How to Manage Pests
Pests in Gardens and Landscapes
Scale insects can be serious pests on trees, shrubs, and other perennials. The impact of infestations depends on the scale species, the plant species and cultivar, environmental factors, and natural enemies. Populations of some scales can increase dramatically within a few months, such as when honeydew-seeking ants or dusty conditions interfere with scale natural enemies. Plants are not harmed by a few scales, and even high populations of certain species apparently do not damage plants. Soft scales and some other species excrete honeydew, a sweet, sticky liquid produced by insects that ingest large quantities of plant sap. Sticky honeydew and the blackish sooty mold growing on honeydew can bother people even when scale populations are not harming plants.
Scales are unusual looking and many people do not at first recognize them as insects. Adult female scales and most immatures (nymphs) are immobile, wingless, and lack a separate head or other recognizable body parts. Immature scales and adult females have a characteristic round or oval to elongate and flattened or humped appearance. Immature males are often a different color and shape than females, especially in later nymphal stages (instars). Adult male scales are tiny, delicate insects with one pair of wings. Adult males are rarely seen, do not feed, and live only a few hours.
Scales insert a tiny strawlike mouthpart into plants and suck fluids. Scales can occur on bark, fruit, or leaves. Armored scales and soft scales are the most common groups (families). Common species of scales and their tree and shrub hosts are listed in Tables 1–3. Excellent color keys for scale insects in California are available from the California Department of Food and Agriculture; see References for titles.
It is important to correctly distinguish the scale family to which your pest species belongs. For example, a popular systemic insecticide (imidacloprid, discussed below) controls European elm scale and most soft scales but does not control armored scales or cottony cushion scale. Imidacloprid can dramatically increase cottony cushion scale populations because it is very toxic to one of its natural enemies, the vedalia beetle, Rodolia cardinalis. This important cottony cushion scale predator is poisoned by feeding on scales that have ingested the insecticide.
Armored scales, family Diaspididae, have a flattened, platelike cover that is less than 1/8 inch in diameter. The actual insect body is underneath the cover. The covers often have a differently colored, slight protuberance (exuviae, or “nipple”). Concentric rings form as each nymphal stage (instar) secretes an enlargement to its cover. Armored scales do not excrete honeydew. Damaging species include California red scale, euonymus scale, oystershell scale, and San Jose scale.
Soft scales, family Coccidae, can be smooth, cottony, or waxy and are 1⁄4 inch long or less. They are usually larger and more rounded and convex than armored scales. Their surface is the actual body wall of the insect and cannot be removed. Soft scales feed in the fluid-conducting phloem tissue of the plant and excrete abundant honeydew, which is sugary water that drips from their bodies. Soft scales include black scale, brown soft scale, European fruit lecanium scale, and various Pulvinaria species.
Other Species of Scale
Species in other insect families include cottony cushion scale and sycamore scale (the woolly sac scales, family Margarodidae), European elm scale (felt scales, Eriococcidae), and oak pit scales (Asterolecaniidae).
Various other organisms may be confused with scales. These include California laurel aphid (Euthoracaphis umbellulariae), coconut mealybug (Nipaecoccus nipae), cypress bark mealybug (Ehrhornia cupressi), parasitized whitefly nymphs, and psyllids such as lemongum lerp psyllid (Cryptoneossa triangula) and redgum lerp psyllid (Glycaspis brimblecombei). Diamond “scale” infesting palms is actually the fruiting bodies of Sphaerodothis neowashingtoniae, an Ascomycetes fungus!
Females of many scale species reproduce without mating (there are no males). At maturity, adult females produce eggs that are usually hidden under their bodies or covers. Eggs hatch into tiny crawlers (mobile first-instar nymphs), which are yellow to orangish in most species. Crawlers walk over the plant surface, are blown by wind to other plants, or can be inadvertently moved by people or birds. They settle down and begin feeding within a day or two after emergence.
Settled nymphs may spend their entire life in the same spot without moving as they mature into adults. Nymphs of other species can move slowly but rarely do, such as when species that feed on deciduous hosts move from foliage to bark in the fall before leaves drop. For species with multiple generations, all scale life stages may be present throughout the year in areas with mild winters.
Most armored scales have several generations a year. Armored scales overwinter primarily as first-instar nymphs and adult females. Except for crawlers and adult males, armored scales spend their entire life feeding at the same spot. Settled armored scales lose their legs, molt, and form their characteristic covers, which they gradually enlarge as they grow.
Most soft scales have one generation each year and overwinter as second-instar nymphs. The multi-generational brown soft scale is an important exception. Brown soft scale females and nymphs of various size can be present throughout the year. Most immature soft scales retain their barely visible legs and antennae after settling and are able to move, although slowly. At maturity, females of certain soft scales, the woolly sac scales (Margarodidae), and some other species produce distinct external cottony or wax-covered egg masses.
When plants are heavily infested with scales, leaves may look wilted, turn yellow, and drop prematurely. Scales sometimes curl leaves or cause deformed blemishes or discolored halos in fruit, leaves, or twigs. Bark infested with armored scales may crack and exude gum. Certain armored scales also feed on fruit, but this damage is often just aesthetic. Soft scales infest leaves and twigs but rarely feed on fruit. A major concern with soft scales is their excretion of abundant honeydew, which contaminates fruit, leaves, and surfaces beneath plants. Honeydew encourages the growth of black sooty mold and attracts ants, which in turn protect scales from natural enemies.
When numerous, some scale species weaken plants and cause them to grow slowly. Branches or other plant parts may die if they remain heavily infested with scales. If plant parts die quickly, dead brownish leaves may remain on branches, giving them a scorched appearance. Several years of severe infestations may kill young plants. Certain armored scales may be more likely to kill plants. Soft scales reduce plant vigor, but seldom kill trees or shrubs.
Scales are often well controlled by beneficial predators and parasites, except when these natural enemies are disrupted by ants, dust, or application of persistent broad-spectrum insecticides. Preserving (conserving) the populations of parasites and predators (such as by controlling pest-tending ants) may be enough to bring about gradual control of scales as natural enemies become more abundant. If scales become too numerous, a well-timed and thorough spray using horticultural (narrow-range) oil applied either during the dormant season or soon after scale crawlers are active in late winter to early summer should provide good control. Complete spray coverage of infested plants (such as the underside of leaves) is needed to obtain good control. Thorough spray coverage is especially critical when treating armored scales and oak pit scales, as these scales are generally less susceptible to pesticides than soft scales.
Inspect plants to determine whether female scales, nymphs, honeydew, or sooty mold are present. When assessing whether scales or their damage are abundant enough to prompt you to manage them, distinguish live scales from dead or parasitized ones by flipping over the female scale body or cover using a sharp tool. The dead scales from previous generations can remain on plants, and sometimes a large proportion of scales are dead or parasitized by natural enemies. During the growing season, inspect trunks for ants. If the descending ants have swollen, almost translucent abdomens, they are probably feeding on honeydew produced by scales or other insects that suck plant juices. Tracing back trail-making ants can lead you to colonies of the honeydew-producing insects.
Action thresholds before spraying have not been established for scales in most situations. Monitor and record scale densities and use the density that caused damage (dieback or unacceptable honeydew) as your preliminary control action threshold. Refine this threshold over time for your local situation as you gain experience.
Tape traps for crawlers discussed in the Foliage Sprays section and honeydew monitoring are useful in certain situations. Honeydew dropping from plants can be efficiently monitored using water-sensitive paper, which is commonly used for monitoring insecticide droplets and calibrating sprayers. Products include bright yellow cards that produce distinct blue dots upon contact with honeydew or water. Regularly monitoring honeydew beneath plants (such as the number of drops during four hours, once a week) can help to develop thresholds and evaluate effectiveness of the treatment. Honeydew monitoring is useful where there is a low tolerance for dripping honeydew, when managing many trees (such as along city streets or in parks), and on tall trees where the honeydew-producing insects may be located too high to easily observe. For more information on monitoring honeydew, see the Aphids section in Pests of Landscape Trees and Shrubs.
Provide plants with good growing conditions and proper cultural care; especially appropriate irrigation, so they are more resistant to scale damage. Prune off heavily infested twigs and branches to eliminate scales when infestations are on limited parts of the plant. Pruning to open up tree canopies helps to control black scale, citricola scale, and possibly other species in areas with hot summers, such as the Central Valley of California. This pruning increases scale mortality as a result of heat exposure.
When landscaping, choose plants that are relatively pest-free and well adapted to local conditions. Consider replacing problem-prone plants. Most pests are highly host specific. Scales that can feed on many different plants usually damage only certain of these plant species and though present, do not damage other species or cultivars in the same plant genus. Notable exceptions include California red scale (which can severely infest and damage all Citrus species) and San Jose scale (which damages many different fruit and nut trees).
Scales are often controlled by small parasitic wasps and predators including beetles, bugs, lacewings, and mites. Predatory Chilocorus, Hyperaspis, and Rhyzobius species lady beetles (ladybugs) can easily be overlooked because many are tiny, colored and shaped like scales, or feed beneath scales. Hyperaspis species are tiny, shiny, black lady beetles with several red, orange, or yellow spots on the back. Rhyzobius lophanthae has a reddish head and underside, and a grayish back densely covered with tiny hairs. The twicestabbed lady beetle, Chilocorus orbus, is shiny black with two red spots on its back, and reddish underneath. The multicolored Asian lady beetle, Harmonia axyridis, is a relatively large, variably colored species. It is mostly orange, red, or yellow with 19 large dark spots, or fewer, smaller, or no spots on its wing covers. The alligatorlike larvae of lady beetles often occur openly on plants. In certain species, small larvae are hidden under the female scale body or cover where they feed on scale eggs and crawlers.
Parasitic wasps are often the most important natural enemies of scales. Parasites include many species of Aphytis, Coccophagus, Encarsia, and Metaphycus. The female wasp lays one or several eggs in or on each scale, where the tiny maggotlike wasp larvae feed. When parasitized, some scales are darker-colored than normal. Estimate parasite activity before making a treatment decision. Check for discolored (parasitized) scales and scale covers with round exit holes made by emerging adult parasites. Lift the covers of armored scales and examine beneath them for immature parasites.
Natural enemies, or beneficial insects, are commercially available for release against California red scale and perhaps certain other scales. However, conserving resident natural enemies is a more efficient and longer lasting strategy than buying and releasing beneficials in gardens and landscapes.
Ant control, habitat manipulation, and pesticide management are the key conservation strategies. Grow flowering plants near scale-infested trees and shrubs to help attract and support natural enemies. Adults of predatory bugs, lacewings, lady beetles, and parasitic wasps live longer, lay more eggs, and kill more scales when they have plant nectar or pollen and insect honeydew to feed on. Minimize dust, which interferes with natural enemies. For example, wash plant surfaces midseason, or when the foliage is covered with dust.
Depending on the scale species and the extent to which biological control has been disrupted, it will take several months of conservation efforts (such as controlling ants and dust and avoiding application of persistent insecticides), or until the next season or longer, before scale populations are reduced by biological control. If current levels of scales are intolerable, use a short residual insecticide such as oil or soap to reduce scale populations while conserving natural enemies.
Control ants if large numbers of them are climbing up trunks to tend scales. Deny ants access to plant canopies by pruning branches or weeds that provide a bridge between buildings or the ground and by applying a sticky material (such as Tanglefoot) to trunks. Wrap the trunk with a collar of fabric tree wrap, heavy paper, or masking tape to avoid injury to bark; wedge pliable wrap snugly into cracks and crevices; and coat the wrap with the sticky material. A barrier band about 2 to 6 inches wide should be adequate in most situations. Increase the longevity of the sticky material by applying it higher above the ground to reduce contamination by debris and sprinkler wash-off.
Do not wrap trunks too tightly. Inspect wraps at least several times a year for damage to bark, such as constricted growth or injured tissue. Remove and relocate any wrap at least once a year to minimize bark injury. Check sticky material at least every 1 to 2 weeks; stir it with a stick to prevent ants from crossing on collected debris. Avoid applying sticky material to horizontal surfaces where birds may roost.
Alternatively, or as a supplement, place enclosed pesticide baits (insecticide mixed with an attractant) such as ant stakes near nests or on ant trails beneath plants. Effective bait insecticides are slow-acting, so that before they die, workers will spread the toxicant among many other ants during food sharing. Boric acid, fipronil, and hydramethylnon are examples of insecticides used in ant baits. Although baits act slowly over a period of hours to weeks (requiring users to be patient), baits can be much more effective than sprays. Sprays only kill foraging workers, while ant baits are carried back to their nests, where reproductive queens and the entire colony underground can be killed. See Pest Notes: Ants for more information.
Horticultural oil (where plants can be sprayed) and certain systemic insecticides are preferred chemicals for most situations when scales are numerous enough to cause damage.
Dormant Season Sprays
Dormant season application of a narrow-range or horticultural oil is effective against most scale pests on deciduous woody plants. A good time to apply oil is when no rain or fog is expected within a day. On leafless host plants, application can be made any time when weather is suitable during dormancy to control lecanium scales, San Jose scale, walnut scale, and many other species. For oak pit and sycamore scales, spray during the delayed-dormant period, which is after the buds swell but before buds open. Do not spray oystershell or olive scales during the dormant season because susceptible stages of these pests are not present during winter.
Oil Use and Precautions
Horticultural oils are specially refined petroleum products, often called narrow-range, superior, or supreme oils. Some botanical (plant-derived) oils are also available. Thoroughly spraying plants at the proper time with oil alone usually provides adequate control. One study of sycamore scale found that organophosphates (e.g., malathion) or organophosphates combined with oil were no more effective than a thorough spray of oil or insecticidal soap alone, if properly timed during the delayed-dormant period. Another study of citricola scale on hackberry found spraying oil on foliage once during late spring after most crawlers emerged was as effective as using a systemic organophosphate (acephate).
Avoid products called dormant oil or dormant oil emulsions. These are more likely to injure plants. Do not apply oil when it is foggy, freezing (under 32°F), hot (over 90°F), or rainy, or within a day of when these conditions are expected. If leaves are present, to avoid oil injury, be sure only to apply to well watered plants. Do not apply horticultural oil to deciduous trees within 30 days before or after application of captan, sulfur, or certain other fungicides to avoid damage to trees.
Horticultural oil is effective in spring or summer on deciduous plants when sprayed soon after most crawlers have emerged. Late spring and summer are also the times to spray avocado, citrus, and many other broadleaf evergreens. Foliage-season application requires more spray volume on deciduous plants than a dormant treatment because leaves as well as bark must be thoroughly covered. Spring or summer sprays should thoroughly cover plant parts where most scales occur (typically twigs and the underside of leaves) and be well timed to occur when most scales are crawlers or young nymphs, the most susceptible stages.
To effectively time a spray of leafy plants, use traps made of double-sided sticky tape to determine when crawlers are hatching. Before crawlers begin to emerge in spring, tightly encircle several infested twigs or branches with transparent tape that is sticky on both sides (this tape is available at stationery stores). Change the tapes approximately once a week and examine the tapes with a hand lens to identify the crawlers. Once scale eggs begin hatching, scale crawlers get stuck on the tapes and appear as yellow or orange specks. Spray after crawler production has peaked and the number trapped each week has definitely declined, which is soon after most crawlers have settled. For more information on sticky tape monitoring, consult Pests of Landscape Trees and Shrubs.
In addition to narrow-range oil, insecticidal soap or a mixture of oil and soap can be sprayed. Carbamate insecticides (carbaryl or Sevin), organophosphates (e.g., malathion), and pyrethroids (e.g., bifenthrin and cyfluthrin) are other contact insecticides registered for scale control. However, these more persistent insecticides cause greater disruption of biological control than oil or soap because persistent residues continue to kill or intoxicate beneficial insects and mites migrating in after the application. It is also best to avoid the more persistent sprays because of problems from their runoff into urban surface water and contaminating municipal wastewater.
Systemic insecticides are absorbed and moved within plants. If the plant is infested with a scale species susceptible to systemic insecticide, systemics are particularly useful where it is not practical to spray because plants are large or spray may drift to unintended areas. Depending on the product, systemic insecticide may be applied by spraying foliage, injection or implantation into trunk vascular tissue, or applying on or into soil beneath trees and shrubs, where the insecticide is absorbed by roots.
A soil application of the systemic insecticide imidacloprid can provide season-long control of soft scales and certain other species such as European elm scale. Imidacloprid is not effective on armored scales and certain other species such as cottony cushion scale. Imidacloprid is available to both homeowners (Bayer Advanced Garden Tree & Shrub Insect Control) and professional applicators (Merit and others). Imidacloprid can be effective when applied to soil during late winter to early spring or before rainfall or irrigation are expected to facilitate root absorption of the insecticide. Summer application to stressed, heavily infested trees is less likely to be effective and is not recommended.
Certain other systemic insecticides are also available, but only to professional applicators. For example, dinotefuran (Safari) is a neonicotinoid (in the same chemical group as imidacloprid) that can be sprayed onto foliage or applied onto soil or through certain irrigation systems, especially to control cycad scale and soft scales.
When using systemics, consider making a soil application whenever possible instead of spraying foliage or injecting or implanting trunks. It is difficult to place insecticide into trunks at the proper depth. Also, unsterilized injection tools contacting internal parts of multiple plants may mechanically spread certain plant pathogens, including bacteria (e.g., slime flux or wetwood), fungi (e.g., vascular wilt pathogens), and viruses. The physical act of penetrating trunks during injections injures plants. Especially avoid methods that cause large wounds, which may lead to wood decay. Do not inject or implant trunks more than once a year.
Dreistadt, S. H., J. K. Clark, and M. L. Flint. 2004. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, UC Statewide IPM Program. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3359.
Flint, M. L. 1998. Pests of the Garden and Small Farm: A Grower’s Guide to Using Less Pesticide. UC Statewide IPM Program. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3332.
Flint, M. L., and S. H. Dreistadt. 1998. Natural Enemies Handbook: The Illustrated Guide to Biological Pest Control. UC Statewide IPM Program. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3386.
Geisel, P., and E. Perry. 2004. Pest Notes: Oak Pit Scales. UC Statewide IPM Program. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7470.
Gill, R. J. 1982. Color-Photo and Host Keys to the Armored Scales of California. Scale and Whitefly Key #5. Sacramento: Calif. Dept. Food Agric.
Gill, R. J. 1982. Color-Photo and Host Keys to the Soft Scales of California. Scale and Whitefly Key #4. Sacramento: Calif. Dept. Food Agric.
Gill, R. J. 1988-1997. The Scale Insects of California Parts 1-3. Sacramento: Calif. Dept. Food Agric.
Grafton-Cardwell, E. E. 2003. Pest Notes: Cottony Cushion Scale. UC Statewide IPM Program. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7410.
Klotz, J., and M. Rust. 2005. Pest Notes: Ants. UC Statewide IPM Program. Oakland: Univ. Calif. Div. Agric. Nat. Res. Publ. 7411.
UC Statewide IPM Project. 2000. Pest Notes: Sycamore Scale. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7409.
Pest Notes: Scales
Authors: S. H. Dreistadt, UC Statewide IPM Program, Davis; J. G. Morse, Entomology, UC Riverside; P. A. Phillips, UC Statewide IPM Program, Ventura Co.; R. E. Rice, Entomology, emeritus, Kearney Agricultural Center, Parlier
Produced by University of California Statewide IPM Program
PDF: To display a PDF document, you may need to use a PDF reader.