A long-tailed silverfish (Ctenolepisma longicaudata) examines a small piece of wood. (Photo: Shutterstock / NTB Scanpix)
A long-tailed silverfish (Ctenolepisma longicaudata) examines a small piece of wood. (Photo: Shutterstock / NTB Scanpix)

You can combat long-tailed silverfish efficiently and safely, it turns out

It’s possible to get rid of silverfish by using bait with small amounts of cockroach poison, according to recent studies from the Norwegian Institute of Public Health.

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Recently, many despairing homeowners in Norway have had their homes infested by pesky silverfish.

Several home buyers have taken legal action against the sellers’ insurance companies, in order to reduce the purchase price or cancel the purchase of their home. They have argued that the little pests are impossible to get rid of.

The Norwegian-Italina blogger Jørgine Massa Vasstrand, known as Funkygine, is one of the affected homeowners. She was however unsuccessful in her demand of a NOK 640 000 price reduction. Recently she and her husband appealed the silverfish decision in their case, according to an article on E24.no (in Norwegian).

But now several studies from the Norwegian Institute of Public Health (FHI) are showing that the silverfish can be effectively combatted. You can read about the study on the FHI website.

Poisoned bait tested in different settings

Researchers at the Department of Pest Control first conducted several laboratory studies to find an effective and safe method to combat the silverfish.

They followed up with tests in various settings, including an apartment complex, a townhouse, a kindergarten, a shop and a library.

The method involves putting out poisonous bait that the silverfish eat. The pesticide indoxacarb was added to the bait in very small amounts; it is an agent that the pest control industry has so far used against cockroaches.

“Now we have evidence from the field that getting rid of silverfish in homes is possible,” says researcher Anders Aak, who is an expert on pests at FHI.

Studies are still ongoing, but preliminary results after seven months are promising.

Formal approval has been lacking

“Bureaucracy has held up using this method on silverfish before now. The poison wasn’t approved for use against silverfish in Norway," says Aak.

Pest controllers have had the poison available, but they have only been allowed to use it for cockroach infestations.

“Norway formally received approval for use of the remedy against flying insects in March of this year,” Aak says.

Entire building needs to be treated

One of the tests was conducted in a complex of 38 apartments. There the infestation of silverfish was so pervasive that it had become a big nuisance for the residents.

FHI researcher Anders Aak is an expert on fighting pests. (Photo: FHI)
FHI researcher Anders Aak is an expert on fighting pests. (Photo: FHI)

First the rooms were vacuumed.

Then they were spot-treated. That involved placing many small drops of bait along mouldings and in natural hiding places in all the rooms of each apartment.

Pest control technicians had access to all the apartments on the same day, both at the start and at follow-up three to four months later. The silverfish population dropped quickly.

After eight weeks, the number of silverfish was reduced by 90 per cent. After seven months, no silverfish were observed in any of the apartments.

Silverfish can easily move from one apartment to another, so the technicians also treated the technical rooms and common areas.

“Without a comprehensive approach that encompasses the entire building, it often takes longer to achieve the desired result,” says Aak.

Combatting the pests in townhouses

The Institute of Public Health also conducted a similar study in townhouses. Here, they combined spot treatment with pre-fabricated cockroach bait stations.

Four weeks after treatment, the number of silverfish was reduced by 76 per cent.

The residents described the change as follows: from being a visible, daily annoyance, they eventually saw no critters at all.

After 12 weeks, the percentage of silverfish was reduced by 90 per cent.

Cannibals

Secondary poisoning was extensive. The silverfish ingested additional poison when they ate other silverfish that had already died from the poison.

“It's a myth that silverfish only eat paper. They need protein, which is why they eat their own species when they die, just like they do with other dead insects they find,” Aak says.

This strengthens the effectiveness of the treatment, even with tiny amounts of poisoned bait.

No guarantee they’re gone once and for all

Only time will tell whether these successful exterminations will lead to fewer lawsuits against home sellers. When you work with biology, you can never guarantee that you’ve solved the problem forever,” says Aak. He believes this is what many home buyers have been speculating on when they sued to have their home purchase price reduced.

Although the researchers have now shown that they can drastically reduce the amount of silverfish in homes, you can never know if they’ll return.

Aak notes that people cannot get rid of these pests on their own. The best toxins are reserved for professional pest control technicians.

“The general population is only allowed to use glue traps, and they are far too ineffective to get rid of the problem in the long run,” he says. “EU regulations govern this process.”

Reference:

Examples of control of long-tailed silverfish with bait in different premises. FHI, 5 September 2019.

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Read the Norwegian version of this article on forskning.no