Don't put citronella in your diffuser.

No — using citronella in a diffuser for indoor mosquito control is a bad idea for several reasons.

Citronella works by masking scents that attract mosquitoes, but it needs to be burned (as in candles or torches) to release its active compounds effectively. A cool-mist ultrasonic diffuser just sprays a diluted mist of the oil into the air, which doesn’t create the same concentration or dispersion. Even if you feel like it smells strong, the actual repellent effect indoors is negligible.

Beyond being ineffective, citronella oil can irritate your lungs and eyes when aerosolized in an enclosed space. Diffusers are designed for lighter essential oils like lavender or eucalyptus. Citronella is thick and viscous — it’ll gum up your diffuser, void the warranty, and leave a greasy film on your furniture.

If you want indoor mosquito control, use a fan (mosquitoes are weak fliers), a proper mosquito trap, or a plug-in repellent like those with metofluthrin. For comfort, lavender or peppermint oil in a diffuser will smell nicer and won’t mess up your machine.

Save the citronella for the patio candle.