Very interesting. I read on the ThermaCell site that "repellent is a synthetic copy of a substance found in chrysanthemum flowers". That rang a bell, as IIRC permethrin (which is a synthetic) shares similar chemistry to compounds in chrysanthemums. Turns out that is the case after all.

So it seems like the ThermaCell folks have developed a way to use permethrin for treating a volume of space, instead of only a fabric surface. Glad to see it.