Insect identification > Homoptera > Aphids

Aphids


Family Aphididae (aphids or plant lice). - This is one of the most important groups of insects from an economic standpoint, as all its members are injurious, often very abundant, and a species usually doing little harm may at any time become a serious pest.

Aphids are tiny, soft-bodied insects, the largest being less than a third of an inch long, generally with long legs and antennae, and are of various colors, green, black, various shades of red and brown, white and gray being the most usual ones. Some are more or less completely concealed beneath long, white waxy threads, giving them a "woolly" appearance; others have a sort of dust or "bloom," like that on a plum, coating their bodies; but the majority are without any covering. Many species of aphids have a pair of tubes, called cornicles, projecting upward from the top of the abdomen. These were formerly believed to be the exit ducts through which honeydew, abundantly produced by the insects, escapes, but it is now known that this substance is expelled through the anus, often in such quantities that when the insects are abundant it forms a sort of fine rain which can be heard falling on the leaves and ground. This fluid, which is sweet and sticky, is eagerly fed upon by ants.

Falling on twigs and leaves it dries there and a fungus grows in it turning it black, and plants where aphids have been abundant often show this by their black appearance. Some aphids produce galls within which they live for at least a part of their lives, but most of them are not thus enclosed, living on leaves, twigs, succulent plant stems or roots.

Though there are great variations in the life histories of different aphids, certain general facts hold for most of the group. In most cases eggs are laid in the fall, on a food plant of the species concerned, and these hatch the following spring. The nymphs soon become full-grown and are known as "stem mothers" and without fertilization (there are no males in the spring generations) produce eggs, or in most cases living young which like the stem mother are all females and on reaching maturity produce young in a similar way. The production of young without fertilization of the parent is not uncommon in insects and is called parthenogenesis or agamic reproduction. In this case the production of these young alive rather than from deposited eggs introduces the additional fact that these insects are also ovoviviparous except in (generally) one generation.

The number of young produced by each parent varies but will perhaps average about 10, a few being born every few days, and the number of generations is variable but is also likely to be about 10, though the firstborn young in each generation, being a week or two older than the last-born young, will gain enough time during the season to produce more generations than the others. In fact, in some species a range from 8 to 21 generations for late- and early-born individuals has been observed, and an average number of 28 young produced per parent, so that the figures given above may be regarded as conservative.

But even with this moderate estimate, allowing only 10 young to a generation and 10 generations a season, the total product from a single egg hatching in the spring, and itself counted as the first generation, would be 1,111,­111,111, and this would be far below the actual number in most cases, were it not for the enormous destruction of these insects by their enemies and by unfavorable weather conditions.

In many species instead of 10 young being produced per female as an average, the number is likely to be nearer a hundred, and in those species which also have more than 10 generations the total number of individuals which would theoretically be produced in a season "would be sufficient to completely cover the entire world with a continuous layer of plant lice."

With such a marvelous reproductive power as this it becomes evident that, despite natural checks to their increase, plants infested are liable after a few weeks to be entirely unable to provide food for the hordes of aphids upon them. Accordingly we find that in most of the generations winged individuals may be produced so that they can migrate to other plants. Winged and wingless forms may therefore be found at almost any time during the summer, and a wide distribution of the insect is obtained in this way.

When cold weather approaches in the fall, a generation appears consisting of both sexes, and the females of this generation lay fertilized eggs which winter over and hatch the following spring. In some cases this does not happen until the second fall and, in a few species at least, sexual individuals have not been discovered and may occur only at long intervals, if at all. This is particularly true of species found in green­houses, where exposure to winter conditions does not exist.

Many aphids do not feed entirely on one kind of plant but spend a part of the year on one species, and the rest on another. One of the species which is injurious to the apple remains on this tree from fall until May or June when it migrates to grain and spends the summer months there. Another species, living on the elm during the fall, winter and spring, passes to the apple for its summer residence, and a long list of aphids having alternating food plants is now known.

Aphids suck the sap from plants and often produce curling or malformation and even wilting of the leaves, frequently accompanied by discoloration. Root-attacking forms produce knots and deformities affecting the health of the plant, and young fruit becomes hard at the attacked spots and remains small. The punctures aphids make often enable the spores of fungi and bacteria which cause plant diseases to enter the plants, and aphids may even transfer these from one plant to another.

Among the diseases transferred thus are an oat blight, fire blight of the pear and cucurbit wilt. Indirectly by the honeydew in which spores can live for several days, it is probable that the diseases can also be widely distributed through the agency of other insects which visit and feed on honeydew. In general a year when aphids are abundant over a large part of the country is certain to result in great injury to plants of all kinds affected by these insects.

Ants not only gather the honeydew the aphids produce, but in some cases the relation is closer, particularly with root-feeding species. Aphids have many enemies which destroy great numbers of them. They are also affected by the weather, cloudy, wet periods being favorable, though driving rains destroy many.

Some aphids attack evergreens and produce rather soft, fleshy galls, generally at the bases of the outer shoots. These appear during the spring months and are of full size by midsummer. They then dry and crack open, showing little cavities occupied by the plant lice, which now leave the galls for other parts, either of the same or of some other kind of tree, according to the species concerned.

The gall formation interferes with the growth of the tree by preventing wholly or in part the circulation of the sap in the shoot at the base of which the gall is located, and this results, by the death or checking of the growth, in trees which look thin rather than dense, and in some cases they may become worthless as lawn ornaments. In the East the spruce is often seriously injured in this way.

Many kinds of aphids often become seriously abundant for periods of 2 or 3 years, then disappear for a time. The potato aphid, the beet root aphid, the cabbage aphid, cherry aphids and others are often destructive for a year or two at a time and outbreaks of these or others may be expected any year.