Diluting citronella makes it weaker.

Yes, citronella loses effectiveness when diluted, especially with water.

Citronella works by releasing volatile compounds that mask scents insects find attractive. Water dilutes those compounds and also doesn’t help them stick to skin or surfaces. An oil-based dilution retains more of the repellent power because the carrier oil helps the citronella evaporate more slowly and stay put. But even with oil, dilution means you’re applying less active ingredient per square inch, so it won’t work as well or as long.

If you’re making your own repellent for skin safety, you basically have to dilute it — pure citronella can irritate. Just know that a 5% dilution (common in DIY recipes) is nowhere near as effective as commercial sprays that use higher concentrations (10–15%) or synthetic repellents.

Bottom line: you can use diluted citronella for a pleasant scent, but don’t count on it for real mosquito protection.