Do marigolds actually repel pests?

Yes — French marigolds (Tagetes patula) release thiophene from their roots, which deters root nematodes. Their scent repels whiteflies and aphids from nearby tomatoes and brinjal. Plant one marigold per 3–4 vegetable pots. African marigolds attract hoverflies that prey on aphids. Both varieties are widely available and low-cost across Indian nurseries.