Gardeners' lore has it that tomato plants can be protected from whiteflies, their most commercially damaging pest, by seeding marigolds among them. The whiteflies don't just avoid eating the marigolds, they also stay away from nearby plants. Now, scientists have not only confirmed the truth of this belief, but they've found the explanation for why this happens and proposed harnessing it to control major pests without damaging the environment.

"We spoke to many gardeners who knew marigolds were effective in protecting tomatoes against whiteflies, but it has never been tested scientifically," University of Newcastle (UK) PhD student Niall Conboy said in a statement. 

Conboy reports in PLOS One that marigolds' weapon is limonene, a chemical that slows down and confuses whiteflies without killing them. In large-scale greenhouse trials, Conboy found marigolds release so much limonene that the flies will avoid anything in a small radius around the flower, allowing them to act as protectors for the tomato plants the whiteflies love to feed on. Many other plants produce limonene, which is a major component of citrus peel, but most don't release enough of it to protect anyone other than themselves.

Commercial tomato growers could take a leaf out of amateurs' book and plant marigold rows between the tomato plants, Conboy concluded, or have pods that waft limonene over their plants. The latter was more effective for emergency treatment when whitefly infestation was well advanced. Moreover, Conboy demonstrated that the marigolds are effective even in the presence of other companion species, such as basil.

Because limonene doesn't kill the pests it affects, resistance should build up more slowly than for lethal pesticides and there should be little damage, if any, to beneficial species. In particular, Conboy found that bees aren't harmed by limonene exposure as they feast on the marigolds' nectar.

Whiteflies feed on the sap of many plants, including other important crops. The products they secrete interfere with the productiveness of leaves and make the fruit hard to sell. Moreover, they act as a vector for plant viruses and increase vulnerability to mold. Given the threat whiteflies pose to other major crops, marigolds may be of interest to non-tomato horticulturalists.

For the research to really be world-changing, however, it would probably need to be useful against a wider array of pests. We know limonene deters mosquitoes (even if not as well as we would like), and now we know it deters whiteflies. Since mosquitoes and whiteflies are not all that closely related, there are likely to be other species of insects that are also affected by it. Indeed, different marigolds have been found to control aphids and some pestilent moths. Nevertheless, this is no universal cure. Onion thrips, the only other pest found in any numbers in the greenhouses Conboy studied, were apparently unaffected by the marigolds.