Aphids EL 60
G.B. MacCollom, Former Extension Entomologist, Plant and Soil Science
Aphids, or plant lice, attack all sorts of annuals and perennials in the flower garden and are present from early spring until late fall. Some are general feeders, while others are restricted to a particular kind of plant. Aphids are small insects, usually less than 1/8-inch in length and often much smaller. They have sucking beaks, and their antennae are generally long and slender. Some aphids have such long legs that they appear to be walking on stilts as they walk about on a plant.
In spring and summer, most aphids give birth to living young, so aphids are generally found in all stages of development feeding on the same plant at the same time. Considering the small size of these pests, one wonders how they can do so much harm. But when one remembers the enormous numbers feeding on a single plant, it is easy to understand how a plant must be weakened by them.
The stickiness noticed where plant lice are present is due to a sweetish substance known as honey dew, which is excreted by the insect from the posterior end of its alimentary canal. Ants are fond of this honey dew and often follow aphids about and tap them on the back in an effort to make them give off more of it. Honey dew imparts a shiny appearance to the foliage and provides a base for the growth of sooty mold.
During the summer, aphids reproduce parthenogenetically; that is, males and females do not need to mate in order that young may be produced. In the summer, every new aphid is a female and a potential mother. For example, the geranium aphid, in from 10 to 12 days after its birth, becomes mature and ready to produce young of her own. Each can give birth to 30 or 35 new aphids, and each of these is full-grown and ready to produce new aphids of her own 10 to 20 days later. The same is true of many kinds of aphids present in the garden. In the fall, the true sexual forms appear; males and females mate and overwintering eggs are laid.
Aphids also produce both winged and wingless forms. In general, it is the wingless aphids that attack garden plants. When a plant becomes over-crowded, however, aphids are developed that have frail, gauze-like wings. These winged forms fly to other plants, thus relieving the crowded condition in one place but starting a fresh infestation in another part of the garden.
One of the worst pests on roses early in the season, and a serious enemy of potatoes later, is the pink and green potato aphid1. This insect, one of the larger plant lice, feeds also on aster, coreopsis, cosmos, dahlia, gladiolus, hollyhock, iris, sweet pea, and zinnia. Its eggs overwinter on roses. The rose aphid2, another large and common plant louse, may be found on roses throughout the entire year. In spring, summer, and early fall, it feeds on the succulent leaves, stems, and buds of roses, while winter is spent in the egg stage on rose twigs and stems. This plant louse sometimes attacks other garden plants.
The green peach3, or spinach aphid, is a pest of fruit trees and of vegetable crops. It is common also in the greenhouse where it is known as the greenfly. In the flower garden, this aphid has been found on aster, calendula, crocus, dahlia, Dianthus, forget-me-not, iris, lily, nasturtium, poppy, Primula, rose, snapdragon, tulip, verbena, and violet.
Other plant lice common in flower gardens, and some of the plants on which they feed, are the melon aphid4 (which attacks aster, hollyhock, lily, morning-glory, and verbena) and the foxglove aphid5 (which feeds on beard tongue, Campanula, columbine, chrysanthemum, false dragonhead, foxglove, gladiolus, lily, Oenothera, pansy, scarlet sage, verbena, and violet). Still other aphids feed on lettuce, lilies, sweet peas, and garden peas. In fact, almost everything grown in the flower garden is subject to attack by some species of aphid.
Certain aphids feed on the roots of plants instead of on the foliage and flowers. Root aphids generally have short legs and short antennae, and, thus, are especially adapted for life in the soil. Where large numbers of these insects are present, they suck out so much sap from the roots that the plants do not receive sufficient nourishment. As a result, the foliage turns yellow and practically no new growth is produced. Root aphids attack many garden plants, including aster, Braille, calendula, primrose, and sweet pea, as well as corn and strawberries. These little aphids are often attended by ants. The ants not only look after the overwintering aphid eggs, but, in the spring and summer, sometimes actually carry the lice through the soil from one host plant to another. In fact, if a plant, especially an annual, looks sickly and ants are found on the ground nearby, it might be well to hunt for root aphids.
In the spring, it is not unusual to find plant lice on gladiolus corms that have been stored over winter, as well as on crocus and tulip bulbs. Where many aphids are feeding, so much sap is removed that the bulbs and corms are considerably weakened and perfect blossoms cannot be produced.
A strong force of water from the garden hose is helpful in removing aphids from plants with stiff stems. Most of the lice that fall to the wet soil below are unable to crawl back onto the plant. The water should strike the under surface of the leaves, because that is where aphids are likely to congregate. This treatment is not practical for soft-stemmed annuals such as nasturiums.
Aphids can be readily killed by spraying or dusting with a recommended insecticide. As different brands vary in strength, the manufacturer's directions should be followed.
Root aphids are hard to control because they are underground pests.
When considering pesticides, consult your local lawn and garden dealer for details.
Edited in January 1997, based on material developed in 1985.
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