Coneflowers such as the black-eyed Susan are often placed in home gardens because they thrive under adverse conditions. These perennial plants attract wildlife such as butterflies and birds, and are relatively deer resistant. However, coneflowers are susceptible to aphid infestations which can affect their health and vigor.
Aphids are small, soft-bodied pests that are usually oval in shape. These insects come in a variety of bright colors such as red, orange, yellow and green. Many species of aphids are white, black or brown. Many plants and flowers are hosts to this pest, including coneflowers, pansies, snapdragon, red salvia, flowering maple, Shasta daisy, rose mallow and zinnia. Certain plants are extremely susceptible to aphid feeding and suffer serious damage from heavy infestations of this pest.
Aphids feed on coneflower leaves with their piercing mouthparts, that they use to extract sap from the plant. Feeding causes distortion of plant leaves, especially on young tender shoots and stippling of leaves. Coneflowers with aphid infestations may also develop leaf puckering. Aphids excrete a liquid substance known as honeydew after ingesting plant sap. Honeydew is very sticky and causes fungi to stick to the coneflower. This condition is known as sooty mold, which is characterized by the black, velvety fungus on host plants. Although sooty mold is not usually harmful to coneflowers, it can cause them to become unsightly and have reduced vigor.
Aphids are often kept under control by predatory insects that feed on them. Green lacewings, lady beetles, syrphid flies and parasitic wasps, all feed on aphids. Avoid using chemical control products if natural predatory insects are present on your coneflowers. Spraying your coneflowers with water several times each week can keep insects such as aphids under control by knocking them from the plant. This process is known as syringing and is most effective when aphid populations are low.
There are several types of chemical control methods available at your local garden center designed to work on aphid infestations. Horticultural oils work by coating the insect with oil, causing it to suffocate. Insecticidal sprays are also effective in eliminating aphids, but be certain to apply these contact chemicals to your coneflowers thoroughly for best results.