Ornamental pest insects may be divided into two groups by the way they feed: (1) sucking types (scales, aphids, mealybugs, whiteflies, true bugs, thrips, and mites); and (2) chewing types (grasshoppers, beetles, sawflies, and caterpillars).
Some types of insects, such as aphids, thrips, and leafhoppers, can spread diseases to healthy plants.
Aphids—Aphids (ex. Aphids), or “plant lice,” are small, soft-bodied insects that usually cluster on stems or undersides of terminal leaves. Aphids may be green, black, or red, but sometimes their color is hidden by a white waxy coating. Much of the sap that aphids suck passes through them undigested and is excreted as “honeydew.” Honeydew makes the leaves sticky, and sooty mold may grow on these deposits. Feeding by some kinds of aphids will cause leaves to pucker, curl, or twist.
Scales—During most of their lives, scale insects are legless and motionless and do not resemble insects at all (ex. Scale Insects). They may be circular, oval, or pearshaped. Some are flat, others convex. Two major groups of scales are most common in Kentucky. The armored scale produces a waxy shell that gives the soft-bodied insect under it some protection. The unarmored or soft scales do not produce a shell, but their bodies may be tough.
Scales reproduce by giving birth to “crawlers” or by laying eggs that hatch into crawlers. Crawlers have legs, eyes, and antennae, all of which allow them to move out from under the mother’s shell or body and seek a suitable place of their own on the plant. Soon after inserting their beak to feed, they molt and lose their legs, eyes, and antennae and remain motionless for the rest of their lives.
Plants infested with scales may lack vigor and appear sickly. Unarmored scales are honeydew producers like aphids and cause the same symptoms as mentioned for aphid honeydew.
It is easier to control most scales while they are in the crawler stage because they are not protected by a shell or waxy coat. Treatment applied for scale control should coincide with crawler activity. A second treatment in 2 to 3 weeks is usually recommended. Timing of systemic insecticide applications is not so critical. On oil-tolerant plants, oil sprays can be used to control all stages of scales, including eggs. Summer oils may be effective during the warmer months. Apply dormant oils in winter. Insecticidal soaps are another alternative for controlling scale crawlers as well as aphids, mealybugs, whiteflies, thrips, and mites.
Mealybugs—Mealybugs differ from typical scales in that they can move about slowly throughout their lives. They are soft-bodied and covered with a white, powdery, cottony, or mealy wax-like material (ex. Mealybugs). Mealybugs suck juices from stems and leaves, an activity which stunts or even kills plants. Black, sooty mold often grows on honeydew deposited by mealybugs. The stressed look of azaleas is often due to mealybug infestation.
Whiteflies—Whiteflies are small, powdery-white insects that resemble tiny moths. The greenhouse whitefly is the only pest species in our area. The immature stages of whiteflies resemble scale insects. Both the adults and immature stages suck sap from the leaves of host plants. When an infested plant is disturbed, the adult insects flutter off but settle back down very quickly. Besides ornamentals and flowers, the greenhouse whitefly also infests many different vegetables, shade trees, and weeds. Infested plants lack vigor, turn yellow, wilt, and may die. Leaves may also be covered with sooty mold growing on honeydew these insects produce.
Thrips—Thrips are tiny, slender insects with rasping-sucking mouthparts. Adults may be yellow, brown, or black and have feathery wings held flat on the back. Immature thrips resemble adults but are lighter in color and wingless. Some species feed primarily on foliage while others feed primarily on blooms. The foliage of thrips-infested plants may be streaked or silvered. Flowers may be deformed or have brown-edged petals, or the flower buds may drop off or fail to open. The protection of plants from flower thrips is difficult during May and June when the thrips are migrating and continually reinfesting plants. Thrips are known vectors of some viral plant diseases.
Mites—Mites are not insects, but their damage and the methods of control are similar to those of insects. They differ from insects in that they have 8 legs, not 6, and have only 1 body region instead of 3. They never have wings (ex. Two Spotted Mite). All mites are tiny and usually cannot be seen without the aid of a magnifying lens. By tapping infested twigs over a sheet of white paper, the dislodged mites are much easier to detect. They vary widely in color. Some mites spin fine, delicate webbing on the host plant. This webbing is usually easier to detect than the mites themselves. Mite damage often appears as a bronzing of the foliage, which sometimes gives it a dusty appearance. Leaf drop may also occur.
Many kinds of plants are attacked by the two-spotted spider mite, and almost all coniferous plants are hosts to the spruce spider mite. The Southern red mite is primarily a pest on broadleaf evergreens such as azaleas and camellias.
Beetles may attack any part of a plant and in various ways. Some are typical leaf feeders and bite off pieces of leaf, while others are leaf miners or skeletonizers. With some beetles, the adults and larvae both are leaf feeders on the same plant; other beetles may be foliage feeders as adults and root feeders on other plants while in the larval stage. The Japanese beetle is an example of this type of beetle, causing serious damage to the foilage of many landscape plants while the grub is a serious pest of turfgrasses. Some feed during the day and some feed only at night, such as the May beetles.
Some beetles, such as the bronze birch borer, feed as the larval stage in the cambium of trees and shrubs. This boring activity leaves “galleries” underneath the bark, usually causing serious damage to host plants. Girdled plants usually die.
Caterpillars—Caterpillars are the worm-like immature stages of moths and butterflies. They range in size from tiny to 5 inches long. They usually have a distinct head and 4 pairs of fleshy legs on the middle of the body (Figure 26). The body may be fuzzy, naked and smooth, or spiny.
Figure 26. Caterpillars
Caterpillars are primarily foliage feeders and eat out irregular areas or they may entirely strip the leaves.
Some caterpillars, because of their special habits, are also referred to as webworms, tent caterpillars, leaf rollers, leaf folders, skeletonizers, bagworms, and leafminers. Some feed as individuals; others feed in groups or colonies.
When only a few large caterpillars are present, handpicking is effective. Webworms and tent caterpillars can either be pruned out or burned out with a torch. If pruning would adversely affect a plant or if the infestation of any caterpillar is generally distributed over a plant, a single treatment of an approved insecticide applied when the caterpillars are young will usually give control.
A group of small moths, usually referred to as clearwing moths, cause serious boring damage to certain plants. The active adults often resemble wasps. The larvae bore through the cambial layer, causing stress, decline and, occasionally, death of plants. Dogwoods, lilacs, and ash are affected by clearwing borers.
Sawflies—Sawflies are wasp-like insects and are related to typical wasps, bees, and ants (Figure 27). The larval stages of most sawflies resemble naked caterpillars, but they have more than 5 pairs of fleshy legs on the body while caterpillars have only 4 or fewer pairs. Some sawfly larvae are slug-like in appearance, such as the pear slug and rose slug.
Figure 27. Sawflies
Most sawfly larvae are foliage feeders that eat the entire leaf, but slug sawflies are skeletonizers. A few types of sawflies are wood borers or leafminers. These differ further from typical sawflies in that they do not have fleshy abdominal legs.
The most serious sawfly pests in our area are those that attack coniferous shrubs and trees. They feed in groups and can quickly defoliate a plant. This defoliation often leads to the plant’s death. A single application of an approved insecticide is usually sufficient for control.
When some insects or mites feed on or lay eggs in plants, they may inject a chemical into the plant that causes it to grow abnormally and produce a gall. Each species of gall-producing insect or mite produces a characteristic gall on a certain part of a particular type of plant. Each gall may harbor one to many gall insects. The stimulus to formation of the gall is usually provided by the feeding stage of the insect or mite. A plant gall may have an opening to the outside (galls of mites, adelgids, psyllids), or may be entirely enclosed (galls of larval insects). The Hackberry nipplegall maker, which is a psyllid, can be a nuisance in the fall when adults hatch and tend to gather around entries to homes.
Oak Gall Wasps—Hundreds of species of Cynipid wasps cause galls on many species of oaks. Plant parts affected include roots, crown, bark, branches, twigs, buds, and leaves. Some of the more common galls are the many small leaf galls and the large brown galls known as oak apples, each of which is a leaf deformed by a larva. Usually, the galls do very little harm to oaks. No control measures are known, except for pruning out infested twigs and branches.