INSECTS
many species of aphids that infest our common field and garden plants (Fig. 101) and cultivated shrubs and trees, to say nothing of those that inhabit the weeds, the wild shrubbery, and the forest trees. Almost every natural group of plants has its particular kind of aphid, and many of them are migratory species like the rosy and grain aphis of the apple. There are root-inhabiting species as well as those that live on the leaves and stems. The Phylloxera, that pest of vineyards in California and France, is a root aphid. Those cottony masses that often appear on the apple twigs in late summer mark the presence of the woolly aphis, the individuals of which exude a fleecy covering of white waxy threads from their backs. The woolly aphis is more common on the roots of apple trees, being especially a pest of nursery stock, but it migrates to both the twigs and the roots of the apple from the elm, which is the home of its winter eggs.
An underground aphid of particular interest is one that lives on the roots of corn. We have seen that all aphids are much sought after by ants because of the honey dew they excrete, a substance greatly relished and prized by the ants. It is said that some ants protect groups of aphids on twigs by building earthen sheds over them; but the corn-root aphis owes its very existence to the ants. A species of ant that makes its nests in cornfields runs tunnels from the underground chambers of the nests to the bases of nearby cornstalks. In the fall the ants gather the winter eggs of the aphids from the corn roots and take them into their nests where they are protected from freezing during the winter. Then in the spring the ants bring the eggs up from the storage cellars and place them on the roots of various early weeds. Here the stem mothers hatch and give rise to several spring generations; but, as the new corn begins to sprout, the ants transfer many of the aphids to the corn roots, where they live and multiply during the summer and, in the fall, give birth to the sexual males and females, which produce
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