Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 421-A   May 29, 1971
Forest Preserve District of Cook County
George W. Dunne, President
Roland F Eisenbeis, Supt. of Conservation

Aphids, or Plant Lice, are tiny defenseless insects that have soft bodies but needle-like beaks with which they puncture plants and suck the sap. They weaken or even kill many plants and also may infect them with virus, bacterial or fungus diseases. Aphids, unless controlled, multiply enormously and cause serious damage in orchards, vineyards, truck farms, gardens greenhouses, and field crops such as corn, cotton, small grains, clover and alfalfa.

There are hundreds of species of aphids distributed over the world and there is scarcely a kind of plant, wild or cultivated, that is not infested by one or more kinds of plant lice. Some feed on stems and leaves, some on the roots, and some on both. Others feed on buds, and a few -- like the Hickory Aphid which infests hickory, maple and other forest trees -- feed on bark underneath the limbs. The hickory aphid is about one-quarter of an inch long, and one of the largest, but most kinds are about the size of a pinhead: less than one-twentieth of an inch in length. Most species are green but many are pink, white, brown or blackish. The woolly aphids, which feed on apple, pear, hawthorn and elm trees, are reddish or purplish but cover themselves with a cottony white secretion of wax.

Aphids have a strange life cycle. It differs considerably according to the species but, in our northern climate, most kinds pass through winter as eggs laid on a particular kind of plant. When warm weather comes, these hatch out small nymphs which soon grow to full size but never have wings. All of them are females, called "stem mothers" because they have the remarkable ability to produce, without mating, young which are born alive and which are wingless like themselves. In a few days these, too, are producing wingless females at the rate of several per day. That goes on and on. There may be 20 or more such generations in a single year, with each female producing as many as 50 or eve 100 young.

However, during summer, some females develop wings and fly to other plants where, without mating, they produce young and start new colonies. Some species select plants like the one on which they were born which includes both males and females, all with wings. In some species these have no mouth parts and cannot eat. After mating, before they die, the last females each lay from one to four eggs which hatch next spring and start new colonies.

In addition to controlling them with sprays and dustings of nicotine compounds, DDT and some newer insecticides, aphids are often kept in check by natural enemies such as lady beetles, the larvae of "sweat flies" and lacewings, and parasitic wasps even smaller than the aphids

Among the many other interesting things about aphids is how they are used, with almost human intelligence, by ants Aphids secrete a sweet sticky fluid called "honeydew". The Corn Root Aphids, and the Strawberry Root Aphids, for instance, are nurtured by the small brown Cornfield Ant which "milks" them of their honeydew, moves them to new pastures when necessary, and stores their eggs during winter. When the young aphids hatch in spring, they are moved to the roots of grasses or smartweeds to feed until young corn or strawberries are available. The only way to control the aphids is to destroy the ants and their nests.

Aphids is dumb but ants is smart!

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