Long-tailed silverfish
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The long-tailed silverfish (also called grey silverfish; Ctenolepisma longicaudatum) has become established in Norway. A marked increase in the number of control treatments has been recorded since 2016, and the number now appears to have stabilised. They are most often found in newer buildings. They resemble the common silverfish, but are somewhat larger, hairier, and have longer tail filaments. Long-tailed silverfish can be a nuisance, but otherwise cause little damage in homes. They can be controlled with poisoned bait.
Appearance
Long-tailed silverfish are wingless insects. They are covered with scales, dark grey, and slightly mottled in colour (Figures 1 and 2). They therefore appear less silvery-shiny than the common silverfish. The maximum body length, excluding antennae and tail filaments, is 18 mm (1), but the normal adult size is 10–14 mm (2).
The body shape is the same as in the common silverfish. At the end of the flattened, elongated, and clearly tapering abdomen there are three long tail filaments. The middle filament points straight backwards and is approximately as long as the body, while the lateral filaments often point outwards at right angles. The head has two long antennae, and in addition two shorter palps can be seen.
Nymphs are about 2 mm when they hatch (Figure 3). Through 14 developmental stages they grow larger and become more like the adults. The youngest individuals lack scales and are therefore lighter in colour (Figure 4).
Life cycle, habitat and behaviour
In Norway, long-tailed silverfish live only indoors, and they can be encountered all year round. They have been observed in buildings throughout the country, and many findings are from modern apartments (3).
They are omnivorous, and even small amounts of breadcrumbs, flour, food scraps, dead insects, dry leaves, or other organic material will be sufficient to sustain a population. Adults can survive for a long time without taking in food (4), and they can manage on simple materials such as paper and wallpaper (5).
Complete development from nymph to adult requires a more varied diet (unpublished data). Development time depends on temperature and ranges from 18 months to 3 years (Figure 5). After reaching sexual maturity, adults can live for a long time, and they lay around 50 eggs per year.
Long-tailed silverfish are nocturnal and are therefore often discovered when the light is switched on at night. They thrive best at high humidity, but compared with the common silverfish they also cope well in drier environments (2;3). They are therefore also found in rooms without piped water, floor drains, or waste pipes.
Both adults and nymphs are sensitive to low and high temperatures. Development and activity are strongly limited at temperatures below 16 °C and stop completely at 11 °C (4). Temperatures above 26 °C also markedly reduce lifespan, and they will survive only a few hours at temperatures above 40 °C (4).
Damage
Under normal conditions, long-tailed silverfish cause little damage, and any material losses will be minimal. They are nevertheless regarded as pests because many people experience them as a nuisance (3). Museum objects, stamp collections, textiles made from cotton, linen, or other plant materials, and old books can be damaged if long-tailed silverfish are allowed to develop freely. Museums, historical collections, and libraries should therefore be especially vigilant, as valuable items may deteriorate (6).
Long-tailed silverfish can be difficult to get rid of, and for a period one may have to tolerate a small number of individuals—similar to, for example, the brown carpet beetle or the common silverfish. If the number of insects is not minimised through the use of poisoned bait and other measures mentioned below, there may be very large numbers of long-tailed silverfish in the building.
Prevention
Long-tailed silverfish do not occur in nature in Norway and therefore have to be transported into buildings via objects in order to establish themselves. Preventing sporadic introductions is difficult, since it would require constant vigilance and inspection of all items brought into buildings.
Self-storage facilities, parcel terminals and bonded warehouses for post and freight, commercial warehouses, second-hand dealers, and flea markets probably play a role as spread pathways, since they involve high goods turnover, intermediate storage, and distribution to shops or customers. It is easy for long-tailed silverfish to hitchhike with pallets, crates, cardboard boxes, and furniture, allowing them to establish themselves in new places, reproduce, and then spread further.
Key actors in retail can demonstrate social responsibility regarding the long-tailed silverfish problem by carrying out regular inspections and, if necessary, control using poisoned bait.
Long-tailed silverfish can also hitchhike in handbags, bags, and backpacks from, for example, workplaces, schools, kindergartens, or private homes; however, since long-tailed silverfish are inactive during the daytime, the risk of bringing unwanted guests in items that are carried to and from infested buildings on a daily basis is probably low.
Detection
If long-tailed silverfish are suspected, this needs to be confirmed with a reliable species identification. Bathrooms, kitchens, and laundry rooms are “hot spots” for long-tailed silverfish (2;5;7). They can be found behind skirting boards, in baseboards, under door thresholds, or in built-in boxes for lights, cables, and pipes. To catch individuals for species identification, it is effective to place small glue traps that are collected after a couple of weeks. If the building consists of several units, e.g., apartments, it is sensible to take a building-wide approach— using detection traps in the so-called “hot spots” throughout the entire building to obtain an overview of the situation.
Control
Control of long-tailed silverfish should be carried out according to the principles of integrated pest management (IPM), where several control methods work together to solve the problem. The strategy needs to be adapted to the different environments in which long-tailed silverfish may occur, but the main method is poisoned bait (8–10). Glue traps, temperature, and environmental changes are used as supplements. Zero tolerance—i.e., removing all individuals—can be demanding to uphold, since their ability to spread means new individuals can easily be introduced, and their hidden lifestyle means small remaining populations are not always detected. By keeping populations down to a low number of individuals, the risk of spread is reduced to a minimum.
Poisoned bait
Several bait products are available for the control of insects in buildings (8). The advantage of such bait is that a very small amount of poison can be used, with controllable placement (9). Bait is therefore preferred over spray insecticides, and has been shown to be effective in apartments (Figure 6), commercial premises, libraries, houses, and kindergartens.
Laboratory studies show that at least four of the baits available to the pest-control industry have good effect, while others have only limited impact (Figure 7). It is therefore important to use the correct product. Bait with the active ingredient indoxacarb is especially effective. An important explanation for this positive effect is probably a high degree of secondary poisoning. When long-tailed silverfish die from the bait, they are quickly eaten by other individuals of the same species. Those killed by the bait contain enough poison to also affect those that only eat the dead individuals (8).
An important explanation for this positive effect is probably a high degree of secondary poisoning (Figure 8). When long-tailed silverfish die from the bait, they are quickly eaten by other individuals of the same species. Those killed by the bait contain enough poison to also affect those that only eat the dead individuals (8).
The bait should be placed as very small droplets (10–20 mg; spot treatment – Figure 8) in many locations along the walls in rooms and throughout the building, since this increases the likelihood that long-tailed silverfish will find the bait (10). A density of one droplet per m2 is a good starting point for treatment (9). Cracks, corners, and crevices should be used to protect the bait and to focus treatment on places where insects are expected to move while searching for food. Placement close to potential hiding places will also increase the likelihood that they find and eat the bait. Bait should be applied in a way that minimises the chance that residents and pets come into contact with the product. Extra caution is used in kindergartens or housing units with small children to prevent accidental ingestion of the bait.
By using bait treatment while also removing competing food sources through thorough vacuuming, one creates such strong “food-poisoning pressure” on long-tailed silverfish that they can be eliminated in many types of buildings (9). It takes time before all individuals eat the bait, and follow-up treatments may be necessary if the bait is washed away or disappears for other reasons.
However, it has been shown that the applied bait adheres well to the surface and remains fully functional for at least 4 years (Figure 9) (11). In most cases, this limits the need for repeated treatment. In addition, it is important to be aware that their ability to spread within a building is good, and reinfestation from untreated parts of the building may occur. A systematic and coordinated control approach that addresses the entire building (building-wide control) will therefore be the most appropriate (9).
Restrict freedom of movement
Once long-tailed silverfish have settled in, they have relatively good dispersal ability. They are small and flat and are able to move between rooms, squeeze through tiny openings, and use pipe penetrations, ventilation ducts, and cable conduits as pathways. Restricting their freedom of movement by caulking and sealing cracks, pipe and cable penetrations, and using fine-mesh screening on ventilation ducts can give them fewer opportunities to seek favourable conditions and reduce the number of hiding places.
Cleaning and environmental changes
Good cleaning routines that limit access to food for long-tailed silverfish are an important element in curbing population growth and improving the effect of poisoned bait (10). Vacuuming and dry mopping are preferred, since this can limit local humidity while removing potential food sources such as crumbs, food scraps, and similar. Dry conditions give long-tailed silverfish fewer opportunities for reproduction, and a low building temperature will prolong development time.
Glue traps
Glue traps are a suitable tool for monitoring and mapping, but in themselves have a limited effect on population development (Figure 10). Removing individuals from a population will always be positive in a control context, but under normal conditions traps will not do a good enough job on their own.
Spray insecticides
The use of spray insecticides is unnecessary in the vast majority of cases, since poisoned bait has been shown to provide at least as good an effect while reducing the risk of poison exposure for users or residents. The use of spray insecticides is undesirable in kindergartens, schools, dining rooms/kitchens, offices, bedrooms, hospitals, and other healthcare institutions. Repeated spraying entails a risk of chronic exposure for building users and is also unfavourable with regard to the development of resistance in grey silverfish. Any use of spray insecticides is therefore a very small part of an IPM solution.
Heat and cold treatment
If temperatures above 45 °C are reached, long-tailed silverfish will die quickly (unpublished data). Treating entire buildings is nevertheless challenging, since long-tailed silverfish can easily retreat to places that the heat treatment does not reach. Local treatment using hot air in cavities therefore seems more relevant and cost-effective, and is used as part of an IPM solution. Steam treatment can increase local humidity and trap moisture inside structures and cavities. Since steam can contribute to better conditions for individuals that survive, dry hot air appears to be a more sensible control method. Heating cavities and hiding places can only kill long-tailed silverfish if the individuals are exposed to direct heat. All life stages of long-tailed silverfish die if exposed to -10 °C for 24 hours (unpublished data). Cold treatment is most relevant in connection with object treatment in museums, libraries, and archives.
Other lepismatid species
In addition to long-tailed silverfish, Norway has the common silverfish, and pearl silverfish (Ctenolepisma calvum) and four-lined silverfish (Ctenolepisma lineatum) have also been observed (11). Pearl silverfish and four-lined silverfish resemble long-tailed silverfish in shape, but not in colour. The pearl silverfish appears white or mother-of-pearl coloured, while the four-lined silverfish has longitudinal light stripes on its back. Both species can occur together with long-tailed silverfish and are treated in the same way.