[dice]

[essential oils]

Handle with care. Many essential oils are "sensitizing" (cause
a local allergic reaction) to the skin, and they can burn the heck
out of you in their undiluted form (cinnamon oil and oregano oil
come immediately to mind in that context; get a drop of cinnamon
oil on you and it feels like someone touched you with a red-hot poker).
They are not water soluble, so they need to be mixed with some kind
of solvent or surfactant to mix up well. Castille soap and Murphy's
Oil Soap both work to let the oil disperse evenly in solution so
that you can spray it. Cheap liquid dish soap probably works,
too, but what all is in it? Castille and Murphy's are both
vegetable-derived soaps.

One formula for neem oil, for example, is 1 tablespoon of raw
Neem Oil and 1 tablespoon of Murphy's in a gallon of water,
mix well, spray on for bug repellent. Neem oil needs to be
at 80F/27C temperature for the fats in it to stay liquid, by
the way. It is not distilled like an essential oil, it is simply
pressed out of neem seeds. Unlike many essential oils, neem
oil will not burn on contact with your skin (in fact it is a pretty
good skin conditioner).

Edit:
Completely off-topic, but Vetiver is one of the best essential oils
around simply for the scent. It is expensive as essential oils go,
and its anti-bacterial properties cited in one study are a fiction,
but as an aroma it is wonderful. Not sensitizing to the skin
(does not burn).