The next generation of pesticides might smell as sweet as rosemary, cinnamon and thyme.
These spices, among others, are proving to be potent insect-fighters that are gentle on the environment and safe for humans, said entomologist Murray Isman, of the University of British Columbia, Vancouver.
The concept is not new: For centuries, people have been using oil extracts from pungent plants to protect their food. Now, scientists are finally figuring out how these essential oils get their pest-battling powers and how people might better harness them.
"There was some magic in how nature has gone about doing this," Isman said. "Our research is trying to figure out which essential oils work best for which applications."
Spice plants get their strong odors from their chemical structure: They consist of small molecules that evaporate easily. Oil extracts from these plants are commonly used to flavor foods and add fragrance to shampoos, candles and other products.
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.
Isman and colleagues tested the toxic effects of each of those top 10 by creating artificial oil extracts that contained the other nine, then applying each mixture to spider mites. The researchers were able finger three chemicals that were most potent, three or four that had small effects, and three or four that didn't seem to do much to the mites at all.
When the scientists tested a mixture of just the top three chemicals, though, the result was only 30 percent as toxic as real rosemary oil.
"There is some internal magic to that mixture of compounds that defies trying to tease them apart," he said. "There is some benefit to using the natural mixtures as they come out of the plant."
Over time, insects also appear unable to develop tolerance to the synergy involved in nature's complex combinations of chemicals, even though they become rapidly resistant to conventional pesticides.
There are already insect repellants on the market that contain rosemary, peppermint, cinnamon, thyme and other odiferous oils, but scientists still can't predict which extracts will work on which insects. Even closely related insects respond differently to the same extracts.
To know for sure what will work best in any given situation, scientists might have to test each type of extract on each type of insect. Coats is also working with computer software that predicts which extracts will be most potent based on their chemical structures.
As experiments continue, data already show that mixing essential oils with conventional pesticides allow farmers to use half as much of the conventional stuff while still getting the same results.
For the organically-minded, the future of pest control keeps smelling better and better.