Fig. 87. Goats Eating Foliage, New Mexico. U.S. Forest Service.
Fig. 88. Sheep Grazing in Forest, Idaho. U.S. Forest Service.
The most destructive animal enemies of the forest are the insects. The average annual loss of trees in the United States from this cause alone has been estimated to be one hundred million dollars.
Insects have two objects in their attack on trees, one is to obtain food, as when they are in the larval stage, and the other is to provide for offspring, as do certain beetles.
The number of insect enemies of the forest is enormous. At the St. Louis Exposition, there were on exhibit nearly three hundred such insects. These belong to some twenty orders, of which the beetles (Coleoptera), which have horny wings and biting mouth parts, and the moths and butterflies (Lepidoptera), with membraneous wings and sucking mouth parts, are the most destructive. Insects attack every part of the tree, the seed, the shoot, the flower, the root, the leaf, the bark and the wood, both standing and cut.
Of the fruit and seed pests, the most destructive are weevils, worms and gall insects.
Of the twig and shoot pests, beetles, weevils and caterpillars are the worst.
Among insects that attack roots, the periodical cicada (17 year old locust) may be noted.