Not all trees are equally seriously affected by insect pests and in the list below where a long list of insects and diseases are given after a plant name it does not signify that all those insects and diseases commonly occur, but merely that they are known to occur on one or more species of the genus. The list is given merely as a guide showing along what lines to look for further information, and it also indicates the sort of treatment required as outlined in the chapter on Maintenance, under the spraying discussion (See Page 75).
Considerable study throughout New York and Ohio in regard to the immunity of trees from insect injury points to the following conclusions: The arborvitae, tree of heaven, and the ginkgo are practically immune from injurious insects. The American and Oriental planes, the red and scarlet oaks, and the tulip tree and junipers are occasionally attacked. The red, Norway, sugar, and sycamore maples, the white and bur oaks, the honey locust, catalpa, the birches, the spruces, and the white pine have each one serious insect pest. Among the trees very likely to be attacked by insects are the European and American lindens, the American and the water or red elms, the soft maple, the horse-chestnut and buckeyes, and the hackberry. The European and Scotch elms and the willows are very seriously injured by insects, while the yellow locusts and poplars and cottonwoods are so seriously injured as to make it doubtful whether they should ever be used in ornamental planting.
In the following alphabetical list of plants all the defoliating larval forms of insects are classed as caterpillars for the sake of brevity, and because they are all treated in the same way when spraying is resorted to; namely, by a poison or a contact insecticide.
Alder (powdery mildew, borers, leaf roller, caterpillars, maple scale).
Apple (canker, aphids, caterpillars, scales, and bark louse).
Ash (trunk rot, canker, leaf spot, borer, caterpillars, and scales).
Aster (wilt and blister beetle).
Azalea (leaf spot and caterpillar).
Barberry (rust, plant louse, and caterpillar).
Beech (leaf diseases, anthracnose, mildew on leaves, maple scale, and caterpillars).
Birch (anthracnose, heart rot, canker, borer, and caterpillars).
Bitter-sweet, (evonymus scale).
Box Elder (canker, scales, and caterpillars).
Boston Ivy (caterpillars).
Buckeye (scales, bark louse, and caterpillars).
Catalpa (leaf blight, powdery mildew, and caterpillars).
Cherry (black knot, scales, aphis, leaf spot, mildew, and caterpillars).
Chestnut (anthracnose, bark disease, weevil, and caterpillars).
Chrysanthemum (leaf spot).
Daphne (magnolia scale).
Dogwood (San Jose scale, oyster shell scale, dogwood scale, and bark louse).
Elm (leaf diseases, leaf beetle, canker, scale, leaf gall, and caterpillars).
Englemann's spruce (red spider).
English Ivy (leaf blight).
Evonymus (evonymus scale, and cottony maple scale).
Fir (saw fly, timber beetle, borer, and caterpillars).
Hackberry (scale and caterpillars).
Hickory (leaf spot, borers, shuck worm, tussock moth caterpillar, bark beetle, and canker worm). Hollyhock (anthracnose and rust). Hornbeam (caterpillars).
Horse-chestnut (leaf blight, rust, tussock moth, bag worm, scales, and bark lice). Hydrangea (leaf blight and rust). Iris (bulb spot, root rot, and leaf blight). Juniper (cedar rust, red spider, and caterpillars). Larch (canker, saw fly, and tussock moth).
Lilac (powdery mildew, San Jose scale, bark lice, and caterpillars). Linden (borers, leaf beetle, caterpillars, scale, and red spider). Locust (heart rot, borers, leaf beetle, maple scale, and caterpillars). Magnolia (magnolia scale).
Maple (anthracnose, tip burn, sun scald, borers, caterpillars, scale, and twig pruner). Mulberry (bacterial disease, cottony maple scale).
Narcissus (aphids and eel worms).
Oak (anthracnose, caterpillars, powdery mildew, scale, leaf beetle, and twig primer).
Osage Orange (scale, bark louse, and caterpillar).
Peach (scales, caterpillars, and borers).
Peony (stem rot and leaf spot).
Phlox (leaf spot fungus and powdery mildew).
Pine (rust, leaf spot, leaf scale, bark louse, saw fly, and weevil).
Poplar (anthracnose, rust, leaf beetle, scales, bark louse, borers, and caterpillars.)
Privet (anthracnose, twig blight, and webworm).
Quince (anthracnose, black rot, and San Jose scale).
Rhododendron (lace-wing fly and borers).
Shad-bush (red spider).
Snapdragon (anthracnose, stem rot, and wilt).
Snowball Bush viburnum (aphids).
Solomon's Seal (leaf fungus).
Sour Gum (caterpillars).
Spice Bush (scale and caterpillars).
Spruce (leaf spot, red spider, bug worm, caterpillars, and weevil).
Sumac (canker, scale, and caterpillars).
Sweet Gum (bagworm and other caterpillars).
Sycamore (anthracnose, powdery mildew, leaf folders, caterpillars, and scales).
Thorn (caterpillars, scales, plant lice, aphids, and leaf beetle).
Tree of Heaven (rose scale).
Tulip Tree (scale and caterpillars).
Veronica (leaf diseases).
Viburnum (see Snowball Bush).
Violet (nematodes and leaf spot).
Virginia Creeper (leaf spot, caterpillars, and scales).
Walnut (anthracnose, mildew, canker worms, caterpillars, and scales).
Willow (caterpillars, leaf beetles, borers, and bark louse).
Wisteria (leaf beetle).
Witch Hazel (caterpillars).
Yucca (caterpillars and grubs.)