Soft Scales

    Soft Scales

    brown scale on a houseplant Brown scale on a house plant.
    Cottony maple scale Cottony maple scale.
    european elm scale European elm scale.
    fletcher scale on yew European elm scale. 
    lecanium scale and honeydew Lecanium scale.

    HOSTS

    • Beech
    • Elm
    • Linden
    • Maple
    • Oak
    • Pine
    • Yew, and other conifers

    DESCRIPTION

    Soft Scales are immobile insects that anchor a straw-like mouthparts to feed on plant phloem or sap. The most common soft scale in Utah is the European fruit lecanium scale. In general, adult scales are hemispherical, with either a glistening-smooth, waxy body, or white cottony surface. Newly hatched scales are called crawlers. They are pale cream-colored, almost flat, and require a 20x hand lens to view.

    BIOLOGY

    Life cycles vary among soft scale species. Most species have one generation per year, but several life stages can be present at one time. In Utah, immature or unmated females overwinter on bark or twigs, and males overwinter in a pupal case. In spring, they resume their growth, feeding and mating. In some species where males do not exist, female scales reproduce asexually through parthenogenesis. Females produce about 200-1,000 eggs, either under her body or in an egg sac at the hind end of her body that is covered with a thick, fluffy wax. Some soft scale species give birth to live young, similar to aphids (i.e., viviparous birth). The female dies shortly afterward, but her body can harden to protect the crawlers. After hatching, crawlers migrate to foliage for the summer and before leaf drop, migrate back to the bark for the rest of their lives.

    SYMPTOMS

    • Yellowing of leaves
    • Premature leaf drop
    • Death of twigs and limbs
    • Excretions of copious amounts of honeydew

    GENERAL MANAGEMENT

    A heavy soft scale infestation is usually a sign that the tree or shrub was already under some kind of stress. Therefore, keeping plants healthy will minimize the negative effects caused by feeding from scales. Monitor soft scale crawler activity by attaching and examining double-sided sticky tape to limbs where scales occur. Scrub infested limbs with a mesh dish sponge to remove adults and old waxy caps.

    Soft scales have many natural enemies, such as predators and parasitic wasps. In most cases, natural enemies will help regulate soft scale populations. Lady beetle adults and larvae and lacewing larvae will eat crawlers and nymphs. Parasitoid wasps lay a single egg within the scale and the developing wasp consumes the scale contents.

    INSECTICIDES

    Low toxicity products will conserve more natural enemies, and include oil, soap and azadirachtin. Dormant oil applied at bud break will also provide a good (and safe) level of control.  

    Conventional insecticides are generally broad-spectrum and will kill beneficial insects. Use these products (pyrethroids, carbaryl) with discretion.


    Precautionary Statement: Utah State University and its employees are not responsible for the use, misuse, or damage caused by application or misapplication of products or information mentioned in this document. All pesticides are labeled with ingredients, instructions, and risks, and not all are registered for edible crops. “Restricted use” pesticides may only be applied by a licensed applicator. The pesticide applicator is legally responsible for proper use. USU makes no endorsement of the products listed in this publication.