Real Solutions For Ornamental Pests: Insect Management

Ornamental pests, such as scale, hemlock woolly adelgid and bronze birch borer, cause aesthetic damage as well as physical injury to landscape trees and plants. Researchers constantly explore new ways to prevent, reduce or eliminate them. One key to successful control is understanding the biology of these troubling insect pests and the damage they cause. That information, combined with sound control recommendations, can lead to healthy trees for years to come.

STRUGGLING WITH SCALE. Scale insects appear in every U.S. state but do not always damage plants to the point where treatment is necessary. Soft scale insects produce a sugary, liquid waste product called honeydew, which serves as a growth medium for sooty mold, a black fungus that eventually covers honeydew-laden areas.

Honeydew attracts nuisance insects such as ants, flies and wasps, and creates an unsightly mess when it drips on parked cars, sidewalks and benches, which makes it an annoyance and a threat to plant health. In large numbers, scale feedings may cause plant injury by reducing vitality and vigor, which often leads to limb dieback and, in some cases, kills the plant.

However, not all scale species ingest sap and produce honeydew. Two general types of scales commonly appear on U.S. ornamental trees and shrubs: soft scale and armored scale. Both scales spend most of their lives immobile, feeding on the same spot on a plant. After eggs hatch beneath females, crawlers - called such because they are in the mobile stage - move to new sites, where they settle and feed. Crawlers are small and flat, looking like dust particles. Scale infestations spread to other plants as crawlers hitch rides on animals or are blown by the wind.

Armored scale insects flatten after feeding and produce a clear wax shell, which covers their bodies and is difficult to penetrate with insecticides.

On the other hand, soft scale insects are not covered by waxy shells. They hatch in mid-summer and crawl directly to leaves, where they begin feeding and excreting honeydew. Then, they return to twigs and bark where they spend the winter as settled second-stage scales, and continue to grow on twigs in the spring until they mate.

Milder winter temperatures have made soft scale more of a problem in many areas of the U.S., according to Dr. Clifford Sadof, extension entomologist at Purdue University, West Lafayette, Ind. "Scales are usually a problem in disturbed habitats, in warmer temperatures and where plant health is compromised," he noted. "For instance, if you want to find pine needle scale, go to a fast food place and look on the south side of the parking lot - the more gravel, the better."

Natural enemies can control scale, added Sadof. "If you increase numbers of flowering plants near trees or shrubs affected by scale, you can enhance populations of natural enemies in the immediate area," he observed. "But, a customer with cottony maple scale dripping honeydew all over the picnic table and causing yellowjacket wasps to swarm does not want you to tell him to plant a few flowers and wait two years for natural enemies to take care of the problem."

Sadof recommended the following steps:

• Identify the species. "Flip over suspicious looking bumps on twigs and branches with your thumbnail," he advised. "Tissue usually remains intact beneath a scale. If the tissue rips, it’s probably a gall."



• Once you have identified scale, ask yourself: Is it a problem? Unless you see accumulations of honeydew, there’s no reason to do anything about soft scale.



• Consider applying horticultural oil. This material works by smothering scales, therefore good spray coverage is essential for acceptable control. Unlike other pesticides, horticultural oil can control scales after they have settled while the scale body is still somewhat clear.



• Consider applying insect growth regulators. These materials kill scales as they molt.



• Consider pesticide applications. Conventional foliar-applied pesticides cannot penetrate a scale’s tough skin or waxy cover. To achieve maximum control, apply pesticides at the beginning of the crawler period. Thorough coverage on tall trees is difficult, and these materials do not control scales after they settle.



• Consider applying soil-applied materials. Applied from spring to early summer, these products can be translocated throughout medium-sized evergreen and deciduous trees in two to three months or throughout medium-sized shrubs within approximately a month.

HEMLOCK WOOLLY ADELGID. Easily recognizable because of the woolly masses clinging to young twigs, hemlock woolly adelgid infestations cause hemlock needles to dry out, turn grayish-green to yellowish and drop from the tree. Since the pest prefers to feed on new twig growth, buds never develop and little new growth is produced. Once affected, hemlocks gradually lose vigor, and the formerly lush canopy turns sparse. Major limbs wither, progressing upward from the bottom of the tree. Trees often die in four to six years.

The hemlock woolly adelgid has a complex life cycle, with overwintering adults laying 50 to 300 eggs on young twigs in dry cottony masses during March and April. Nymphs hatch in April and May and migrate to new growth, where they lose their legs and feed at the base of the needles. They remain in the same spot until they reproduce and die. The spring generation matures by the middle of June and begins laying eggs. After the second generation hatches by early June and settles on new growth, it hibernates until the middle of October, when feeding resumes. Nymphs feed and develop through the winter, reaching maturity by spring.

"The hemlock woolly adelgid is a huge problem in the eastern U.S.," remarked Dr. Mark McClure, chief scientist at the Valley Laboratory of the Connecticut Agricultural Experiment Station, Windsor, Conn. He said the insect has moved north from Mid-Atlantic states and has the capability to move up to New England states.

McClure offered the following control measures:

• Locate early infestations and remove branches or limbs to reduce further colonization. Pruning also promotes new growth by increasing sunlight exposure.



• Improve tree health by keeping trees as stress-free as possible. Proper watering is critical to hemlock health.



• Plant resistant hemlocks. Western and Japanese hemlock species tend to be more resistant to hemlock woolly adelgid.



• Use insecticidal soaps and horticultural oils. Complete coverage is crucial to achieving control. Although one yearly application may be sufficient on small, isolated trees, two annual applications are usually needed on large trees where thorough coverage is unlikely or on trees near infested hemlocks that are not being treated. Sprays can be applied anytime from April through October.



• Use soil applications so the material moves through medium-sized hemlock trees within two to three months. "When injected into the soil before trees are damaged to any great extent, control products can be very effective," McClure said. "But, if you make the soil treatment after trees are suffering, you may not get the same degree of control."

BATTLING BRONZE BIRCH BORER. Borers are among the most destructive pests to ornamental plants and shade trees, causing girdling, branch dieback, structural weakness and eventual death. The bronze birch borer is the larva of a beetle native to North America that feeds on the phloem and cambium layers of birch trees, creating galleries under the outer bark. Since the bronze birch borer cannot survive in healthy trees, successful larval development depends on the host being in a weakened condition from injury, adverse weather or old age.

From mid-June to early July, adult birch borers lay eggs on the surface or in crevices of the outer layer of bark, often at the site of a recent mechanical injury to the tree. Larvae burrow into the tree and feed on the phloem throughout the summer and into the fall. The insect has a one-year life cycle. In winter, the bronze birch borer hibernates in the phloem of birch trees, completes development and pupates in the xylem, emerging as an adult beetle in late May to mid-June.

While native birch species are resistant to the bronze birch borer, the susceptible European white birch is common throughout the eastern and midwestern U.S. "Native species don’t grow as fast or turn white at as early an age," remarked Dr. Dan Herms, assistant professor at The Ohio State University’s Ohio Agricultural Research & Development Center, Wooster, Ohio.

In addition to selecting native, resistant species, such as paper birch and gray birch, Herms offered the following suggestions:

• Do not plant birch trees in the shade, as they are not a shade-tolerant species. Plant trees so roots are shaded but the canopy is in full sunlight at all times.



• Maintain tree vigor through proper planting, mulching and adequate irrigation during drought periods. "Irrigating is critical to limiting susceptibility," Herms added. "My research showed that you can make paper birch trees immune to bronze birch borer with as little as 1 inch of water per week during summer months."



• Prevent injury to young trees, as stressed trees are particularly vulnerable to bronze birch borer.



• Initiate a pesticide program for high-value trees. "The use of soil injection products looks promising, though more research is necessary."

The soil injection method of pesticide application offers some advantages over bark sprays when treating for bronze birch borer, Herms said. "Bark sprays require three precisely timed applications at two-week intervals," he noted. "With soil application, arborists and landscape managers can tailor treatments to follow a more relaxed application schedule. They also reduce the chance of drift and, for the most part, avoid the public-relations problems generally associated with spraying in residential neighborhoods."