Cottony cushion scale may look like a fungus, but it's an insect. Partial to tropical and subtropical climates, cottony cushion scale can be found on a wide range of woody ornamental plants and fruit trees, most often on their stems and trunks. The insects can be readily seen with the naked eye and recognised by the sticky substance they secrete, called honeydew. While they don't generally kill the host plant, cottony cushion scale can reduce the vigour of the plant and make it unsightly.
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Look for clusters of cottony cushion scale egg sacs on twigs and bark in summer. Sacs are about 3/8-inch long, with a white, waxy, protective coating that helps it adhere to the branch. The inside is filled with a cottony fluff. When the eggs hatch, nymphs are tiny and red, with dark legs and antennae. Adult female scale insects are 4.5mm long, while the males are 3mm long and have blue wings.
Cottony cushion scales can produce two to three generations of offspring per year. Once eggs hatch, first nymphs emerge and crawl toward leaves, which they feed on. After several weeks, the nymphs moult and become second nymphs, and then third nymphs. At this point, the female third nymphs crawl back to the twigs, moult into adults, and mate. Then they lay eggs and spin sacs and the life cycle begins again. Unless winter temperatures plunge below -12.2 degrees Celsius, the life cycle continues year-round, though more slowly in winter.
Look for the signature secretion of these insects, called honeydew. It looks and feels like sticky sap. Cottony cushion scale pierce leaves and bark with their mouth parts, sucking out juices from the plant. As they digest, they secrete honeydew on leaves and twigs. With a heavy infestation, it can feel like a tree is raining honeydew if you stand under it. Damaged leaves may darken, yellow and drop off early. While cottony cushion scale rarely kills a tree, it can weaken one to the point where it can die from other stressful conditions like drought or extreme heat.
Control and Management
Fortunately, there are both natural and chemical remedies for cottony cushion scale. Vedalia beetles and their larvae feed on scale insects by laying their egg sacs right under the scale's egg sac, and with a life-cycle of five to six weeks in summer, it can keep up with the rapidly reproducing scales. Another natural enemy, the parasitic fly, actually lays its eggs inside female scale insects, consuming them from the inside out.
Chemical control is not as effective as biological control, but organophosphate sprays like malathion and acephate will kill cottony cushion scale. Keep in mind that pesticides often kill beneficial insects as well as their targets, so use them only as a last resort.
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