What is the chemical in marigolds that repels insects?

French and African marigolds are often cited as the most powerful Tagetes species for repelling insects. Both contain chemical compounds called thiopenes in their seeds, roots and other plant parts that are offensive to nematodes, aphids, cabbage maggots, white flies and other pests. French marigolds (T. patula) and African marigolds (T. erecta) are also pretty additions to the garden.

Organic gardeners say that marigolds keep the soil free of plant-damaging nematodes, and discourage many insects in the garden. They may, however, attract spider mites and slugs. They also discourage whiteflies from attacking tomatoes when planted in a companion planting. Marigolds also deter beetles.