It's just an essential oil I have here, it's an old bottle from Elizabeth Van Buren. Any good geranium oil should do though. Can't hurt to try it! I tried using DEET myself one year and it was awful plus which it didn't work all that well. It probably also depends on what type mosquitoes you have as I know there are several biting kinds that each have different "preferences" as to what odors they are drawn to and what repelled by.... I did a (very little) bit of research into this once in hopes of finding a better way than just to give up! I didn't find it, but I also never tried pure geranium oil before this.
If you do try it, you do not need much, really, just a drop on your limbs. It doesn't have to cover at all as it is the scent the repels them.
