Tipping the scales
Scale insects are among the more unusual — and mysterious — insects associated with landscape plants. Sedentary creatures, a few kinds are economically useful, such as the lac scale (the source of shellac) and cochineal scale (from which a red dye is made). Unfortunately, far more are pests of our valuable ornamentals. Despite their inert appearance, some can cause substantial injury and even kill plants. They have an extraordinary ability to multiply and can be very difficult to control. Reviewing some basic information on scale insects can help you recognize them and successfully control infestations.
Identification and life cycle
Mature scales may resemble tiny seashells, raised blisters, tufts of cotton, plant galls or other curious manifestations, depending on the species. Many kinds are well camouflaged against bark or twigs and easily overlooked. Adult males are rarely observed, resemble tiny flies and live but a few hours.
In some species, males are unknown and females reproduce without mating. They typically lay eggs either under the cover or in a cottony mass of waxy threads. Some kinds of scales dispense with egg laying altogether and produce live young.
The newly hatched nymphs or youngest scale stage is known as a crawler. These are quite tiny, about the size of a pinpoint, and vaguely resemble a yellowish or orange flattened aphid. The aptly named crawlers move around the plant or may be carried by wind or other means to another host plant, where they find a place to settle and start to feed.
The immature scales molt through several stages. In the process, some kinds, such as armored scales, secrete a characteristic hard or felt-like cover. In other species, such as soft scales, the insect's own outer skeleton becomes the cover. There are one or more generations a year, depending on the species and location. References listed at the end of this article include color photographs and useful information for identifying a particular species. Some mature scales, such as calico scale and European elm scale, are relatively easy to recognize based on appearance. Some kinds are hard to identify using photographs or descriptions, so you are better off consulting an Extension service office or university diagnostic laboratory. The most effective controls are usually timed to affect the crawler stage, so determining the species will help to predict when you'll need to plan for an application.
Scales are sucking insects that feed on plant juices. Plants usually tolerate low populations of scales and natural enemies help keep the pests in check. Evidence of larger infestations are often first detected as deposits of sticky honeydew on foliage or objects below, followed by the growth of unsightly sooty molds. Some scales form unattractive white, cottony masses, coverings or powdery coatings on leaves, twigs or bark. Occasionally, infestations are so heavy that the scales themselves form obvious large encrustations on branches or twigs, typically accompanied by thinning or yellowing foliage and possibly branch dieback. In severe cases, they can even kill plants.
Types of scales
Most landscape professionals have heard the terms “armored scale” or “soft scale.” These are just two among the several scale groups or taxonomic families. Some insecticides are labeled specifically for controlling scales in a certain group, so it is helpful to be aware of these groups and to which family a particular pest species belongs. Following is a partial list of the more common groups of scale insects (and taxonomic family) and some familiar representatives.
Mealybugs (Pseudococcidae): Close relatives and classed with scale insects, these insects are typically covered with a white, mealy wax. Species include taxus mealybug on yew and other hosts, and maple mealybug on maples. A yellow spotting on azalea foliage has also been attributed to maple mealybug. Sooty mold is evidence of infestation. Mealybugs are often found in bark crevices or twig crotches.
Soft scales, wax scales and tortoise scales (Coccidae): Scales in this group produce copious honeydew and some may form large encrustations on twigs or limbs. Cottony scales form unattractive white egg masses under foliage. Common species include cottony camellia scale on camellia, yew, holly and a few others; calico scale and European fruit lecanium on numerous deciduous ornamentals; black scale, a southern species, on many plants; magnolia scale on magnolia; tulip-tree scale on magnolia and tulip tree; Fletcher scale on arborvitae and yew; cottony maple scale and cottony maple leaf scale on dogwood and many other hosts; and Indian wax scale on camellia, hollies and others.
Armored scales (Diaspididae): These leave no honeydew deposits and are among the more difficult scales to control. Unlike soft scales, the covers of armored scales are easy to detach. Common species include hemlock scale and elongate hemlock scale on hemlock; juniper scale on juniper and Chamaecyparis; pine scale and pine needle scale on pines; Florida red scale on citrus and many southern ornamentals; oystershell scale on many deciduous hosts and boxwoods; white peach and white prunicola scales on Prunus, lilac and others; euonymus scale on several hosts, such as euonymus, daphne, camellia and pachysandra.
Felt scales or eriococcids (Eriococcidae): Azalea bark scale settles on bark of azalea, rhododendron, andromeda and Kalmia; European elm scale is often found by twig crotches on elm. Infestations of either are often accompanied by heavy sooty mold.
Margarodid scales (Margarodidae): This group includes the notorious cottony cushion scale, an exotic which launched classical biocontrol in California during the late 1800's, and the unusual Matsucoccus scales which may be very damaging to pines.
Bark crevice scales (Cryptococcidae): Beech scale forms tiny cottony masses in bark cracks of beech trunks and limbs. Infestations commonly precede devastating beech bark disease caused by invasions of Nectria fungi.
Gall-like scales or kermesids (Kermesidae): On oaks, pin oak kermes and northern red oak kermes sometimes cause twig dieback, which may be mistaken for injury caused by twig girdlers, twig pruners or cicadas. Check for scales at the base of affected portions.
Pit scales or asterolecaniids (Asterolecaniidae): Oak pit scales, such as golden oak scale, cause pits in bark of oak twigs and sometimes decline and dieback. Holly pit scale on certain hollies and pit-making pittosporum scale on many woody ornamentals are other common species that cause distorted growth and dieback.
Visual inspection is the best way to detect scale insects. Several good references are available with excellent color photographs and details on scale biology. A hand lens is helpful for seeing smaller stages and noting scale covers with holes, which may indicate parasitism. Trails of ants, which feed on honeydew, may be a clue to honeydew-producing scale or mealybug infestations. Regional or local diagnostic labs can help you identify species when needed. When you do discover infestations, wrap double-sided sticky tape around twigs to detect the presence of crawlers, which get trapped as they attempt to walk across. Double over the end to make it easy to remove and check every week or so.
Scales have numerous natural enemies, including various beetles, wasps and even critically timed adverse weather, which help keep populations in check. Small holes in scale covers are evidence of parasite or predator activity. On Long Island, N.Y., we have seen cottony maple leaf scale infestations rise and fall in cycles due to natural enemies or other factors. Infestations that are not generally a serious threat to plant health can often be tolerated. A few scale natural enemies can be purchased from suppliers, but most releases have been done in orchards and greenhouses, and there is little experience with landscape plants. If you are contemplating introductions of natural enemies, consult with suppliers to see if there is a suitable match for the pest, because beneficials may be fairly specific to certain scales.
Little information exists on scale-resistant cultivars. ‘Manhattan’ euonymus (E. kiautschovicus ‘Manhattan’) is rarely troubled with the notorious euonymus scale, as one example, and the literature reports a few other cases. Problem-prone plants might also be candidates for replacement with less susceptible or resistant species. Culturally, take care to limit fertilizer applications around infested plants, because excess nitrogen may be associated with increased scale problems. Prune off heavily infested branches. Some professional arborists lightly scrub off heavy infestations of white prunicola and other bark scales. Before leaves begin to expand in spring, you can also use a forceful jet of water to “power wash” scales from bark and limbs.
Insecticides are useful tools for managing scale insects, but timing, as they say, is everything. For sprays, at least two periods are critical. Dormant-stage applications of horticultural oil (3- to 4-percent rates) are used for many scale species, just before budbreak in early spring or late winter when plants are nearing the end of their dormant period. However, this sometimes yields poor results against certain armored scales (such as euonymus scale). Horticultural oils should not be used when temperatures are expected to drop below 40°F or go above 90°F, or when plants are under drought stress. Some plants, such as walnut and Cryptomeria are known to be sensitive to horticultural oil. Oil also removes the bluish coloring from conifers, such as blue spruce, and should not be mixed with certain pesticides. Fall dormant applications may increase susceptibility to damage during winter, although some southern applicators apply oils well after plants have fully hardened off in early winter. Check with local guidelines and read labels for specific cautions and further information.
The second control period targets the scales' crawler stage, which is quite susceptible to many insecticides. Usually, you should apply sprays after the majority of crawlers have emerged, although you may need to repeat applications where emergence occurs over a long time. Sticky tape, mentioned earlier, is one method to detect periods of crawler activity. Pheromone traps are available for a few scale species, (such as San Jose and California red scales) to attract and trap males. Sprays for crawlers are scheduled based on regular trap counts. You can also time applications using growing degree-days, a method of accumulated heat units that predicts expected crawler emergence. You can find details on calculating growing degree-days from daily temperatures in university pest management guides or at the Grounds Maintenance website at www.grounds.mag.com (search articles by typing keywords “Degrees of freedom”). Inexpensive data loggers are now available that collect this information.
Currently labeled insecticides include a range of products (see table, above). When choosing an insecticide to control crawlers, be aware that some persistent insecticides may be also harmful to the natural enemies of scales, and also lead to an increase of mites or other pests. Products for nursery production should have appropriate WPS labeling (the ‘Agricultural Use Requirements’ box) which includes appropriate post-spray re-entry intervals.
Soil application and trunk injection are other methods for delivering systemic insecticides to their target. Imidacloprid (Merit) is labeled for soil application to control soft scales or provide suppression of armored scales. While the topic of timing is still open to some debate, application around late fall or early spring may provide the best results. However, local experience might suggest a different, more optimal timing.
Imidacloprid also is formulated for trunk injection (Pointer or Imicide). Bidrin (Inject-A-Cide B), oxydemeton-methyl (Harpoon or Inject-A-Cide) and acephate (Dendrex for needle scale on redwood and most pines; Acecap 97) are other products for trunk injection. Trunk injections should provide fairly rapid results, so labels generally recommend application when crawlers appear.
Read them, follow them
Before applying any pesticide, be sure to read the label and follow all directions carefully. Some pesticides, formulations, labels or uses may not be approved in all states, so verify state registration status before use.
Dan Gilrein is a horticulture specialist with the Cornell Cooperative Extension Long Island Horticulture Research and Extension Center (Riverhead, N.Y.)
Proper identification is key to defining control options for scales. The following references contain useful ID sections.
Johnson, W. T. and H. H. Lyon. 1988. Insects that feed on trees and shrubs, 2
nded. Cornell University Press, Ithaca.
Kosztarab, M. 1996. Scale insects of northeastern North America. Virginia Museum of Natural History, special pub. No 3. Martinsville
Hamon, A. B. and M. L. Williams. 1984. Soft scale insects of Florida. Arthropods of Florida v. 11. Florida Dept. of Agriculture and Consumer Services, Gainesville.
Dekle, G. W. 1965. Florida armored scale insects. Arthropods of Florida v. 3. Florida Dept. of Agriculture and Consumer Services, Gainesville.
Dreistadt, S. H. et al. 1994. Pests of landscape trees and shrubs. Univ. of California Div. of Agriculture and Natural Resources publication 3359, Berkeley.
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