All insects go through this larva stage. Then they do nothing but eat. Bees feed their babies with honey, so they do no harm at all, and are very useful to us. But most insects die when they have laid their eggs, and they leave their greedy babies to eat plants that men work so hard to grow. They always lay the eggs where the larva can find their favorite food, and they lay hundreds and even thousands of eggs, most of them too small for you to see.
When the eggs of butterflies, moths and flies hatch, they come out as caterpillars with six legs, hairy or smooth worm-like bodies and chewing mouths. The larva of beetles are usually footless grubs. Some of them look much like the parent insects, but are less active. They all begin to eat ravenously. Inside there is little but stomach and material for making cocoons. Leaf-eaters grow to full size in a few weeks. Orchard fruit and nut eaters stay in the fruits until they fall. The larva of some boring beetles live in the wood of trees for two or three years. They honey-comb solid trees with little tunnels.
When grown to full size the larva of all insects spin cocoons or make horny or papery cases Some roll up in leaves, using a very little silk to close the openings. They use the hairs from their own bodies, sometimes, to mix with silk, or with plant fibres. These cocoons nestle in the ridges of bark, hang from stems or leaves, or lie in the ground. Cocoons are often so near the color and texture of the thing they are fastened to, that you may look at hundreds of them and never see them at all.
There is no living plant or animal that these little creatures do not prey upon. As insects they sting, suck blood and sometimes kill the higher animals. But it is as grubs and caterpillars that they eat and injure millions of dollars worth of grains and fruits and garden crops every year.
Hessian Fly which causes great damage to wheat.
Wheat has three insect enemies—the chinch bug, the Hessian fly and the wheat midge. When ground into flour the meal worm often hatches out and makes it unfit for use. On the potato plant is the Colorado beetle that eats the leaves, so they cannot make plant food. The cabbage head is burrowed into by the larva of the cabbage moth. Big green caterpillars feed on the tomato, and grape vines. A moth makes
