Common Insect Pests
of Trees and Shrubs
in North Dakota

E-296 (Revised), October 1995

Phillip Glogoza, Extension Entomologist

• Common Pests with Piercing-Sucking Mouthparts
  Aphids
  Plant Bugs and Leafhoppers
  Scale Insects
  Spider Mites
  Gall Forming Insects and Mites
• Common Pests with Chewing Mouthparts
  Leaf Feeding Beetles
  Leaf Feeding Caterpillars and Sawflies
  Wood Boring Insects
• Products recommended for the management of tree and shrub pests

Every homeowner has had problems with insects attacking shade trees and shrubs in their landscape. It is difficult to predict when many of these pests will reach destructive numbers. Some are present nearly every season and may require annual control. Many insects are found yearly but their feeding causes little or no harm to the plant.

Managing these pests requires a basic understanding of the insect itself. Become aware of which insects may be found on specific plants. Knowing the life cycle can help plan management activities when they would be most beneficial. Recognizing feeding injury and evaluating its severity helps decide if control is needed. Insecticidal control should be the last management strategy used after other options have failed or are impractical.

Symptoms of feeding injury can be generalized into two categories based on the type of mouthparts present on the insect. Insects with piercing-sucking mouthparts feed by removing fluids from plant tissue. Typical symptoms include discoloration, wilting, distortion or curling of leaves. Pests with piercing-sucking mouthparts include aphids, plant bugs, scale insects, leafhoppers, and spider mites. Insects with chewing mouthparts consume plant tissue. Typical symptoms include holes in leaves, wood or fruit, skeletonizing of leaves, and tunneling in wood. Pests with chewing mouthparts include caterpillars, beetle adults and larvae, sawfly larvae, and grasshoppers. Understanding this relationship will help diagnose problems, identify the offending pest, and select the proper management strategy.

The time of year and the frequency of defoliation by pests is critical in the long term health of trees and shrubs. Leaves are critical for production of energy for the growing plant, so leaf loss impacts negatively on plant energy levels. Defoliation by pests in the spring and early summer can result in net energy losses to the plant for the season. Repeated defoliations from season to season can cause loss of stored energy reserves. Branch and root die back may occur, possibly leading to the death of the tree. Managing pests to prevent extensive leaf loss early in the season is important in maintaining an actively growing plant. Late season defoliators have less impact on overall tree energy levels and growth.

Aphids are small insects, about the size of a pinhead, or 1/10 of an inch long. They are soft-bodied and pear-shaped. On the rear of the aphid, two "tail-pipes" known as cornicles can be seen. Color is variable, ranging from yellow to green to black, depending on the species. Populations of aphids can increase very rapidly due to their ability to bear live young and to reach the adult stage in as little as seven days. Often their colonies are found on new plant growth, clustering on the underside of the leaves. Aphids may be winged or wingless. They feed by sucking juices from the plant. Infested plants may lose their natural color, wilt, or become disfigured due to the aphid feeding. Aphids excrete a clear fluid called honeydew, which becomes sticky and accumulates on foliage (and other objects) below the feeding sites.

Aphid infestations on large shade trees are difficult to control with foliar applications of insecticides. In these cases, their feeding is rarely considered important. Infestations on smaller trees and shrubs are more successfully managed when populations are unsightly and cause significant feeding injury. Light infestations of aphids can be removed from foliage by using a forced water spray delivered under high pressure. Lady beetles are frequent predators of aphids and should be protected when found feeding on aphid colonies.

Plants Attacked

Shrubs: snowball, high bush cranberry, dogwood, honeysuckle, rose, cotoneaster, caragana, and others.

Trees: elm, boxelder, maple, poplar, birch, aspen, plum, willow, apple, hawthorn, crabapple and others.

Examples

Woolly elm aphid -- Bluish grey with white powdery coating. Feeding causes curling and distortion of leaves. These aphids are responsible for the honeydew raining down on cars parked beneath elms. Most common during early and late summer. Control is difficult due to spray coverage problems.

Honeysuckle aphid -- Pale green to cream. They begin feeding in spring when new leaves appear, moving to new foliage all season. Damaged branches form ugly "witches' broom" with folded and dwarfed leaves. Select and plant resistant cultivars such as Emerald Mound, Clavey's dwarf, Miniglobe, Sakhalin, Freedom, Honeyrose and some of the Arnold Red.

There are many insects which can be described by these common names. The plant bugs and leafhoppers are all very active insects which move quickly when disturbed. The young nymphs move by walking rapidly or jumping short distances. Adults may fly from plants when disturbed. Many are small, ranging in size from 1/8 to 1/2 inch long. They remove fluid from plant tissues. Plant bugs frequently cause spotting on leaves, or the death and loss of buds. Leafhoppers generally cause spotting of leaves or browning of leaf margins (hopper-burn). Leafhoppers frequently carry plant viruses that infect leaf tissue and result in additional injury symptoms.

Plants Attacked

Shrubs: Caragana and ornamental vines

Trees: Ash, oak, and elm.

Examples

Ash plant bug -- Nymphs are green to tan and adults are tan with pinkish markings on the back. Nymphs hatch in the spring as new leaves appear. There are two generations per season. Damage appears as yellow spotting on leaves. Severe feeding may result in deformed leaves and possibly defoliation. Control is justified when leaf injury is easily found throughout the canopy.

Lace bugs -- Adults are dark and flat with lace-like wings. Infested leaves have brown or yellow damaged areas; black, tar-like fecal spots are visible on leaves. Damaged leaves turn brown and drop early, especially during dry weather. Two generations occur each year.

Female scale insects have a protective covering of wax that is secreted from their bodies. The eggs are laid under the protective scale covering. The eggs hatch and the young "crawlers" move about for a short time, then settle down to feed, sucking the plant sap. It is during the "crawler" stage that insecticidal and horticultural oil treatments are most effective. To determine when "crawlers" are active, shake branches over a piece of white paper and inspect for their presence; magnification helps with detection. Once the insects are under the protective covering, most insecticidal applications are ineffective. Heavy scale infestations weaken the trees or shrubs. Infestations isolated on individual branches may be pruned from the plant in an attempt to reduce scale populations.

Horticultural oils are petroleum-based products. When applied, these oils suffocate insect and mite pests and their eggs. Thorough coverage is required for effective control. There are summer oils and dormant oils. There are many temperature and host plant restrictions for applying these products; read all labels carefully before using horticultural oil sprays to treat for labeled pests.

Examples

Oyster shell scale -- The protective shell resembles miniature oyster shells. They are 1/8 inch long and shiny brownish-grey in color. Their eggs hatch in late May or early June. Plants infested include cotoneaster, apple, lilac, spirea, snowball, high bush cranberry and others.

Scurfy scale -- The protective shell is pear-shaped and whitish in color. They are 1/10 inch long. The scales cluster on stems and branches. Heavy infestations give the bark surface a flaky appearance. Eggs hatch in mid-May. Plants infested are elm, ash, apple, maple, willow, aspen, and cotoneaster.

Cottony maple scale -- The covering is oval, flat, and pale to dark brown. In the spring a white cottony egg sac two to three times the length of the scale is produced. The eggs hatch in late June or July. The crawlers are tiny and brownish. They feed on the undersides of leaves, moving back to the branches in the fall. Plants infested are maple, poplar, basswood, and elm.

European lecanium scale -- The shell is bowl-shaped, 1/8 to 1/4 inch long, slightly longer than wide with a smooth surface. They are a shiny speckled brown color. Eggs are laid in the spring and hatch in late June or July. The crawlers feed on the leaves, returning to the twigs in the fall where they overwinter in cracks in the bark. Plants infested are willow, ash, elm, rose, fruit trees, and poplars.

Galls are abnormal growths on leaves, twigs and flowers, appearing as warts, bumps, or balls and are caused by insects and mites. Mechanical damage or salivary secretions from the pest initiate production of plant growth hormones that stimulate the growth of the gall. The shape, size, and color are extremely variable, but appearance of the galls is unique to the insect causing them. Most gall-making insects and mites are not harmful to the host plant. Control attempts are rarely justified and usually are unsuccessful because the gall protects the insect within it. Some of the galls are quite interesting, adding a touch of uniqueness to plants.

Plants Attacked

Shrubs: rose

Trees: oak, hackberry, maple, cottonwood, willow, chokecherry, ash, and honey locust.

Examples

Hackberry nipple gall -- Caused by jumping plant louse. Galls are light green and nipple-shaped.

Hackberry bud gall -- Caused by jumping plant louse. Small, round, multi-celled growths of the bud.

Maple bladder gall -- Caused by mite. Galls are bladderlike, red, and found on the top side of leaves.

Poplar petiole gall -- Caused by aphid. Green, marble-shaped galls found on the leaf petioles.

Poplar bud gall -- Caused by mite. Form cauliflower-like growth on leaf buds.

Poplar vagabond gall -- Caused by aphid. Infested leaves are reddish and twisted, later turning brown.

Ash flower gall -- Caused by mite. Infest male flower clusters causing irregular growths.

Honey locust pod gall -- Caused by fly midge. Leaflets form hollow pods which eventually turn brown.

Oak leaf galls -- Caused by flies and tiny wasps. Various warty growths found on leaves, petioles, and twigs.

Oak bullet gall -- Caused by tiny wasp. The light tan, round galls are found on twigs in clusters of three or more.

Mossyrose gall -- Caused by tiny wasp. Galls are large, round, light green, hairy masses; turn brown later. Prune out galls before spring.

Leaf feeding beetles are quite diverse in size and appearance. In some cases only the adult beetle feeds on foliage; in other cases, both the adult and the larvae, or worm stage, feed on the plant. Sizes range from 1/4 to 1 inch or more. Leaf feeding beetle adults may be drab (brown or black) or brightly colored. The grubs may be cream with no markings to colored with stripes and spots. All leaf feeding beetles have chewing mouthparts. When adults feed, leaves are skeletonized; the larvae feed on the under-sides and leave the upper leaf surface intact, damage described as "window feeding."

Plants attacked

Shrubs: caragana

Trees: cottonwood, willow, elm, aspen, and poplars.

Examples

Cottonwood leaf beetle -- Adult beetles are 1/2 inch long and have black heads and yellowish wing covers with black stripes. Young larvae are black; older larvae are yellow with black and white spots down the side. Both stages feed on foliage. Adults begin feeding on leaves and twigs in the spring. Clusters of yellow eggs are laid on the undersides of leaves soon after. There are two generations per year.

Blister beetles -- Adult blister beetles are foliage feeders. These beetles have a narrow, elongate, soft body. They range in size from 1/2 to 1� inch long. The head is wider than the first body segment giving the appearance of having a "neck." Their color is black, grey, or striped. These beetles frequently are found in clusters, where they have been attracted to a particular group of plants to feed.

Many kinds of caterpillars and sawflies feed on leaves. They all have chewing mouthparts so damage will always be characterized by holes or skeletonized leaves. When feeding symptoms are seen, inspect plants for the presence of larvae. For some caterpillar pests, conspicuous webs may be present. Frequently, larvae consume a high percentage of the foliage during their last growth stage. If you fail to notice the infestation until this time, controls may not be warranted because the larvae will be preparing to form the pupa, a non-feeding stage. Also, the larger the larvae, the greater the difficulty to control the pest.

Plants Attacked

Many trees and shrubs

Examples

Fall and spring cankerworms -- The larvae vary from green to reddish-brown to black with pale stripes. Larvae of both species hatch in the spring when new leaves appear. Female moths are wingless and must crawl up the bark of trees to reach branches for egg laying. (For additional information see extension publication E-999, Biology and Control of Cankerworms in North Dakota)

Spiny elm caterpillar -- The larvae are velvety black with small white dots and bear a single row of red spots on the back. There are several rows of large branched spines the length of the body. Eggs are laid in early spring. When feeding they can often be found in clusters.

Prairie tent caterpillar -- The larvae are pale blue with an interrupted white stripe bordered by two reddish-orange stripes down the center of the back. Eggs that were laid in the fall hatch early in the spring as new leaves emerge. The larvae live in colonies and construct large silk tents around a fork or branches of trees. Larvae feed outside the tent.

Fall webworm -- The hairy, pale yellow larvae have red heads and reddish-brown spots, or some other color variation. Eggs are laid in early summer. The newly hatched larvae feed in groups enclosed by a webbed nest at the ends of branches. Webs are enlarged to enclose more foliage as needed. Larvae feed within the web, making control with insecticidal sprays difficult.

Elm sawfly -- Older larvae are yellowish-green with a blue and black stripe running the length of the back. Adult sawflies emerge in early summer and lay eggs. Larvae finish feeding in August, migrating to the base of the tree trunk to pupate and overwinter.

Pear slug -- The sawfly larvae is slug-like, dark olive-green, and covered with slime. Eggs are laid in June and July. The larvae feed on the upper leaf surface, consuming leaf tissue while avoiding the the main veins. they are found on hawthorn, pear and cotoneaster.

Birch leafminer -- The cream-colored sawfly larva feeds between the epidermal layers of the leaf, causing leaves to dry and turn brown. There are two generations per year. The first adults emerge and lay eggs beginning in late May; the next generation of adults emerges in mid July.

Wood boring beetles and caterpillars are most injurious to trees weakened by transplanting, drought, disease, poor fertility and other stress factors. The larvae of borers are difficult to control because they are protected within the tree during most of their lives. Proper care and maintenance to promote a healthy tree help prevent the attraction and establishment of the borers. Pruning dead or infested wood from trees helps prevent destructive borer populations. When insecticides are used, they act as a protectant, controlling some adults and reducing the risk of egg laying. Applications should be made just prior to the appearance of the adult.

Examples

Poplar borer -- Adults are elongate grayish beetles with long, slender antennae. They emerge during mid summer. The white, legless larvae tunnel in trunks and branches for two to three years.

Ash and lilac borer -- The wasp-like moths are black and yellow to orange and brown and have clear wings. They emerge in late spring (May-June). Leaves on infested branches turn brown; branches die back and break easily.

Bronze birch borer -- The slender adult beetles are olive green to black with metallic bronze reflections. They emerge in late June and can be active until August. Early infestations of trees start in 1 inch diameter branches, later including larger branches and the trunk as the tree's health weakens.

The "alternative" products represent those that would be generally classified as "organic," representing botanicals, microbials, and soaps. Always read and follow the label. Before treating ornamentals, check the label for comments pertaining to plant sensitivity to the chemical. (&127; indicates listed pest is found on the pesticide label)

E-296 (Revised), October 1995

NDSU Extension Service, North Dakota State University of Agriculture and Applied Science, and U.S. Department of Agriculture cooperating. Sharon D. Anderson, Director, Fargo, North Dakota. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. We offer our programs and facilities to all persons regardless of race, color, national origin, religion, sex, disability, age, Vietnam era veterans status, or sexual orientation; and are an equal opportunity employer.
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