SCALE INSECTS
General Description
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.
Important Species
Three hundred-fifty species of armored scales occur in the United States and about 175 species are present in Florida. Eighty-five species of soft scales occur in the United States, with 60 species in Florida.
Some of the more common scales on woody plants in Florida include:
Armored Scales:
Soft Scales:
Biology
The armored scale life cycle is generalized as follows. The eggs are laid underneath the waxy covering and hatch over a period of one to three weeks. The newly hatched scales (called crawlers) move about over the plant until they locate succulent new growth. They insert their piercing-sucking mouthparts into the plant and begin feeding. Female scales lose their legs and antennae during the first molt. They molt a second time before reaching maturity and do not pupate. The cast skins (exuviae) are incorporated in the scale cover. Male scales go through two additional molts and pupate underneath the wax. Adult males are tiny two-winged, gnat-like insects without mouthparts. In some armored scales the adult stage is reached in six weeks, and there are several generations per year.
In the females of the soft scales the antennae and legs are not lost, but are reduced to such an extent that though the adults can move about somewhat they seldom do. The wax when secreted, usually forms a sac at the rear end of the body enclosing the eggs, and the scale on the back of the insect becomes much thickened, forming a thick fluffy mass. The life cycle is similar to the armored scales except some soft scales require one year to reach maturity.
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.
Seasonal Abundance
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.
Damage
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 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.
Sampling
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.
The easiest and most effective method to detect the crawlers and time sprays is to use sentinel plants as follows. Select two to four plants infested with the target scale species and place them in an isolated spot on the side of the bed. The sentinel plants must be maintained under conditions similar to the rest of the plants in the bed. Use double-stick tape along with visual inspections one to three times a week to detect crawler emergence. Make management decisions about the scale infestation based on the data gathered from the sentinel plants. You may or may not want to spray the sentinel plants along with the rest of the bed. Each species of scale present should be treated the same with separate groups of sentinel plants. The seasonal abundance of a given species of scale will not often correlate with another species even when they are in close proximity on the same plant species.
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 such as the Putnam, tesserata scale, cyanophyllum scale, mining scale and lesser snow scale are highly parasitized by tiny wasp parasites. Pin-sized holes in the wax are evidence of parasitism. When scouting the nursery, be sure to take into account the amount of parasitism before applying an insecticide. Scale crawlers are also preyed upon by numerous beneficial insects.
Cultural Control
Be sure plants are free of scales before they are placed in the production area of the nursery. Scales cannot fly, therefore they do not readily infest plants as do most other insects.
Chemical Control
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 local county office of the Cooperative Extension Service. If you have access to the World Wide Web you can consult the University of Florida's Insect Management Guide at:
http://edis.ifas.ufl.edu/TOPIC_GUIDE_Insect_Management_Guide
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