Most rodent repellents do not work well enough to rely on. A University of Arizona Cooperative Extension article on ultrasonic pest control says several researchers have concluded that these devices do not effectively repel or eliminate pests from homes, and that rodent populations kept living and reproducing in attics where the devices were installed. Scent products like peppermint oil have little evidence behind them. Mothballs are pesticides with label rules, and using them to repel animals outdoors is a misuse.
If you want rats and mice gone, sealing entry points, trapping and removing food and cover work better. Here is what the research and the labels say.
Ultrasonic devices
Plug-in devices that claim to scare rodents away with high-pitched sound are everywhere. The research is not on their side.
- The University of Arizona extension article says animals placed in cages next to these devices continued to live normal lives.
- The same article says researchers placed devices in rat- and bat-infested attics and the animals kept living and reproducing there.
- The Internet Center for Wildlife Damage Management cites a 1985 study by Howard and Marsh that found rodent control with ultrasonic units was just as successful when the units were turned off.
Money spent on a plug-in device is better spent on door seals and hardware cloth.
Peppermint oil and other scents
Peppermint oil, cayenne, dryer sheets and similar products are popular because they are cheap and smell pleasant to people. The evidence that they keep rats or mice away from a house is weak, and the scent fades. At best, they might make one spot slightly less attractive for a short time. They will not close the gap a mouse is using.
Mothballs
Mothballs come up often in advice about pack rats and cars. A University of Florida IFAS fact sheet, The Facts about Mothballs, makes the main points.
- Mothballs are registered pesticides containing naphthalene or paradichlorobenzene, and as with any pesticide, the label is the law.
- Labels typically direct you to use them in a tightly closed container.
- Most misuse involves outdoor placement, and using them to repel wildlife is not approved.
- They can harm people, pets and wildlife that touch them or breathe the vapors, and one mothball can seriously harm a child who eats it.
In an El Paso garage or under a car hood, mothballs put vapors where people and pets breathe without solving the rodent problem.
Predator urine, lights and other tricks
Products made from predator urine, flashing lights under car hoods and similar ideas have the same problem as scents: rodents get used to them, or the effect is too weak to matter when food and shelter are close. If they work at all, they work only alongside the steps below.
Why repellents are tempting
Repellents promise a fix without traps, bait or cleanup, and they are easy to buy. The problem is that rodents follow food and shelter. A garage with cardboard boxes on the floor and a bag of dog food in the corner will attract mice no matter what is plugged into the wall. That is why the CDC's advice focuses on removing food, water and shelter and sealing the house.
If you still want to try one
Use it alongside proven steps, not instead of them. Seal the gaps, set snap traps and clean up food sources. If you use any chemical product, read and follow the label. Keep anything scented or chemical away from pets and children. If you are dealing with pack rats in a car, see our pack rats in car engines article for steps that work better than repellents.
Pets and repellents
Scented oils and chemical repellents can bother pets, and mothballs are toxic if eaten. Keep anything you try out of reach of dogs and cats.
What works better
The CDC's approach to rodents comes down to three things: seal up, trap up, clean up. Removing food, water and shelter is the base of it.
- Seal gaps a quarter inch and larger for mice and about a half inch for rats, using steel wool with caulk for small holes and metal, cement or hardware cloth for large ones.
- Trap along walls with snap traps, checked daily.
- Store food, pet food and seed in sealed containers.
- Move woodpiles and clutter away from the house and off the ground.
- Keep garbage in cans with tight lids.
Our exclusion page and trap guide go step by step. For pack rats and cars, see pack rats in car engines.
Frequently asked questions
Do ultrasonic mouse repellers work?
Extension sources say no. Research found rodents kept living and breeding near the devices.
Does peppermint oil keep mice away?
The evidence is weak, and the scent fades quickly. Sealing and trapping work better.
Can I put mothballs in my garage or car?
Mothballs are pesticides and should only be used as the label directs, usually in a tightly closed container. Outdoor use to repel animals is a misuse.
What is the best rodent deterrent?
Closing entry points and removing food and cover. Nothing you plug in or spray replaces that.
Do cats keep mice away?
A cat may catch some mice, but it will not close entry points or stop new mice from coming in. Sealing and trapping are still the fix.
Sources
- University of Arizona Cooperative Extension: Ultrasonic Pest Control (Backyard Gardener)
- Internet Center for Wildlife Damage Management: Ultrasound repellents
- University of Florida IFAS: The Facts about Mothballs (PI289)
- CDC: How to seal up to prevent rodents
- CDC: How to trap up to remove rodents