APHIDES.—Plant-lice.
These are the most extraordinary insects, being found upon almost all parts of plants, and there is scarcely a species which does not support one or more kinds peculiar to itself. Then they are so exceedingly prolific! Reaumur proved that one individual, in five generations, may become the progenitor of nearly six thousand millions of descendants. Most of these insects, which we find so abundant upon our trees, are wingless females. Winged insects, both male and female, appear later in the season, and after laying their eggs, they soon perish. Some lay in the fall, others wait till spring. When these eggs hatch, the brood consists wholly of females, which are wingless, and do not lay eggs, but are viviparous and produce from fifteen to twenty young lice in the course of a day. This second generation are also wingless, and at maturity produce their young, and so on to the seventh generation, without the approach of a single male, until the autumn, when a brood of males and females appears, which are both winged at maturity, and then the eggs are laid for the next year's brood, and the parents die.[27]
The injuries occasioned by plant-lice, are much greater than would at first be expected, from an observation of the small size and extreme weakness of the insects; but these make up by their numbers what they lack in strength individually, and thus become formidable enemies to vegetation. By their punctures and the quantity of sap they draw from the leaves, the functions of these important organs are deranged, or interrupted, the sap is withdrawn or contaminated, and unfitted to supply the wants of vegetation. Plants are differently affected; some wither and cease to grow, their leaves and stems become sickly, and die from exhaustion. Others, not killed, are greatly impeded in their growth; the tender parts, which are attacked, become stunted and curled. The punctures of the lice appear to poison some plants, producing warts or swellings, which are sometimes solid, sometimes hollow, containing within them a swarm of lice, descendants of a single individual.[28] These last are often seen upon the leaves of the Elm, and upon some Poplars, and other trees; but I have not found any upon the foliage of our cultivated fruits, unless it be those on the grape.
Aphis mali, or the Apple Leaf-louse, is a small, green insect without wings, accompanied by a few black and green ones having wings. These are all crowded together upon the green tips of twigs, and under the leaves, sucking the sap. The eggs remain in deep cracks of the bark during the winter, and hatch as soon as the buds expand in the spring. The most successful treatment is to scrape off the loose bark, and to apply to the stems of the trees alkaline or lime washes. Many of our familiar little winter birds consume these eggs. In the spring and summer, alkaline solutions may be used with advantage, syringed or sprinkled upon the affected shoots and foliage.
The smell of these insects is peculiar, which, indeed, is generally characteristic with bugs. Each sort seems to derive a special flavor from the tree or plant upon which it feeds. Most insects of this family secrete copiously a sweetish fluid, called the honey dew, which is ejected from two little horns or nectaries, that project, one on each side of their bodies. This sweet material attracts a great many flies, and other insects, particularly ants, which are the constant attendants of these creatures, and are said to protect them from their enemies in order to obtain their sweet secretion. Some entomologists have called Aphides the Ants' cows.
No one, who is acquainted with the Aphides, and the various insects which prey upon them, will ever permit a valuable plant to suffer injury from these pests. He will collect some of the Aphis' enemies alive, carry them to the affected plant, and set them free to do their work; there they will remain while the food lasts. The Aphides have more numerous, more active, and more inveterate enemies than insects of any other group—these are the means by which their wonderful fecundity is kept in check. Among them are the Aphis-lions, which are the larvæ of the Golden-eyed and Lace-wing flies, belonging to the order Neuroptera. They are reddish-brown, with a dark stripe down the middle, and a cream-colored one on each side; bodies long, narrow, and wrinkled transversely. Their jaws are long, curved like sickles, projecting forward from their heads horizontally.[29]
The Coccinellidæ, mentioned as useful members of the order Coleoptera, on a previous page, are among the most active enemies of the Aphides. The eggs are laid in clusters of twenty to forty on the under side of a leaf, to which they are closely glued; they are oval, and light yellow. They hatch into small blackish larvæ, which are active, and which boldly attack an Aphis much larger than themselves, leaving only the empty skin. They consume hundreds while in the larval state, about two weeks, when they attach themselves by the tail, and go into the pupa state. One of the largest of these Lady-birds is the Mysia 15-punctata; the larva is a clear white, the middle of the back tinged with red, and two or three black spots on each segment—nearly a hundred species of Lady-birds are found in this country. The perfect insect, as well as the larvæ, feed upon Aphides, and instead of being destroyed, they should be cherished and encouraged.
Besides these, there are other inveterate enemies of the plant-lice in the Syrphidæ, which are two-winged flies, resembling the common house-fly, but handsomer. They deposit their eggs where Aphides exist; the maggot, which hatches from these, seizes upon the first Aphis that comes within his reach, and sucks its fluids. A medium-sized worm will consume a hundred lice in an hour. They are always found in a colony of Aphides.[30]
Aphis prunifoliæ, or the Plum Leaf-louse, is black, with pale green abdomen. It is found on the under side of the leaves, which become wrinkled and distorted. It is not so abundant as some other species, but its habits are similar.[31]
Aphis cerasi, (Fabric.), or the Cherry Plant-louse, is very common, very numerous, and very black. They appear with the first expansion of the leaves, and continue or are renewed when destroyed, and remain until mid-summer, when they generally disappear. Their numbers are almost incredible, and they give a young cherry tree a wretched appearance. On the under surface of a small leaf, three-fourths of an inch long, Mr. Fitch counted one hundred and ninety lice, on one side only of the midrib. Their natural enemies come to the rescue to check their wonderful increase, and sometimes will utterly rout the Aphides in a single week.[32]