Sap-sucking insects can be damaging to landscape trees and shrubs. While they may look quite different, this group of pests has one common denominator: they feed by sucking sap from the stems and leaves of woody ornamentals.

PHOTOS BY JAMES KALISCH, UNL.
A photo of a leafhopper.

Nearly all species of woody ornamentals are attacked by one kind of sap-sucking pest or another. Effective management of these pests requires a sound understanding of the growth habits of the plant, knowledge of the biology, behavior, life history and type of damage caused by potential pests, and information on the plant growth stage or environmental conditions under which pest damage is most likely to occur. Accurate pest identification is also critical. Incorporating plant diversity into the landscape and maintaining healthy vigorous plants are among the best preventive control strategies available.

Scale insects

Scale insects produce a protective shell under which they live. They are the least mobile of all the sap-sucking insects, but that doesn’t mean they are the least injurious. Because of their small size and lack of movement on and near the plant, they often go unnoticed and eventually cause more injury than other sap suckers. Scales feed by inserting a narrow stylet into the stomates, the natural openings in the bark and leaves, and withdrawing plant fluids.

Nearly all tree species are affected by scales. Scales vary in color from white to grey to brown, but most have the same general size, about 1/16 to 1/10 inch. Because they are so small, and often take on the color of the tree’s leaves, bark or needles, they can be very difficult to spot at first glance. Routine inspection is an important step in scale insect control.

A close-up photo of leafcurl ash aphids.

Scales are best controlled during the brief crawler stage of their life cycle. Crawlers emerge from eggs and move to new feeding sites before covering themselves with a new layer of cuticle. Many products are available for treatment of scale crawlers, including insecticidal soaps, horticultural oils, acephate (Orthene), carbaryl (Sevin), deltamethrin (DeltaGard) and permethrin (Astro). In some situations, a soil drench of chlorantraniliprole (Acelepryn), clothianidin (Arena), imidacloprid (Merit) or thiamethoxam (Meridian) may be a good choice.

Aphids

Aphids occur on almost all species of woody ornamentals. They feed predominantly on the undersides of leaves, but are also found on the tender shoots of plants. Most are green, but can vary in color from pink to black. Like scale insects, aphids possess piercing-sucking mouthparts that serve to draw out cell sap.

The best time to control aphids is early in their life cycle for three reasons. First, smaller aphids, succumb to treatments more readily than older and larger ones. Second, as aphid colonies develop, leaves often curl around the aphids protecting them from predators and against the effects of insecticide applications. Finally, all aphids excrete a sticky substance called honeydew, which collects on lower leaves and branches and creates fertile growing areas for sooty mold. The end result is not only unsightly, but also reduces the photosynthetic surface of the tree leaves.

For greatest effectiveness, insecticidal sprays should be directed towards the undersides of the leaves. Because aphids have a relatively short life cycle and can regenerate quickly, multiple treatments may be necessary for acceptable control. The foliar applied products listed earlier for scales are also good choices for control for aphids.

Thrips

Thrips (singular and plural) belong to the insect order Thysanoptera or “fringe-winged” insects, an apt name when viewed up close. When observed with a 10x hand lens (something every arborist should have handy), these tiny insects appear to be mostly fluff because their wings are clear and are bordered with whisper-like structures similar to miniature eyelashes.

Thrips feed in a slightly different manner than aphids or scales in that they have rasping-sucking mouthparts that they use to scrape the plant tissues. As sap is released from the wound, the thrips uses a sponging action to draw the liquid into its mouth. A good place to look for thrips is under leaf sheaths or in branch or flower terminals, as they tend to prefer young, succulent tissue.

Other than “treat as soon as you see them,” thrips don’t seem to have a window in their life cycle when they are most vulnerable. Insecticidal soaps, horticultural oils and the insecticides mentioned above top the list of effective control products.

Leafhoppers and treehoppers

Leafhoppers are easily identified by their characteristic wing pattern, size, shape and color. About .25 inch in length, leafhoppers hold their wings at a sharp angle, similar to a small camping tent. They often match the color of the plant tissue upon which they are feeding, most commonly green, yellow or gray. Treehoppers also have remarkable pronotum patterns, which can resemble spines, horns or keels. Some species are thorn-like in appearance.

Unlike many other sap-sucking insects, leafhoppers are reasonably good fliers, and easily move from tree to tree to escape predators and arborists. They inhabit most broadleaf trees, but seem to prefer honeylocust, ash and oak.

Because of their ability to fly, insecticidal soaps and horticultural oils are not always effective. Residual insecticide applications are preferred, as well as systemic treatments. Fortunately, damage from leafhoppers is usually not severe enough to require treatment.

Spider mites

Although not technically an insect (insects have six legs and three body parts, while adult mites have eight legs and two body parts), spider mites are the prototypical sap suckers. Under cool and moist springtime conditions, Spruce spider mites multiply rapidly on conifers such as spruce, juniper and others. Damage is not usually noticed until the weather becomes hot and dry later in the summer. As the summer progresses, two-spotted spider mites can become a problem on essentially all landscape ornamentals.

Leafcurl aphid damage on an ash tree.

Mites can be detected in two ways: First, use a 10x hand lens to look for moving critters on the underside of a broadleaf tree leaf or on the new growth of a conifer. The body and legs of motionless mites should be clearly visible, while moving mites in motion may simply appear as moving dots. Second, place a white sheet of notebook paper under a branch or group of leaves and rap them with a small stick. If mites are present, they will fall onto the paper where they can be easily seen.

If one or two mites are found on a leaf, immediate treatment is usually not necessary. However, continue to monitor the tree for any changes in the mite population. If more than six to 10 mites appear on the sheet of paper, consider a treatment with a miticide such as bifenthrin (Talstar).

While sap-sucking insects may have a common method of feeding, keep in mind that each should be considered individually when it comes to selecting the most appropriate control strategies.

John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb. Frederick P. Baxendale is a professor and extension entomologist with the University of Nebraska-Lincoln.