Pests on Ornamentals

Insect Pests of Flowers and Shrubs

C. A. Weigel, R. A. St. George.

Many kinds of insects beset flowers and shrubs about the home and in greenhouses. The injury they do depends on their feeding and egg-laying habits. Some insects feed on the seed as soon as it is planted. Others attack the young seedling as it breaks through the ground. Still others infest the flowers, leaves, stems, or roots.

Injury to the leaves may consist of mining the interior, skeletonizing the surface, and eating part or all of the foliage. Injury to the terminal shoot may be caused by external feeding, by the chewing of holes through the surface, and by tunneling extensive mines in shrubs between the bark and wood or even in the wood itself. Such injury may cause the stunting or death of the terminal growth beyond the point of attack. Certain insects also hollow out the interior of terminal buds and shoots of hardy shrubs. A few make pits in the surface of the bark of the main stem after settling there and feeding for a while. One other type of injury consists of the removal of the cell sap by the feeding of certain insects or mites on the stem, foliage, or other parts of the plant; the part attacked may have a stippled appearance. Injury to the roots may consist of feeding on the young rootlets and on shrubs--of chewing through the bark surface or boring direct into the wood. Further injury consists of the formation of galls, which may occur on any part of the plant.

The effect of the various kinds of injury depends largely upon the intensity of insect infestation and the vigor of the plants. It may range all the way from making the plants only slightly less attractive in appearance to seriously weakening, stunting, or killing them. The maximum effect of such injury is apt to be most marked during or immediately following periods of drought or transplanting operations, when it is difficult for the plants to get adequate moisture and food.

Because insect enemies of flowers and shrubs are of many kinds, it would be difficult for the average person to identify them without first knowing the different groups involved. We therefore group them as leaf-chewing insects, sucking insects, leaf-mining insects, gall-forming insects, tip and stem-infesting insects, and soil- and root-infesting insects. We give a few examples of each group and an account of the injury they cause, their habits, and control.

To control insects outdoors, insecticides are usually applied as sprays, dusts, or baits. In the greenhouse they may be sprays, dusts, baits, fumigants, and aerosols, although aerosols have been made so effective that they have nearly replaced other methods in greenhouses.

LEAF-CHEWING INSECTS bite off the foliage and chew and swallow the plant tissue. Therefore they generally are controlled by a stomach poison. Chief among them are caterpillars, sawflies,and beetles: The bagworm, cutworms, the fall webworm, the juniper web-worm, the eastern tent caterpillar, sawflies, the catalpa sphinx, the rose chafer, the rose curculio, cucumber beetles, the Japanese beetle, blister beetles, flea beetles, and the imported willow leaf beetle.

The bagworm is a caterpillar that lives in a silken, cocoonlike bag, to which are attached bits of leaves from the host plant. It gradually increases the size of the bag as it grows. By late summer it is about 2 inches long. At that time it attaches itself to a twig. The female is wingless and remains in the bag, where she lays a mass of eggs. The winter is passed in the egg stage. The eggs hatch in May in the South and late May or early June in the North. Bagworms prefer to feed on arborvitae and juniper but also infest many other evergreen and deciduous trees and shrubs.

Treatment : Destroy all mature bags in the spring before growth starts in order to kill the overwintering eggs. If that is not done, spray with lead arsenate soon after the caterpillars hatch (about June 1 in Washington, D. C.) , using stronger dosages for the larger caterpillars. A 2-percent emulsion of chlordane or parathion as a wettable powder is also effective. DDT is inferior to arsenate of lead against bagworms, particularly the more mature larvae.

Cutworms are seldom seen. They usually remain hidden under clods of earth or in the topsoil by day. Evenings they emerge to feed. They cut off small plants at or near the ground line, climb the plants, and feed on the foliage or bore into the developing flower buds. Plants are usually ruined overnight. One cutworm can kill several plants.

Cutworms are smooth, plump caterpillars, gray or brownish, and 1 to 2 inches long when full-grown. They hatch from eggs laid by brownish moths late in the summer. By late fall they are nearly full-grown and bury themselves in the ground for protection during the winter. Among the several species, the variegated cutworm probably is the most serious, both under glass and outdoors.

Treatment: Poison bait consisting of a mixture of sodium fluosilicate or paris green mixed with wheat bran is a standard remedy. The moist bait is scattered thinly over the infested area late in the evening when the caterpillars are active. DDT in dust or spray mixtures is said to be superior to baits in greenhouses.

Poisonous caterpillars occasionally eat and injure garden plants, shrubs, and trees. Most caterpillars are not poisonous, although several species have stiff, poisonous hairs or spines that may cause a painful, burning sensation when they come in contact with tender skin.

The saddleback caterpillar, the best known, attacks several kinds of flowers and shrubs. It is brown at each end; the middle is green with a purple center that resembles a small saddle. Poisonous caterpillars are seldom found in greenhouses.

Treatment: Spray the leaves with DDT or lead arsenate. If only a few caterpillars are present, they may be picked by a gloved hand.

Sawfly larvae injure roses by skeletonizing the foliage or chewing large, ragged holes in the leaves. Three species are concerned. They are often called false caterpillars or slugs. The adults are small, wasplike insects. The females deposit their eggs in slits "sawed" in the leaves.

A common species is the bristly rose-slug. The young larvae skeletonize the leaves on the under side and give a glazed appearance to the foliage. As they increase in size they eat large holes and often leave only the larger veins. Full-grown slugs are about one-half inch long and are dirty, yellowish green with a darker green stripe on the back. The body bears stiff hairs, from which the name is derived.

Treatment: Spray or dust with DDT or lead arsenate. If diseases or mites are present, add sulfur to the mixtures. Nicotine sulfate and derris are effective against the young slugs. Frequent spraying or washing of plants with a stream of water under pressure will keep them free of the slugs.

The spotted cucumber beetle is typical of several species that attack flowering plants. It feeds on the leaves, buds, and flowers. Its chief injury consists in eating holes in the blossom petals. If many beetles are present, their excrement often discolors the blossoms. Injury is likely to be most serious in late summer or early fall. At that time many of the more favored host plants have matured or become unpalatable and the beetles migrate to asters, dahlias, and other late-season flowers. The beetles are about one-fourth inch long and yellowish green, with 12 black spots on the wing covers. They winter in the adult, or beetle, stage. The eggs are laid in the ground in the spring. Newly hatched larvae feed on the roots of various garden plants and weeds for about a month before the adults emerge.

Treatment: Spray or dust the plants with DDT or chlordane every 2 or 3 weeks. Repeated applications are recommended because the beetles are apt to be present in the surrounding area and thus are likely to be a recurrent problem.

The rose chafer is an outstanding pest of the blooms of the rose, iris, and peony. It also attacks the flowers of many other plants. It is a long-legged, yellowish-brown beetle about one-third inch long. It often appears in swarms rather suddenly in June or early July and continues its ravages for a number of weeks.

Treatment: If only a few plants are involved, shake the beetles into a pan of water, covered with a film of kerosene or other oil, early in the morning before they become active. Otherwise, DDT or chlordane as a dust or spray is recommended.