Mothballs do not reliably keep spiders away, and using them for that purpose is actually illegal in the United States. Mothballs are registered pesticides designed specifically to kill clothes moths and fabric pests in enclosed containers. Their labels do not list spiders as a target pest, and the National Pesticide Information Center states plainly: “Mothballs are not wildlife repellents.”
Why Mothballs Don’t Work on Spiders
Mothballs contain high concentrations of either naphthalene or paradichlorobenzene. These chemicals slowly release a toxic gas in enclosed spaces like sealed garment bags or storage trunks, where the vapor can build to lethal concentrations for small fabric-eating insects. Spiders, however, are not insects. They’re arachnids with different biology and behavior, and neither chemical has been shown to repel them in any controlled study.
The vapor from a mothball placed in an open room, closet corner, or basement dissipates quickly. Naphthalene is removed from indoor air through ventilation and absorption into walls and surfaces. In a well-ventilated space, the concentration never builds high enough to affect much of anything. You’d need a dangerously high amount of mothball vapor in a sealed area to have any real impact, and at that point the health risks to you and your family far outweigh any theoretical benefit.
Health Risks of Misusing Mothballs
Breathing naphthalene vapor is genuinely harmful. In humans, the primary blood-related concern is hemolytic anemia, a condition where red blood cells break down faster than the body can replace them. Chronic exposure has been linked to increased eye and nasal irritation, swelling of the nasal lining, and reduced lung function. Two case series have documented increased cancer rates in workers and individuals with prolonged naphthalene exposure, including laryngeal and colorectal cancers.
Scattering mothballs around your home means exposing yourself to these fumes every day in spaces where you eat, sleep, and breathe. Children are especially vulnerable because of their smaller body weight and faster breathing rate relative to their size.
The Risk to Pets
Dogs and cats are highly susceptible to mothball poisoning. Naphthalene irritates the digestive tract, causing vomiting and loss of appetite as early symptoms. Larger exposures can progress to hemolytic anemia, difficulty breathing, and pale gums. In rare cases, neurological symptoms like tremors, loss of coordination, and seizures develop. Paradichlorobenzene-based mothballs cause abdominal pain, vomiting, lethargy, and trembling. Chronic ingestion of either type can damage the liver and kidneys. A curious pet that chews on or swallows even one mothball faces a serious medical emergency.
It’s Also Against the Law
Every pesticide sold in the U.S. carries a label specifying exactly where and how it can be legally used. Mothball labels restrict use to enclosed containers for protecting stored fabrics. Using them in gardens, crawl spaces, attics, or anywhere else to repel spiders, snakes, or other animals violates federal pesticide law. The National Pesticide Information Center notes that using mothballs in ways not specified by the label “is not only illegal, but can harm people, pets or the environment.”
What Actually Repels Spiders
A 2018 study published in the Journal of Economic Entomology tested the three most commonly recommended natural spider repellents: lemon oil, peppermint oil, and chestnuts. Researchers used a two-choice setup where female spiders from multiple species could move toward or away from each substance. Peppermint oil and chestnuts strongly repelled brown widow spiders and European garden spiders, and showed some effect on false widow spiders. Lemon oil, despite being the most frequently recommended repellent online (with over a million Google results at the time), had no effect on any species tested.
If you want to try peppermint oil, a few drops diluted in water and sprayed along windowsills, doorframes, and corners where you’ve seen webs is a reasonable approach. It smells pleasant, costs little, and carries none of the health risks of mothballs. Chestnuts placed along baseboards may also help, though they’re seasonal and harder to source in many areas.
Prevention That Actually Works
The most effective spider control comes down to making your home less hospitable. Spiders follow their food supply. If your house has lots of small insects, spiders will move in to eat them. Reducing insect entry points reduces spiders as a downstream effect.
Start with sealing cracks in your foundation and gaps around windows and doors. Check window and door screens for tight seals, since good screening keeps out both spiders and the insects they hunt. Spiders also hitchhike indoors on firewood, cardboard boxes, and potted plants, so inspect these before bringing them inside.
Inside the home, vacuum or sweep regularly in corners, along baseboards, behind furniture, and in storage areas like basements and garages. Vacuuming is surprisingly effective because spiders’ soft bodies typically don’t survive the trip through the hose. One useful detail: a dusty web is an abandoned web. Spiders keep active webs clean to catch prey, so if you see dust on a web, the spider has already moved on.
In storage areas, keep boxes off the floor and away from walls. Seal boxes with tape to prevent spiders from nesting inside. Clear clutter from garages, sheds, and basements, since piles of rarely disturbed items create ideal hiding spots. The less undisturbed shelter your home offers, the fewer spiders will settle in.