Scale insects plant injury and management
Wayne Seidel, University of Minnesota Extension
Figure 7. In severe infestations, armored scales can kill individual branches and potentially the entire tree. This crabapple is in decline from an infestation of oystershell scale.
R. Scott Cameron, Advanced Forest Protection, Inc., Bugwood.org
Figure 8. Sooty mold growing on honeydew on a pine infested with pine tortoise scale.
Scale populations can expand rapidly after just a few seasons of infestation. A plant can literally be covered with scales before the problem is noticed. Injury doesn't usually become obvious until scale populations reach outbreak levels.
Armored scale insects are typically more damaging to their hosts than soft scales. Armored scales feeding on branch tissues kill individual cells. Cell death disrupts the transportation of materials through the tree limb and can cause the branch to die. In heavy infestations armored scales feeding on branch tissues can cause the death of entire plants. Limb death is often the first indicator that there is a scale problem on the plant (Figure 7).
Armored scales that feed on foliage kill individual cells as well. Damage is observed as purpling and yellowing of foliage and loss of foliage to which the scales are attached.
Soft scales are frequently identified due to the honeydew they produce on infested plants (Figure 1). Honeydew will support the growth of sooty mold on the leaves of plants, but it is not directly damaging to the plants (Figure 8). In severe soft scale infestations damage occurs on individual limbs before it impacts the entire canopy. Observable soft scale insect damage can range from leaf deformation and branch tip dieback to limb death and loss of sections of the canopy (Figure 7).
Scale insect species are usually kept below damaging numbers by natural enemies especially lady beetles and tiny parasitic wasps. However, there may be times when biological control is not sufficient and scale numbers become abundant, requiring management.
When dealing with scale insect pests, be sure to properly identify the scale species that is present. Knowing the biology of the scale will give the approximate time when scale crawlers are active. The best time to treat most scale insects is after egg hatch when the crawlers are first active and are searching for feeding sites, but before they begin producing their shells.
Start to visually inspect for the presence of crawlers just before they are expected to appear. This can be done by shaking infested branches over a white sheet of paper or white paper plate to look for reddish or yellowish colored crawlers. Double-sided tape can also be placed on branches where scales are present. Check the tape for evidence of crawlers. Check for crawlers at least once week until they are discovered.
Because of the protective shells that adult scales possess, they are not affected by direct insecticide applications. However, the immature crawlers lack these waxing coverings and are vulnerable to most insecticides.
Low impact products
Insecticidal soap and horticultural oil are effective options for managing scale insects. They have the advantage of having minimal impact on natural enemies, helping to preserve them. When timed correctly, insecticidal soaps and horticultural oils can be used to manage the crawler stage of most scales. Scales with extended periods of egg hatch may require repeat applications to achieve satisfactory management with soaps and oils since these treatments do not have residual effects.
Scales that overwinter as adults or nymphs are also vulnerable to dormant oil applications. These oils suffocate the scales. Scales that overwinter as eggs are not affected by oil treatments. Oils should be applied in spring before plant bud break (March-April). Thorough coverage of affected limbs is essential to achieve good control.
Additional low impact insecticides, available only to licensed applicators, are pyriproxyfen (e.g. Distance) and buprofezin (e.g. Talus). These insecticides are insect growth regulators (IGRs), preventing the immature nymphs from developing into functioning adults. These insecticides do not affect adults. These products are also useful in conserving natural enemies that are present.
Residual contact insecticides
There are a variety of residual insecticides available to residents that can be applied as sprays to infested trees and shrubs. They should be timed to coincide with crawler activity to be effective. These insecticides include pyrethroids (such as permethrin, bifenthrin, lambda cyhalothrin), acephate, and carbaryl. These products generally last one to two weeks after they have been applied as long as they have had time to dry before any rain.
Systemic insecticides are transported by the tree through the conductive tissue (xylem and phloem). Imidacloprid is applied as a liquid drench to the soil around the trunk of the tree (professional applicators can also apply it as a soil injection or a trunk injection). Dinotefuran is applied as granules to the soil directly around the tree (professional applicators can also apply it as a bark spray, soil drench or soil injection). Imidacloprid is effective at managing most soft scales. Dinotefuran is more mobile in the plant and is effective at reducing both soft and armored scales.
Caution: Be careful when applying systemic insecticides to hardwood trees and shrubs that are attractive to bees. This includes linden/basswood, crab apple, sugar maple, as well as juneberry (serviceberry), pagoda dogwood, nannyberry viburnum and many other shrubs. However this does not include evergreen trees and shrubs (To see a list of bee attractive trees and shrubs, click here).
Apply systemic insecticides to trees and shrubs only after flowering has already occurred to reduce pesticide exposure to bees. If the timing to make an application does not occur when crawlers are active, use an alternative method of treating the scales.
Also, do not use systemic insecticides that are applied to the soil when bee attractive flowers are planted directly adjacent to trees or shrubs.
Commercial tree care companies typically have experience in managing scale insects and in handling and applying pesticides. They also have access to products and procedures to manage scale insects that are unavailable or unfamiliar to residents. When treating trees is not practical for residents, they should use licensed pesticide applicators working for professional companies.
Caution: The information given herein is supplied with the understanding that no discrimination is intended and no endorsement by the University of Minnesota Extension. Mention of a pesticide or use of a pesticide label is for educational purposes only.A pesticide label is a legal document. Always follow the pesticide label directions attached to the pesticide container you are using. Pesticide labels may change frequently. Internet labels may not match the label on the container you are using. The site of use or plant to which the pesticide is to be applied must be listed on the label or the pesticide cannot be used. Remember, the label is the law.