The purple coneflower (Echinacea purpurea) is native to the woodlands of the central and eastern United States. Its 2- to 4-inch flowers are actually clusters of small golden flowers surrounded by purple or pink petals. This perennial grows up to 4 feet tall and thrives in United States Department of Agriculture (USDA) Hardiness Zones 3 through 9. Purple coneflowers are susceptible to damage from a variety of pests.
The tiny, soft-bodied aphid sucks plant sap through its long feeding tube. Purple coneflowers are susceptible to attack by the brown ambrosia aphid, the melon aphid, and the green peach aphid. After ingesting sap from the leaves and stems, aphids excrete a sticky, clear substance known as honeydew. Sooty mold grows in honeydew and produces a black discoloration in leaves and stems. Heavy infestations cause wilting and dieback. Effective insecticides include acethate and imidacloprid. Less toxic controls include insecticidal soap, horticultural oil and pyrethrums.
The aster leafhopper carries the aster yellows disease, which is a serious infection in purple coneflowers. Also known as the six-spotted leafhopper, this light green insect pest has six black spots on its head. It measures 1/8 inch long and has a life cycle of 40 days. Symptoms of aster yellows include green flower petals, stunted growth and plant death. To prevent this disease, which overwinters in plants, remove and destroy plants infested with aster leafhoppers.
The microscopic eriophyid mite is a common insect pest in purple coneflowers. This insect pest feeds on nutrients inside flower buds. Infestation symptoms include stunted flowers, as well as green growths emerging from the sides of the flowers. The University of Illinois Extension notes that symptoms of eriophyid mite infestation resemble symptoms of aster yellows disease. Cut back infested plants, and remove and destroy infested plant material.
If the leaves on your purple coneflower have a lacy appearance, the culprit may be the Japanese beetle. An adult measures 1/2 inch long and has copper brown wing covers and green legs. White tufts surround its body. Japanese beetles eat the leaf areas between the veins, creating the lacy leaf appearance. This insect is difficult to control. Large numbers of traps, and weekly applications of carbaryl, malathion or cyfluthrin control adult beetles. Milky spore powder produces the milky spore disease, which is fatal to the larvae; you can purchase the powder at a garden supply center. Carbaryl, trichlorfon and imidacloprid are also effective against larvae.
Sweet Potato Whitefly
The sweet potato whitefly produces honeydew after it feeds on the undersides of coneflower leaves. Also known as the silverleaf whitefly, this pest measures only 1/32 of an inch long and white wings fold into a peak over its back. Infestation symptoms include yellowed leaves, and black sooty mold. According to the Texas A&M University Extension, the best control of sweet potato whitefly is prevention. Remove and destroy infested plants. Natural predators include the common green lacewing and lady beetles. Insect growth regulators, such as azadirachtin, kinoprene and fenoxycarb, are effective controls.