The chemistry behind marigolds’ pest-control power

Glasshouse whiteflies can decimate a tomato crop. The greenhouse pests transmit viruses and snack on the plants’ leaves, resulting in secretions that hinder photosynthesis. It’s lore among gardeners that planting marigolds alongside tomatoes protects the plants from glasshouse whiteflies, but until now, no one knew how the flowers worked their magic. In search of the answer, Niall J. A. Conboy and researchers at Newcastle University studied the volatile compounds given off by French marigolds. Among those is (+)-limonene, a terpene that the pests find repugnant (PLOS One 2019, DOI: 10.1371/journal.pone.0213071). The researchers placed air-freshener-like dispensers loaded with (+)-limonene among tomato plants and found that these deterred glasshouse whiteflies more effectively than marigolds. Inexpensive (+)-limonene is a major component of citrus peels, so it could be a green option for pest control. “Most pesticides are sprayed onto the crops,” Conboy explains. “This doesn’t only kill the pest that is targeted, it kills absolutely everything, including the natural enemies of the pest.” Next, the team plans to explore plants that deter two other tomato pests, spider mites and thrips.