There are many species of aphids, varying in size, color and habitat. As individuals, they do little harm to a host plant, but large infestations can produce severe damage. While their behavior is determined largely by food preference and feeding site, it can generally be said that anytime you observe large groups of very tiny insects hanging onto a plant, they are most likely aphids. Most are seen on the leaves, stems, and foliage of plants, especially on the new growth, but there are species that feed underground on roots and bulbs. Many produce galls or other deformities. Fortunately, there are many effective control methods available.
Aphids live in large colonies and reproduce rapidly, having numerous generations each season. Life cycles vary with species but, generally, only female aphids are present during the summer and can give birth to living young without mating. This wingless form of aphid is known as the stem mother. After one or two generations, winged forms are born and fly off to other plants. Males are produced in the fall, at which time mating does occur and females lay eggs that overwinter in bark or ground litter. Larvae will emerge at the end of May or early June in the Chicago region. On average, an aphid lives for about one month and each female produces 80-100 offspring.
An aphid feeds on its host plant by sucking plant sap through a beaklike feeding tube inserted into plant tissue, thereby weakening stems and leaves. Check the undersides of leaves for small groups of aphids, or you may find them clustered on the new buds, stems, and young leaves of flowers.
The ‘honeydew’ produced by the aphid is a sticky substance and can provide an environment for a black, sooty mold, which not only blocks light from the plant tissue, but also attracts ants. These ants, in turn, may kill any aphid predators. Ants will often bring soildwelling aphids to plants by carrying them through their tunnels to plant roots, and will nurse aphid eggs through the winter. Infested plants often show distorted growth that may be curled, puckered or otherwise stunted. Leaves may show pale or yellow spots or entire leaves can turn yellow or brown. Flower buds and flowers may also become shriveled or reduced in size. Twig growth may be twisted. Under bright sunlight, affected plants will often wilt. Seedlings and tender ornamentals can be severely damaged by an infestation of aphids.
Certain species of aphids live in the soil and attack roots, bulbs and corms. The only way to verify their existence is to examine the roots of plants for knots caused by these aphids. Besides the knotted roots, plants infested with root aphids show the same symptoms as those attacked by the above ground species: stunted and wilted growth, with curled, yellowed foliage. Root aphids damage bulbs by sucking out nutrients and causing wounds through which fungal and bacterial decay organisms can enter. Badly infested bulbs will produce weak and stunted growth and yellowish leaves with brown tips. Those flowers that do mature are usually small, streaked and off-color. If a plant stops growing, it may be a sign of root aphid infestation.
An aphid problem may often be a symptom of too much nitrogen fertilizer or an overuse of pesticides that have eliminated natural predators and parasites. Changing to organic, slowrelease fertilizers and non-toxic pesticides, and encouraging the natural aphid predators listed below, are important steps in reducing aphid populations.
Aphids are attracted to the color yellow. Fill a yellow container with slightly soapy water, or place a yellow board covered with a commercially- formulated “goo” near susceptible plants to serve as traps a week or two before aphids are expected to appear, usually in early May through June. Certain plants, such as nasturtiums, petunias, garlic, coriander, anise, chives and other alliums can act as ‘target crops’, attracting aphids away from more vulnerable plants.
Aphids deposit eggs in leaf litter and twigs, so good garden sanitation in the fall and a thorough clean-up of flowerbeds in spring will help to eliminate sites where eggs may overwinter. Make sure air can circulate around vulnerable plants; stagnant air creates a more attractive environment for infestations.
Another good practice is to encourage winter songbirds to visit your garden because many birds will search tree bark for overwintering aphid eggs. Good varieties are chickadees, nuthatches, purple finches, and warblers. A thorough control of aphids means controlling the ant population as well. Establishing barriers of bone meal or powdered charcoal is an effective method, and cleaning up leaking tree sap will detract ants.
By the time you notice an aphid problem, they are usually too numerous to effectively control by hand removal. Light infestations can be controlled by washing the plants with a forceful jet of water, usually early in the morning, paying particular attention to the undersides of leaves. For best results, spray every other day for a minimum of at least three times to effectively decrease the population.
Fortunately, aphids have many natural enemies that are very effective in controlling infestations. Chief among these is lady beetles, with both adults and larvae usually increasing in number to thoroughly control the damaging population. Other major predators include lacewings, small parasitic wasps, syrphid flies (hover flies), and soldier bugs. Avoid the use of any chemical sprays while predators are present.
Insecticidal soap, such as Safers, is an effective control method and is less environmentally toxic than other chemical sprays. Apply thoroughly to the plant every 3-5 days over a twoweek period to eliminate the infestation.
Foliar insecticides may be applied if aphids become numerous and continue to be a serious problem. Spray foliage thoroughly and repeat as needed. Check for the presence of lady beetles and other natural predators before spraying any insecticides.
Refer to the Illinois Urban Pest Management Handbook (University of Illinois Extension Service) for a complete listing of chemical recommendations. Use pesticides safely and wisely; read and follow label directions. The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used.
The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.