That behavior — coupled with experiments in lab-grown cells that show catnip activates TRPA1 — suggests that insect TRPA1 senses catnip as an irritant. Catnip probably didn’t evolve in response to predation from ancient mosquitoes or fruit flies, he says, since plants aren’t on the insects’ main menu. Though maybe don’t plant catnip in the garden, says study coauthor Marcus Stensmyr, a neuroscientist at Lund University in Sweden.