Aphids are found throughout the United States. These small, soft-bodied insects may be pale green, pink, black, or yellow, depending on the species. Some stages of the life cycle are winged, others wingless. Aphids feed on a wide variety of plants, including most edible and ornamental plants. Clustering on tips of new growth and leaf undersides, they suck plant juices causing leaves to become distorted and yellow.
Aphids secrete a sugary fluid called honeydew that attracts ants and may cause the growth of a sooty black fungus on leaves. In small numbers aphids do little damage, but they reproduce rapidly. They can also spread diseases among plants.
Start by rinsing plants with a strong spray of water to reduce the population. If aphids return, spray with insecticidal soap or horticultural oil. When spraying, be sure to cover the undersides of leaves. A variety of natural insect parasites and predators also reduce aphid populations, which is the main reason to not be overly aggressive with sprays.
Another organic approach to dealing with aphids is by attracting their predators such as ladybugs, lacewings, or one of the best, the hover or syrphid fly. Adult hover flies resemble wasps and yellow jackets, but there's no need for you to be alarmed; they can't sting and are perfectly harmless. The wasplike adults feed primarily on flower nectar, but during their week-long stint as crawling larvae, each juvenile can consume hundreds of aphids.
Researchers at Oregon State University tested many plants to determine which attract hover flies. The winner was sweet alyssum (Lobularia maritima), which is easy to grow from seed or nursery plants, and prefers the cool temperatures of spring and fall. Runners-up include cilantro (Coriandrum sativum), buckwheat (Polygonum fagopyrum), common fennel (Foeniculum vulgare), and yarrow (Achillea).
For best results, keep the plants in flower with successive plantings and regular deadheading.