Their chemical structure also allows spice extracts to easily penetrate cell membranes in places like the brains of bugs. In recent years, scientists have identified two types of messenger molecules in insect brains that essential oils tamper with.
By blocking or over-stimulating these molecules or their receptors, essential oils cause insects to become hyper or fly away. At high enough doses, Isman said, some oils can even fry an insect's nervous system.
People, pets and other animals don't have the same messenger molecules in their brains, making extracts that harm insects safe for us. Scientists are now eyeing these brain processes as targets for synthetic pesticides.
"Some of these essential oils have really outstanding repellency," said Joel Coats, an entomologist and toxicologist at Iowa State University in Ames. "If we can understand how they work at a biochemical level, we may be able to use them more readily, and that's a really important step right now."
Isman has spent a lot of time dissecting the mysteries of rosemary, which is a particularly powerful neurotoxin for many insects. Previous work has shown that rosemary oil contains some 80 or 90 chemicals, 10 of which make up more than 90 percent of what's in there.