Bug spray can kill spiders, but it works far less reliably on them than it does on insects. Spiders aren’t insects, and most consumer bug sprays are formulated with insects in mind. A direct hit will often knock down a spider, but the residual barrier you spray along baseboards and doorways may do little to stop spiders that walk through later.
Why Bug Spray Is Less Effective on Spiders
Spiders are arachnids, not insects, and their biology makes them naturally harder to kill with standard insecticides. The active ingredients in most household bug sprays, typically synthetic pyrethroids, target the nervous systems of insects. Those same chemicals can affect spiders, but the dose needed and the time to kill are often higher.
One persistent myth is that spiders can “hold their breath” to avoid inhaling pesticides. In reality, neither spiders nor insects breathe the way we do. There’s no active pumping of air in or out. Insects rely on passive diffusion through a network of tiny tubes called tracheae. Spiders have a different primary system: book lungs, which are stacked hollow plates surrounded by blood. Oxygen and carbon dioxide simply diffuse through these thin walls. The openings to a spider’s book lungs lack muscles to seal them shut, so a spider couldn’t block airborne chemicals even if it tried.
What matters more is contact. Most household sprays kill by landing directly on the pest’s body, not just through the air. The real limitation is what happens after you’ve sprayed a surface and walked away.
Direct Spray vs. Residual Barriers
There’s a major difference between spraying a spider you can see and spraying a surface hoping a spider crosses it later. Direct application is far more effective, but even then, results vary dramatically by species.
Research testing a common pyrethroid (cyfluthrin) on two spider species showed striking differences. Black widow females hit directly with the spray died at a rate of 100% within one hour. Hobo spider females, by contrast, showed 0% mortality after a full day of direct exposure, and only 33% died after five days. Hobo spider males fared somewhat worse, with 66% mortality by six hours. The takeaway: some spiders are remarkably tough, even when soaked with the product.
Residual barriers, the kind you spray along walls and let dry, are even less dependable. Texas A&M’s integrated pest management program states plainly that interior and exterior insecticidal sprays are generally ineffective at reducing spider populations, though they may provide some repellency. Pyrethroids can help reduce numbers but won’t eliminate all spiders.
One reason is that spiders walk on the tips of their legs, minimizing body contact with treated surfaces. They also don’t groom themselves the way insects do, so they’re less likely to ingest residue. And research on several spider species found that most could detect and avoid fresh pesticide residues, choosing to walk around treated areas. Once those residues aged even one day, spiders no longer avoided them, but the chemicals could still be toxic on contact for species sensitive to pyrethroids.
Where to Spray for the Best Results
If you’re going to use bug spray against spiders, placement matters more than quantity. The University of Kentucky’s entomology department recommends treating the specific spots where spiders live and travel rather than coating broad surfaces. Indoors, that means crevices along baseboards and window frames, plus sills, joists, and rafters in basements, crawl spaces, and attics. Outdoors, focus on door thresholds, garage entrances, crawl space openings, foundation vents, and the bottom edge of siding.
The goal is to create a perimeter that forces spiders to cross treated surfaces as they enter. Even so, expect the protection to fade. Most residual sprays maintain effectiveness for roughly 30 to 90 days before they need reapplication, and that window is likely shorter for spiders than for insects given the reduced efficacy.
Directly spraying any webs you find, along with the spiders in them, is the single most effective chemical approach. You’re guaranteed contact, and even resistant species absorb more product when it’s applied directly to their bodies.
Do Natural Repellents Work Better?
Peppermint oil is the most commonly suggested natural spider deterrent, and it has some scientific support. A study testing the three most frequently cited natural repellents (lemon oil, peppermint oil, and chestnuts) found that peppermint oil and chestnuts did deter spiders from settling in treated areas across two different spider families. Lemon oil, despite being the most popular recommendation online with over a million Google results at the time, had no effect on any species tested.
The catch is that even effective natural repellents work through scent, which fades quickly. You’d need to reapply peppermint oil frequently, and the concentration matters. A few drops mixed into water likely won’t match the levels used in controlled studies. Natural options may help discourage spiders from specific spots like windowsills, but they won’t eliminate an existing population.
What Actually Reduces Spider Numbers
Because chemical sprays have limited effectiveness against spiders, pest control professionals generally recommend a combined approach. Reducing the things that attract spiders works better than trying to poison them after they’ve moved in.
- Remove webs regularly. Physically knocking down webs with a broom or vacuum disrupts breeding and forces spiders to relocate. This is more immediately effective than any spray.
- Control other insects first. Spiders go where the food is. If you reduce flies, moths, and other prey around your home, spiders have less reason to stay. Ironically, bug spray is more useful for killing the insects that attract spiders than for killing the spiders themselves.
- Seal entry points. Caulk cracks around windows, doors, and foundations. Install door sweeps and repair torn screens. A spider that can’t get in doesn’t need to be sprayed.
- Reduce clutter. Spiders prefer undisturbed areas with hiding spots. Clearing storage boxes, woodpiles near the house, and dense vegetation against exterior walls removes habitat.
Indoor Safety With Pesticide Sprays
If you use bug spray indoors, the EPA recommends applying it only to limited areas like cracks and crevices rather than coating entire floors, walls, or ceilings. Open windows and run fans after application. Keep children and pets out of treated areas for at least as long as the product label specifies. Check for the signal word on the label: “Caution” means least hazardous to humans, “Warning” is moderate, and “Danger” means the product is poisonous or corrosive and requires extra care.
Store products in their original containers with childproof caps refastened, in a locked cabinet out of reach of children and pets. Never transfer pesticides into food or drink containers.