Scale Insects

The world of insects and other arthropods offers tremendous diversity, and you cannot help but be amazed by the things you can see if you really take a close look around you. Our garden plants are prime examples, offering up a wonderful array of animal life, along with the life and death struggles they create when insects encounter each other. One group of fascinating, but destructive, insects is the Scale Insects, and yet to look at them you probably have a hard time even perceiving these things as insects. By far, they look more like distortions of the plant itself, perhaps a disease or fungus growth.

Scale insects constitute a group of sucking insect pests in the same Order of insects that includes aphids, whitefly, and leafhoppers, but the scales stand out above all the rest with their curious life styles. There are “soft” scales and “armored” scales, and it is the large, sometimes colorful soft scales that attract our attention the most. As a sucking insect that feeds on the juices of the host plant, the scale insect tends to discharge a great deal of excess sap in the form of a sticky goo we fondly call “honeydew”. This excretion falls as a mist or droplets onto surfaces below, and this may be leaves or it may be sidewalks or cars that are parked under the infested tree. You have probably noticed this at one time or another as you walked along a sidewalk and saw a very damp look to the paving. If you were careless enough to step into this mess you would have enjoyed the sticky film on your shoes for quite awhile after.

Aphids and mealybugs also produce this copious supply of honeydew, and since it is high in sugar it becomes very attractive to insects that feed on sugary liquids, principally ants and yellow jackets, and if you have plants with a heavy invasion of scales or aphids the resulting honeydew is just like clanging the dinner bell for these other annoying bugs. Honeydew can easily be washed off of both plant and sidewalk with a mild solution of water and dish-washing liquid soap. One interesting side note to this is the “lac insects” of Southeast Asia, which are scales that produce huge quantities of resin-like excreta, and this material for many years was collected and processed into the lacquer that was used as a high-polish finish for fine furniture.

So, let’s take a close up look at some scale insects to see just what makes it an insect. By the time you see the large blob on your plants what you actually are seeing is the thick wax cover the female insect has created for herself, and she is hidden below. The male of most scale insects looks like a tiny little wasp, and it does not feed on plants at all. In fact, the male scale insect does not feed on anything, and shortly after mating he will die. Many scale insect species do produce their offspring by mating between males and females, while other species are what is called “parthenogenetic”, and the female is able to produce her eggs and offspring without fertilization from a male.

While we still have not answered that burning question of which came first, the chicken or the egg?, for our purposes here we will begin with the chicken…in this case the fully developed female scale insect. That waxy blob you see on the stem of your shrub or tree contains the female insect and eventually her many hundreds of fertilized eggs. After these eggs hatch the baby scales, called “crawlers”, work their way out from under the wax shell of their mother, and move onto softer plant tissues to feed. This tiny little dots can be seen moving about on the plant, and this is actually the best time to try to control them if insecticides are going to be used, for at this time they are exposed and vulnerable.

These crawlers represent both male and female insects, and for the next few weeks they feed and move about until they have molted to their final nymph stage. At this point they settle in for good, the males molting once more to the winged form and the females molting to the “sessile”, or stationary form. The adult female may not even possess legs or antennae, for she has no use for these clumsy appendages under the wax shell she begins to secrete. The color, size, and shape of the scale she produces will be typical for each different species of scale insect. Some are brown and look like pencil erasers. Some are orange or brown and flattened. Some are black, some fuzzy, and some, like the Cottony Cushion Scale, have large, powdery white areas of wax. The “armored” scales tend to be smaller than the soft scales, and a very common pest species of armored scale is called the Oystershell Scale, due to its resemblance to this marine animal.

The scale insects can be very damaging to a plant. They feed by inserting their mouthparts into the soft tissue of the plant and sucking out the plant juices. The oystershell scale is well known for its attacks on many kinds of fruit trees or ornamental shrubs, sometimes completely covering limbs with the waxy scales, and these heavily infested plants are in danger of being killed. The San Jose Scale is a small species that became a terrible problem in the citrus groves of California in the late 1880’s, having been imported from the Orient. While the threat to the citrus crop was eventually resolved, this species still occurs throughout the United States. The large Cottony Cushion Scale has been another threat to our citrus crops.

The presence of the feeding scale also can cause unusual distortions on a plant. The Oak Pit Scale, for example, feeds on the softer green stems of oak trees, and the stem tissue around the scale swells dramatically to form a dime-sized circle with a sunken “pit” in the center, in which the scale resides. Other obvious scale species include the Black Scale that is common on olive trees, Kuno Scale common to plum trees, pear trees, pyracantha bushes, and other ornamental shrubs and trees, or a number of scale species that invade greenhouses and feast within that warm, humid climate. There are even scale insects called Ground Pearls that live in the soil and feed on the roots of plants. They sometimes cause severe damage to lawns, and the evidence may be large patches of your beloved lawn turning brown and dying.

Another approach is to use an application of an oil insecticide. This still is better done by a professional company, especially if the trees you have a fairly large, for they will have the specialized equipment capable of applying the material evenly over all the branches. These refined oils leave a thin coating over the plant and any insects on them. Since insects, and even insect eggs, have to “breathe”, the oil prevents them from getting their oxygen and they suffocate. There are oils used in the winter when the trees have no foliage on them, and they are referred to as “dormant” oils, meaning for use when the plant is in its dormant winter period. There also are more refined, lighter “summer” oils that can be used in the spring and summer when the plants have their leaves on them. Whichever path is chosen, though, the application must be thorough so that all the scales, their crawlers, and their eggs are contacted and coated with the oil.

If the infestation is very small, especially on a small plant such as a houseplant, you might just choose to use a little time and patience and physically wash them off the leaves and stems. Another common insecticide is called Safer Soap, and it may be applied to potentially sensitive plants to kill a variety of insects, including scales. In the past few years some wonderful new natural products have come onto the market too, most often extracts from trees or other plants, so the arsenal of effective products that pose little to no risk to you is growing.

So, as you wander around your garden with your cup of coffee, admiring the hard work you put in over the previous weekend, look at the stems of the plants to see if you have any of these unusual invaders on them, the parasites we call Scales.