Cilantro and dill are both good examples of umbelliferous plants. When these plants bloom, the flower structure is in a shape called an umbel — scores of tiny flowers arranged on a more or less flat or umbrella-shaped plane. Other umbeliform plants include carrot, Ammi, parsley, parsnip, celery, and fennel. Using these plants for this purpose is known as companion planting with umbellifers.
Because of their flowers, all of these plants attract beneficial predatory insects. Because dill blooms relatively early in the year, it is particularly useful as a companion plant. Watch for ladybug larvae and adults hiding along the stems and leaves of dill. Watch for the tiny parasitoid wasps that drink the nectar of the flowers. Large, bee-sized predatory flies may be identified by their unusually large compound eyes. You may notice an abundance of territorial hoverflies moving from flower to flower in search of prey. Lacewings are also found among the umbels.
All of these insects prey on pest species, from aphids to the caterpillars of the Small White butterfly (cabbage moth). This flower form will increase biodiversity in the garden and provide the balance needed to prevent infestations.
Like many other aspects of organic growing, companion planting exploits naturally occurring phenomena to increase productivity. This strategy enables gardeners to bypass the use of pesticides, and minimize the need for preventative measures like row cover. A simple planting of dill or Ammi near your vegetable bed can naturally control pest insects.