Do Rodent Repellents Work? Ultrasonic and Peppermint

Most rodent repellents do not solve a mouse or rat problem. The Federal Trade Commission has warned makers of ultrasonic pest devices that their claims need scientific support, and Iowa State University Extension says there is no research to support claims that ultrasonic devices eliminate rodents. Scent repellents like peppermint oil fade quickly and do not close the gaps rodents use.

What works is removing food and shelter, sealing entry points and trapping the rodents that are already inside. Here is what the evidence says about the popular options.

Ultrasonic plug-ins

Ultrasonic devices plug into outlets and emit high-frequency sound that is supposed to chase rodents away. In 2001 the FTC sent warning letters to more than 60 manufacturers and retailers, saying claims that these devices work must be supported by scientific evidence. Between 1985 and 1997 the FTC had already brought cases against six companies over claims that the devices eliminate rodent infestations, among other claims.

Iowa State University Extension's review of the topic says there is no research to support the claims. It also notes the FTC's earlier complaints alleged that rodents become accustomed to ultrasound, so any effect would be short-lived.

Ultrasound also does not pass through walls or furniture, so even a device that bothered mice in the open would not reach them inside wall cavities, which is where they nest in older Cleveland houses.

Peppermint oil and other scents

Peppermint oil, cayenne, ammonia and dryer sheets are common home remedies. The idea is that strong smells make rodents avoid an area. The practical problems:

Scents fade within days as the oil evaporates.
Mice that are hungry, cold or nesting tend to tolerate smells to reach food and warmth.
A scent does nothing about the gap the mouse came through.

At best, a scent might shift where mice travel for a short time. It will not remove a population from a house.

Mothballs

Mothballs are pesticides registered for use against clothes moths in airtight containers. Using them in attics, crawl spaces, garages or car engines to repel rodents is not what they are labeled for, and pesticide labels are legally binding. The fumes can also be harmful to people and pets in enclosed spaces. Skip them.

Repellent sprays and granules

Some registered repellent products are sold for use around foundations, gardens and car engines. Read the label carefully. These products may slow rodents in an area for a while, but they do not remove an established population or close entry points. Any pesticide, including a repellent, has to be used exactly as the label directs.

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Cats and dogs

A good mouser may catch some mice, and the presence of a cat or dog might make mice more cautious. Pets do not reach mice inside walls or rats in burrows, though, and some pets ignore rodents entirely. Pets can also be exposed to diseases and bait if they catch or eat rodents.

Why repellents fail in older Cleveland houses

Repellents are sold as a way to make a space unpleasant. In an older Cleveland house, the space mice want is inside the walls, behind plaster and lath, in the basement ceiling and in attic insulation. Sound and scent do not reach those places well, and mice there have warmth, shelter and a short trip to the kitchen.

Cold weather adds pressure. When a hard freeze arrives in late fall, mice outside are looking for any warm structure. A smell or a sound is a minor obstacle compared to that. The gaps in the sill, foundation and utility lines are the real question, and repellents do nothing about them.

What keeps rodents out

The CDC's rodent guidance comes down to three steps: remove food and water, seal up holes and trap the rodents that are already inside.

Seal gaps. The CDC says mice can fit through a hole about a quarter inch wide. Use steel wool held with caulk for small holes and hardware cloth, metal sheeting or cement for larger ones. See steel wool vs copper mesh.
Remove food. Store food and pet food in sealed containers, keep trash carts closed and clean up bird seed.
Trap. Use snap traps along walls on active runways. See best mouse and rat traps.

In a Cleveland house, the sealing step does most of the work. Older foundations, sills and utility holes are where mice and rats get in. A technician can find and seal them. See rodent exclusion.

Frequently asked questions

Do ultrasonic mouse repellers work?

Research does not support claims that they eliminate rodents. The FTC has warned sellers that efficacy claims require scientific evidence, and Iowa State Extension says there is no research to support them.

Does peppermint oil keep mice away?

Not reliably. The scent fades fast, and hungry or cold mice tolerate it. Sealing entry points works far better.

Are mothballs safe to use for mice?

No. Mothballs are pesticides labeled for clothes moths in airtight containers, and using them to repel rodents is off-label. The fumes can harm people and pets.

Will a cat keep mice away?

A cat may catch some mice, but it cannot reach mice inside walls or rats in burrows. Sealing and trapping are still needed.

What is the best way to keep mice out for good?

Seal the gaps they use with materials they cannot chew, remove food sources and trap any mice already inside.

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