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SILVERFISH HOOKED ON BAITS IN LATEST RESEARCH

A team of European researchers has discovered that the long-tailed silverfish has shown susceptibility to gel baits. Could baiting be the answer for controlling a wider range of silverfish species?

A silverfish problem is normally dealt with by using a combination of dusts and sprays, carrying out a general treatment and trying to target potential silverfish hiding places. However, even more so than cockroaches, finding these hiding places is a major challenge for silverfish – especially as they may be hiding in books, documents and clothes, which cannot be treated. The concept of baiting silverfish is therefore very attractive.

However, whilst silverfish are susceptible to most general insecticides, there is limited scientific knowledge about their susceptibility to the various gel baits on the market. These primitive, wingless insects are known to feed on sugars, proteins and fat, so it is logical to assume that baits which target ants or cockroaches might successfully control them, assuming the baits are attractive enough. With the long-tailed silverfish, Ctenolepisma longicaudata (pictured above) recently establishing itself as a pest in several European countries, a team of researchers has tested a range of baits, using four different actives, to determine the most effective for gaining control of silverfish.

In a paper published in the journal Insects in March 2020,1 the research team from the Norwegian Institute of Public Health tested several baits with either indoxacarb, clothianidin, fipronil or imidacloprid as the active ingredient. Four of the six baits were designed for cockroach control, which the researchers believed would prove effective against silverfish due to the pests’ similar lifecycles, involving many juvenile stages, and similar behavioural patterns of time spent in aggregations.

The team then looked at the levels of primary and secondary poisoning; primary poisoning being through ingestion of the bait, and secondary poisoning occurring as a result of excretion, necrophagy (consuming dead silverfish) or emetophagy (consuming vomitus). The researchers also tested the durability of baits to determine which would be the most suitable for use as part of an IPM solution.

Lab-reared colonies of C. longicaudata were starved for three days before being placed in one of several testing arenas. The insects were given the choice between flaked oats – typical of the kind of food they might find in a house where small amounts of dry food will often be accessible on the floor – and one of six commercially available gel/paste baits. The experiment lasted for 18 days, with dead insects being removed from the testing arenas daily.

The results showed that baits using the active ingredients indoxacarb, clothianidin and fipronil killed more than 90% of the experimental insects (by primary poisoning) when presented in competition with food, with the time to kill 50% of the population being in the range of 2-5 days. They were significantly more effective than the imidacloprid bait, which delivered less than 50% mortality.

In the subsequent experiment looking at secondary poisoning, the researchers took the insects that had died from ingestion of the indoxacarb and clothianidin baits and reintroduced the dead insects to the test arenas to investigate the levels of secondary poisoning, primarily through silverfish feeding on the dead carcasses. The fipronil bait was not included in the secondary poisoning trial.

When the silverfish fed on carcasses killed by indoxacarb, more than 75% secondary mortality was observed. By comparison, the clothianidin treatment resulted in only 15% mortality.

It is interesting to note that indoxacarb needs to be ingested and bioconverted before killing an insect, whereas clothianidin acts more quickly. The researchers suggested that in the case of clothianidin, there may not have been enough toxins present in the bodies of the dead insects to achieve horizontal transfer. The findings suggest that a low dose of a slower acting pesticide, such as indoxacarb, may actually be better to control silverfish than a fast-acting pesticide. The results also indicate that necrophagy is an important aspect of the silverfish diet, perhaps due to the high protein content of the dead insects.

The researchers then looked at the performance of the indoxacarb baits aged under field conditions. Bait drops aged in the field were collected after four- and six-month intervals and were brought back to the lab and used as bait for the silverfish. Interestingly, the four- and six- month-old baits maintained their efficacy, significantly reducing the survival rate of the insects. With the indoxacarb baits showing to have such a long lifespan in the eld, it suggests that baiting has the potential to be a highly valuable tool against silverfish, providing the right active ingredient is used.

Furthermore, the very low indoxacarb dose in the ant bait – just one twelfth of the amount in the cockroach bait – was sufficient to kill the primary target and appears to have produced more potent carcasses for secondary poisoning.

It should be noted that these laboratory trials were carried out on the long-tailed silverfish (Ctenolepisma longicaudata), not Lepisma saccharina, the common silverfish found in Australia and no field trials have been carried out. As such it cannot be assumed the same performance will be observed with Lepisma saccharina and it remains to be seen if silverfish claims will make their way onto bait product labels in Australia.

1 Aak, Anders & Hage, Morten & Rukke, Bjørn. (2020). Long- Tailed Silverfish (Ctenolepisma longicaudata) Control; Bait Choice Based on Primary and Secondary Poisoning. Insects. 11. 170. 10.3390/insects11030170.

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