Boil a cup of dried rosemary in a quart of water for 20 to 30 minutes, then strain the water steeped with rosemary into a larger container. Add a quart of cool water and mix well.
Pour the diluted rosemary mix into spray bottles to spritz on mosquitos as a repellent. You can store leftover rosemary water for later until its smell significantly dissipates.
You can also toss a few rosemary sprigs with the green leaves still attached in the hot coals or your fire pit as you enjoy an evening outdoors to deter mosquitos.