Can't say I've seen actual studies, but citronella and catnip are supposed to be effective insect repellents. I keep a sprig of catnip inside a bottle of oil (almond, grapeseed, or another base) that also has a few drops of citronella oil in it. It seems to repel gnats pretty well, but I don't know about ticks and mosquitoes.
Subbing this thread, by the way. I'm growing an arsenal of medicinal herbs this year for the front garden. If I'm going to be motivated to maintain it, it had better have useful things growing in it! I'm growing calendula, wormwood, blue vervain, valerian, astragalus, and a few others. Hopefully having these available will motivate me to learn how to use them effectively.
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