Do plug-in ultrasonic repellers work?

What the devices claim

An ultrasonic repeller plugs into a socket and gives out high-pitched sound above the range of human hearing. Some models add flashing lights or claim to send pulses through the house wiring. The packaging often lists mice, rats, spiders, cockroaches, ants and mosquitoes, and promises a pest-free home within weeks with no traps and no poison.

The appeal is obvious: plug it in and forget it. The problem is that the claims have not stood up when regulators asked for proof.

What regulators found

• United States, 2001. The Federal Trade Commission (FTC) sent warning letters to more than 60 makers and sellers of ultrasonic pest control devices. It said claims such as eliminating rodent infestations, repelling insects or replacing conventional pest control must be backed by competent and reliable scientific evidence. Its staff said rodents become accustomed to ultrasound and return to where they nest and feed, even with a device running.
• United Kingdom, 2013. The Advertising Standards Authority ruled that claims for an ultrasonic mosquito repellent, that it could repel and kill mosquitoes, had not been substantiated and were misleading.

We found no UK public body that recommends these devices. Advice on rodents from the Health and Safety Executive (HSE) names hygiene, physical barriers and traps.

Why sound does not solve the problem

High-pitched sound does not travel well. Walls, furniture, boxes and soft furnishings block or soak it up, so a plug-in in the hall does nothing for a mouse behind the kitchen units or under the floor.

Even where the sound reaches, a rodent that has food and a safe nest has a strong reason to stay, and the FTC's staff said rodents get used to it. The evidence for insects is no stronger. We found no independent evidence that the versions claiming to work through house wiring do any better.

The cost of waiting

The real price of a repeller is the time it costs you. Mice breed fast: a female can have five to ten litters a year and her young can breed at about six weeks. A few mice in October can be a colony by Christmas while you wait for a gadget to work. Rats gnaw cables and pipes in the meantime.

If the sound did bother rodents, it would bother pet rodents too. Keep any device away from rooms with hamsters, gerbils or pet rats.

Reading the box

The packaging tends to make the same promises. A coverage area in square metres leaves out the walls and furniture in the way; a list of every pest from mosquitoes to rats assumes one sound suits them all; and the words "tested" or "proven" carry weight only when a published study is named. A money-back period of a few weeks is not evidence either: mice that go quiet for a fortnight may simply have moved to another part of the house.

If you have bought one, carry on trapping and proofing alongside it rather than waiting for it to work.

What to use instead

Never use glue traps. They are banned for householders in England, and banned outright in Wales and Scotland.

When to call a professional

Call someone if droppings keep appearing after two weeks of trapping, if you hear rodents in the walls or loft, or if you see cockroaches in daylight. See mouse control and rat control for what a visit involves, and what mouse control costs. Many councils treat rats and mice for a fee or free, so check your council first.