Farmers have, for ages, cultivated 'companion plants' — carrot and rosemary, corn and sunflower, marigold and tomato — together for no apparent reason.
By conducting large-scale glasshouse studies and analysing the marigold flower, researchers from Newcastle University have now identified the chemical compound in the flower that helps repel tomato whiteflies.
Limonene, the chemical found in the peels of citrus fruits, were found in the studied French marigold flowers. “This is exciting because limonene is inexpensive, it’s not harmful and it’s a lot less risky to use than pesticides, particularly when you don’t apply it to the crop and it is only a weak scent in the air,” explains Niall Conboy, Ph.D. scholar at the School of Natural and Environmental Sciences, Newcastle University in a release. He is the first author of the study published in PLOS ONE .
“Limonene has been found to have activity against a number of different insects, mites and microorganisms. It is also commonly used as part of mosquito repellents. So there is a clear potential that this could work for other tomato pests,” he adds in an email interview to The Hindu .
Glasshouse studies
In the first set of experiments, marigolds were grown with tomato plants throughout the cultivation period. The team then assessed the whitefly population and found that intermediate marigolds significantly reduced whitefly numbers.
The second experiment, termed ‘emergency measure’, tried to find out if planting marigold after whitefly infestation would help save tomatoes. They concluded that the emergency measure was less effective compared to growing marigold throughout the cultivation period.
The paper also noted that a mixture of edible and ornamental plants would also be economically viable as it reduces the use of additional chemical control measures.
“There must be more of these commonly used “folk” methods for protection of crops. We should look to test these techniques scientifically to potentially use on commercially grown crops in the future,” adds Mr. Niall.
The team has planned to use limonene in the form of dispensers (similar to hanging air fresheners) from the start of the growing season, to see if it has a more pronounced effect on whiteflies.
