In an effort to discourage insects I have tried burning citronella candles but the quality of wax is variable. Some expensive ones still smoke even when the wick is trimmed every hour or so. Googling to find an alternative, there has been academic research on the subject and this is the basic result:
- reed diffusers are many times more effective than candles
- the smell which best deters insects is not citronella but rose geranium (that is, a variant of geranium, not roses, although there is some resemblance in the perfume).
I do not want to spend a fortune on diffusers but I'm willing to punt a few pounds on avoiding mosquito bites. I therefore obtained the following
- A packet of 8 reeds which were down to 50p
- a jam jar with a lid
- Odd oil which was no longer suitable for cooking - a lurking bottle of almond oil and a lonely splash of corn oil. Which ever it is, the oil should not have a strong smell if its own. Baby oil is OK, though.
- surgical spirit or other solvent such as vodka.
- a bottle of geranium oil and one of citronella as I do not like geranium on its own.
Together the perfume oils should cost about £12 and you may be able to find them much cheaper. Any health store will have them.
Mix together the geranium oil, the citronella oil and the cooking oil. Exact quantities do not matter but two small bottles of essential oil will strongly scent at least half a pint of base oil.
Punch 8 holes in the lid of the jar (I used a pointy thing from the screwdriver case and hit it with a hammer) such that the reeds can be inserted. You could put them straight in the jar but if it fell over the gunk would go everywhere.
Put half an inch of surgical spirit in the bottom of the jar, then top up with as much again of the scented oil. The oil does not dissolve perfectly in the spirit but they stir together well enough to soak up the reeds. Some people use vodka instead of surgical spirit which would work but is not as cheap. On the other hand, at least it doesn't smell of surgical spirit.
Put the lid on the jar, insert the reeds, give it a gentle swirl to encourage the oil and spirit to mix up again, and wait. Reverse the reeds after a few hours to start the diffusion properly. You will know it is working because the place will smell like hospital flowers but it is not a bad smell - just peculiar. You would not want to dab it behind your ears. Indeed, you must label the rest of the oil carefully to make sure it does not get confused with cooking oil.
An inch of the mixture in the jar will last at least a month which is bad news if you hate the smell but I can confirm that it has dissuaded all but the most determined bluebottle from coming in. There is enough oil to top up the diffuser for about a year, meaning the cost of a home-made insect repellent is a little over £1 per shot maximum compared to £10 commercially. Give the jar a swirl every now and then to re-mix the solvent and oil.
After about two days the stronger smells go and the remaining level is not noticeable to humans - just a lemony background note - but the insects seem to stay away.
It is worth sticking the jar inside a prettier container - say, a plant pot or jug - because otherwise it looks like you've gone all Howard Hughes and are displaying your own urine samples.