At worst, mosquitoes can carry a variety of diseases and at best, are a nuisance. Standing water provides an ideal breeding environment for mosquitoes. If you do not use your hot tub on a daily basis, you may need to add a mosquito repellent to the water in order to prevent it from becoming a mosquito breeding ground.
There are several products on the market specifically designed to repel mosquitoes and other pests from pools, hot tubs and other bodies of water. Buzz Off Insect Repellent (see resources) is a tablet placed in a floating dispenser/container and allowed to float in your hot tub, dispensing a safe insect repellent much the same way as chlorine tablets are dispensed.
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A properly cleaned, maintained and chlorinated hot tub is unlikely to develop a significant mosquito problem. You can significantly reduce or eliminate the chance of mosquitoes breeding in your hot tub by running the hot tub daily and keeping it clean and well chlorinated. Make sure that your hot tub's filter and pump are in good condition as well.
Citronella is a well known mosquito repellents. You can place citronella candles or tiki torches that burn citronella oil around your hot tub to repel mosquitoes away from the hot tub area. The downside of citronella candles and torches are that you have to stay relatively near the sources to avoid mosquitoes.
Waterproof Mosquito Repellents
If your main concern is being bitten by mosquitoes while you are using your hot tub, you can put on a waterproof mosquito repellent product. Products such as Off! Skintastic insect repellent work to repel mosquitoes, come in SPF 30 and are waterproof so you can still benefit from the product's effectiveness while you are in the hot tub. When you are purchasing spray-on mosquito repellent, read the directions and product information to make sure the product is waterproof.