are passive during the day, the risk of exporting uninvited guests from infested buildings is low.
Detection: Suspected cases of long-tailed silverfish must be confirmed by species determination. Bathrooms, kitchens and laundry rooms are prime spots for long-tailed silverfish (2;5;7). They hide behind skirting boards, in sockets, under door thresholds or in recess boxes for lights, cables and pipes. Set out small glue traps for a few weeks to catch individuals for species determination. If the building has several units, e.g. apartments it makes sense to check the entire building. Set traps in prime spots throughout the building to get an overview of the situation.
Restrict freedom of movement: Once long-tailed silverfish have settled, they spread easily. They are small and flat and can wander between rooms and squeeze through tiny gaps, using plumbing, air ducts and cable ducts as pathways. By grouting and sealing cracks, pipe and cable ducts, and adding fine mesh on air ducts, they find it harder to seek out favourable conditions and shelter.
Carry out eradication according to Integrated Pest Management (IPM) principles where several control methods can be combined to resolve the problem.
Tailor the strategy to each environment but the main method should be poisoned bait (8-10). Use glue traps, temperature and environmental regulations as a supplement. Zero tolerance for long-tailed silverfish (removing all individuals) is difficult to maintain; their ability to spread allows new individuals to be introduced and their hidden lifestyle means that small residual populations are not always detected. Keeping the long-tailed silverfish population as low as possible reduces the dispersal hazard to a minimum.
Poisoned bait: Several bait products can be used to combat insects in buildings (8). Their advantage is that a tiny amount of poison is used in a highly controllable manner (9). Bait is therefore preferable to pesticides and has been shown to provide efficient control in apartment buildings (Figure 6), office buildings, libraries, private houses and kindergartens.
Laboratory studies show that at least four of the baits available to the pest industry have good effect, while others have only limited effect (Figure 7) so it is important to choose the right product. Bait with the active ingredient indoxacarb is particularly efficient.
An important explanation for efficient control is probably a large degree of secondary poisoning (Figure 8). The long tailed silverfish will be consumed rapidly by conspecifics when they die from the bait. The bait-killed individuals contain a sufficient amount of toxins to also kill individuals only feeding on the dead (8).
Place the bait as very small droplets (10-20mg droplets; spot treatment - Figure 9) in several places along the walls of rooms and building (10). A density of one drop per square meeter is a good starting point for treatment (9). Use nooks and crannies to protect the bait and to focus the treatment on places where the insects are expected to move in search of food. Place bait close to potential hiding places to increase the likelihood that they will find and eat it but out of the reach of residents and pets. Take extra care in kindergartens or residences with young children to prevent accidental ingestion of the bait.
It is likely that both using bait and removing competing food sources with thorough vacuuming will exert such a large "food poisoning burden" on the long-tailed silverfish to completely eradicate them in many buildings (9). It takes time for all the individuals to eat the bait and repeated treatment may be needed. Remember that they can spread within a building and a relapse from untreated parts of the building is possible. A systematic and coordinated approach that deals with the entire building is best (9).
Cleaning and environmental changes: Good cleaning practices that limit access to food for long-tailed silverfish help to curb population growth and improve the effect of toxic bait (10). Vacuuming and dry mopping is best because local humidity is limited and potential food sources such as crumbs, food residue and the like are removed. Dry conditions give the long-tailed silverfish fewer opportunities to reproduce and low temperatures in the building extend their development time.
Glue traps: Glue traps are not only useful for monitoring and mapping the situation but also give a limited effect on the population (Figure 10). Removing individuals helps towards eradication but traps alone are not enough.
Pesticides: The need for pesticides must be considered strictly from place to place since poisoned bait has such a good effect. Avoid spraying in environments with a high risk of exposure of users or residents, such as kindergartens, schools, dining rooms / kitchens, offices, bedrooms, hospitals and other health institutions. Since there is currently no resistance to the pesticides, choose the product that is least toxic to humans. Repeated spraying poses a risk of chronic exposure of users in buildings and may allow development of resistance in long-tailed silverfish. Pesticide use should only be a minor part of an IPM solution.
Heat and cold treatment: Long-tailed silverfish die quickly at temperatures above 45 °C (unpublished data). However, treatment of buildings is demanding because long-tailed silverfish can easily hide in places where the heat treatment does not reach. Local treatment using hot air in gaps is therefore more appropriate and cost-effective, and is used as part of an IPM solution. Steam treatment can provide high local humidity and trap moisture inside structures and gaps, which makes life better for surviving individuals, so dry hot air is a more sensible control method. Heating of gaps and hiding places can only kill long-tailed silverfish if they are exposed to direct heat. All stages of long-tailed silverfish die if exposed to -10 °C for 24 hours (unpublished data). Cold treatment is most relevant in connection with the treatment of objects at museums, libraries and archives.
Other lepismatid species
In addition to long-tailed silverfish, we have the common silverfish (Lepisma saccharinum) in Norway, and Ctenolepisma calva and the four-lined silverfish (Ctenolepisma lineata) have been observed (11). Ctenolepisma calva and the four-lined silverfish are similar to long-tailed silverfish in shape, but not in colour. Ctenolepisma calva appears white or mother-of-pearl, while the four-lined silverfish has longitudinal light stripes on the back. Both of these species can occur together with long-tailed silverfish and be treated in the same way.