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Long-tailed silverfish

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Skjeggkre
Figure 1. Long-tailed silverfish can be confused with regular silverfish but are larger, have longer filaments and hair that is more prominent. FHI - Preben Ottesen

Long-tailed silverfish (Ctenolepisma longicaudata) 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.


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Appearance

Skjeggkre
Figure 2. Long-tailed silverfish (Ctenolepisma longicaudata) have become common indoors in recent years. Foto: Kim Abel, Naturarkivet.no
Egg fra skjeggkre
Figure 3. Eggs from long-tailed silverfish (Ctenolepisma longicaudata). FHI - Morten Hage
Andrestadie av skjeggkre
Figure 4. Secondary stage of long-tailed silverfish (Ctenolepisma longicaudata). FHI - Morten Hage

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 10-12 mm without antennae and filaments but they can reach 18 mm. 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. 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 and can eat simple material such as paper or wallpaper. However, full development from nymph to adult requires a more varied diet. 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. 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. Temperatures above 26 °C limit their lifetime and they will only survive for a few hours at temperatures above 40 °C.

Illustration of long-tailed silverfish showing development from egg to adult
Figure 5. Illustration of long-tailed silverfish showing development from egg to adult. The development time for the various stages is taken from Lindsay (1940) and indicates time under constant and optimal conditions, while the span of the total life cycle is based on an expectation of sub-optimal and variable temperature conditions throughout the day and year. NIPH - Preben Ottesen

Damage

Under normal conditions, long-tailed silverfish cause minimal material damage. However, they are considered to be pests because many find them bothersome. 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. 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, just like brown carpet beetles or silverfish. However, the number of individuals should be kept in check using the following 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 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 and mapping: Suspected cases of long-tailed silverfish must be confirmed by species determination. Bathrooms, kitchens and laundry rooms are prime spots for long-tailed silverfish. 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.

Eradication

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. Use glue traps, pesticides and temperature 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.

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 bait. 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.

Eksempel på limfeller som kan benyttes til fangst av skjeggkre
Figure 6. Example of glue traps that can be used to catch long-tailed silverfish. FHI - Morten Hage

Glue traps: Glue traps are not only useful for monitoring and mapping the situation but also give a limited effect on the population (Figure 6). Removing individuals helps towards eradication but traps alone are not enough. Many glue traps would be needed over a very long period, which many people would consider to be impractical.

Poisoned bait: Several bait products can be used to combat insects in buildings. Their advantage is that a tiny amount of poison is used in a highly controllable manner. Bait is therefore preferable to pesticides.

Survival among long-tailed silverfish when bait is placed in small spots in long-tailed silverfish habitats with hiding places, food and water
Figure 7. Survival among long-tailed silverfish when bait is placed in small spots in long-tailed silverfish habitats with hiding places, food and water. The test was performed with 32 individuals per survival curve. Lower case letters (a, b and c) indicate significant differences between products (p <0.05). NIPH - Anders Aak

Laboratory studies show that at least three 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.

The effect of bait eradication in apartment building
Figure 8. The effect of bait eradication (spot treatment) in an apartment building. NIPH - Anders Aak
Secondary poisoning
Figure 9. The figure show survival in a population of 48 healthy long-tailed silverfish feeding on conspecifics killed by bait. NIPH - Anders Aak

Bait for long-tailed silverfish also provides a rapid decimating effects on the population in studies performed in apartment blocks (Figure 8). An important explanation for efficient control is probably a large degree of secondary poisoning. 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 (Figure 9).

Plassering av åte mot skjeggkre
Figure 10. Example of placing a bait spot. Many small drops (10-20 mg) placed along skirting boards, preferably in small gaps and in nooks and crannies can contribute to effective treatment. FHI - Anders Aak

Place the bait as very small droplets (spot treatment - Figure 10) in several places along the walls of rooms and building. 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. Use bait stations or fixed protective skirting to prevent children from coming into contact if necessary.

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. The Norwegian Institute of Public Health will investigate this in the years to come. 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.

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 50 °C.

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. Knowledge about the cold tolerance of long-tailed silverfish is lacking, but deep-freezing over time kills long-tailed silverfish.

Report about long-tailed silverfish

In 2018, the Norwegian Institute of Public Health conducted a literature study on long-tailed silverfish. The findings are gathered in a report entitled Long-tailed silverfish - biology and control. The report contains more detailed information about long-tailed silverfish and more images than are found on this website.