Scale insects are divided into three groups: (1) armored scales, (2) soft scales, and (3) mealybugs. The armored and soft scales are one of the most destructive groups of insects that attack ornamental crops. Mealybugs are not generally considered a problem on most woody ornamentals.
The armored scales secrete a waxy covering over their bodies. This covering is not an integral part of the insect's body. The scale lives and feeds under this covering which resembles a plate of armor, hence the name. They vary in size from 1/16 inch to 1/8 inch in diameter and can be almost any color, depending on the species. Armored scales may be circular, oval, oblong, thread-like, or even pear-shaped. The female's armor is larger than that of the male, while the shape and color may be similar or distinctly different, depending upon the particular species.
Soft scales also secrete a waxy covering, but it is an integral part of their body. Soft scales vary widely in color, size, and shape. They range from 1/8 inch to 1/2 inch in diameter and may be nearly flat to almost spherical in shape.
Virtually every woody plant is subject to attack by one or more species of scales. Some scales attack only a few species of closely-related plants, while other species infest a wide variety of plants.
Scales are present year-round. Scale species may overwinter in any life stage but most overwinter as eggs or mated females which are the stages that best survive low winter temperatures. The egg stage and hatch of the crawlers are often correlated with the flush of new growth in the spring. However, each scale species has its own innate phenology that varies with temperature and also may be affected by the plant host species. Populations build up throughout the spring and summer months until by the end of the season all the life stages are present together.
On many ornamental plants, scale insects are the most serious pests, and most ornamentals are susceptible to one or more species of scales. Scales cause damage by sucking the juices from the plants. Heavily infested plants appear unhealthy and produce little new growth. Scales feeding on the undersides of leaves may cause yellow spots to appear on the top sides, and these spots progressively become larger as the scales continue to feed. If the scales are not controlled, leaves will drop prematurely, sometimes killing portions of twigs and branches. Scales also feed on trunks and stems of plants. Soft scales excrete large amounts of honeydew, which is rich in nitrogenous compounds and sugars. The honeydew excretion is an excellent medium for the
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growth of a fungus known as sooty mold. This black-colored fungus coats the top side of leaves, interfering with photosynthesis and makes the plants unattractive. Armored scales do not excrete honeydew.
Inspect plants closely at weekly intervals, especially plants where scale problems have occurred in the past. Since scale insects may occur on all plant parts, every part of the plant must be checked. Leaves should be examined on both surfaces, and particularly along the midrib of the underside. The use of a 10X hand lens or magnifying glass will aid in their detection. Crawlers may be detected by placing double-sticky tape on some branches of the plant. Stems should be checked around buds, leaf petioles and lenticles or other depressions which may afford a place for the scales to hide and feed. Pay particular attention to old wound scars where the bark is thin. Many times, the scale wax covering blends in with the bark and is difficult to see. The contrast between the bark and scale can be increased by wetting the bark with water before examining with a hand lens.
Dispersal of scales depends upon the movement of crawlers which are motile in all species. Wind may blow crawlers to other plants. Crawlers are also moved about by birds or other insects. The most important means of dispersal is the movement of infested plants by man.
Many species of scales are highly parasitized by tiny wasp parasites. Pin-sized holes in the wax are evidence of parasitism. When scouting, be sure to take into account the amount of parasitism before applying an insecticide. Scale crawlers are also preyed upon by numerous beneficial insects.
Be sure plants are free of scales before they are placed in the production area. Scales cannot fly, therefore they do not readily infest plants as do most other insects.
Scales, especially armored scales are very difficult to control when mature. Spray applications should be timed to coincide with the crawler stage which is most susceptible to insecticides.
Examine plants for live scales by crushing the wax cover. Dead scales do not fall from plants. Select pesticides that have the least effect upon other non-target organisms. For established infestations, apply a second application in two weeks. Horticultural oils are often effective and relatively safe on beneficial organisms. One or two applications of dormant oils should be applied to suppress established overwintering populations.
For the most current insecticide recommendations to control these pests, please contact the Phillips County Extension Office at 870-338-8027.