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. Their behavior is determined largely by food preference and feeding site. 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. Some produce galls or other deformities. Fortunately, there are many effective management methods available.
A soft-bodied, pear-shaped insect, the aphid is generally very small, with most species under 1/8 inch in length. Aphids can be almost any color, but green, black, white, and gray are the most common. Some are covered with a threadlike white material, which makes them appear woolly, while others may be covered in a fine dust. There are both winged and wingless individuals of most species. Aphids can be identified by a beak or rostrum that sits far back on the underside of the head. Their antennae are rather long and placed in the front of the head, between the eyes. The feet are two-jointed and terminate in claws. Many aphids have a pair of projections called cornicles on each side of their posterior through which they emit a sticky substance commonly referred to as ‘honeydew’.
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 blocks light from the plant tissue. The honeydew also attracts ants. These ants, in turn, take care of the aphids, even killing aphid predators. 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, slow release 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.
Aphids generally congregate in groups near the tips of stems. Cutting off the tip of the stem can remove an entire colony at once. 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.
Contact the Plant Clinic (630-719-2424 or firstname.lastname@example.org) for current 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.