Do mothballs keep mice away?

Mothballs release naphthalene or paradichlorobenzene vapor. In an airtight container that vapor builds up enough to kill moth larvae in stored wool. Scattered in an attic, garage, or basement, it never reaches a concentration that bothers a mouse for long, and mice nest right next to them. Every attic we've opened that had mothballs in it also had mouse droppings.

The label is the law with pesticides, and it says exactly where the product can be used (NPIC). Loose mothballs in living space let toxic vapor into the air, and long-term exposure can cause health problems (NPIC). They're also a poisoning risk for kids and pets, who mistake them for candy. Using them outdoors to repel wildlife contaminates soil and water and is specifically called out as misuse (NPIC).

Mice fit through a gap the width of a pencil, and the fix is physical. Find every opening where pipes, cables, and gas lines enter, at the garage door seal, the foundation sill, and vents, and close them with steel wool packed in and caulked, or with hardware cloth and metal for bigger holes (CDC). Foam, plastic, and wood get chewed through. Then trap the ones already inside. Peppermint oil, ultrasonic plugs, and dryer sheets belong in the same category as mothballs: things mice ignore.

Mothballs show up in almost every older attic and detached garage we inspect in Nassau and Suffolk, usually next to an active mouse nest. Sealing before October is what prevents the fall move-in, and it's the part of a mouse job that carries the 60-day guarantee.