Catnip

Researchers report that nepetalactone, the essential oil in catnip that gives the plant its characteristic odor, is about ten times more effective at repelling mosquitoes than DEET — the compound used in most commercial insect repellents. (...)

Catnip Repels Mosquitoes More Effectively Than DEET

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[ed. Why have I never heard of this before? See also: Catnip vs DEET: No Contest (includes a couple of spray recipes).