Having thus summed up the damage done by insects, let us see what may be done to prevent their spread and if possible drive out the most harmful species entirely. Unfortunately, that seems almost impossible; so far all man's efforts have only resulted in saving a larger or smaller proportion of the various crops each year. In insect control we turn first to the natural means of destruction. Chief among these means are birds,--of which we will speak in another chapter,--snakes and toads. Toads live entirely on insects and catch large quantities of them. It is estimated that a single toad is worth almost twenty dollars a year in a field or garden. English gardeners are said to pay high prices for them and to keep as many as possible in their gardens. Toads will eat almost any kind of insect, are absolutely harmless, and should be carefully protected. There is one class of insects which, so far from being an enemy to man, combines with him to kill the harmful insects. Among these are the black beetles which feed on cutworms and other larvae which injure the roots of plants. Lady-bird beetles destroy large numbers of plant-lice, and the Asiatic lady-bird has been found to be the natural destroyer of the San Jose scale. These little insects are now being hatched in this country, and it is hoped through them to stamp out the pest. A number of larger insects prey on the smaller ones. Other insects, such as the Hessian fly, the green-bug or spring grain aphis, the army-worm and various species of grasshoppers are killed by tiny parasitic insects whose eggs are laid in the bodies of the larger insects, but which, after being hatched, feed on them. To these natural methods of control man has added others. Cultivation is one of these methods. As insects flourish when given an unusually large amount of food of a particular kind, and starve when that food is taken away from them, so rotation of crops proves to be one of the best means of getting rid of those insects which can not travel far for their food. Farmers who practise rotation of crops are much less troubled with insects that injure the roots of plants than those who do not. One of the best means of preventing damage from the Hessian fly is to sow a narrow strip of wheat all around the edges of the field several weeks before the main crop is to be sowed. The flies will gather in this strip and lay all their eggs in the early wheat. Just before the main crop is sowed, the narrow strip is plowed up and thoroughly harrowed and the larvae perish for want of food. The best known means of getting rid of grasshoppers is to destroy the eggs. This should be done by plowing and harrowing all roadsides, ditch banks, uncultivated fields and grassy margins around fields in the fall or winter. Fall harrowing and deep spring plowing will prevent many of the bugs and beetles which spend the larval state in the ground from hatching. This method will also destroy the plum-curculio in orchards.