Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
Scale insects are positively insidious. As female scale insects mature, they settle down to feed on a plant, hiding beneath a convex shell-like scale covering. The female scales never move again. Concealed beneath the armor-like scale, they often go unnoticed because they don’t move and they don’t look much like insects.
You might wonder what damage is caused to plants by these sneaky little creatures. Scales feed by sucking sap from plants. While a few scales won’t harm a plant, large numbers weaken it. A heavy scale infestation can make a plant more prone to damage from drought or severe temperatures and make it more susceptible to attack from other insects. Heavy scale infestations can also lead to the death of branches and limbs or possibly an entire plant. Some species of scale inject a toxin into a plant as they feed, causing even greater damage.
Certain types of scale suck out more plant sap than they can use. They expel the excess sap as a mixture of liquid and sugar called “honeydew”. The sticky honeydew is not harmful, but it can serve as a food source for the “sooty mold” fungus. Sooty mold is a black fungus that lives on the secreted honeydew, coating leaves or needles and detracting from a plant’s appearance.
If the adult females don’t move once they start feeding, one might wonder just how they spread from plant to plant. Before we can answer that question, let’s take a look at the life cycle of scale insects. Most scale insects start their life as eggs. These eggs hatch and become immature scales, also known as nymphs. These immature scales are mobile and referred to as “crawlers”. The crawlers have six legs and walk around looking for a place to feed on the new plant growth.
When they are ready to settle down and feed, the crawlers molt and begin forming the scale over their bodies. They produce this scale covering from waxy filaments and feces. The shape, color, and size of the scale are distinctive for each species of scale. The scale covering provides protection from being eaten by other insects... and also from being killed by insecticide sprays.
Once the female scales have settled down to feed and secreted the scale covering over their backs, most species lose the use of their legs. Male scales are different. They develop under thin scales and emerge as winged forms. Their only purpose in life is to mate. They have no functional mouthparts and can’t feed. After they mate, the males disappear. The females then produce eggs (some species produce live nymphs) under their scale covering or in some type of cottony material... and thus the cycle of lives begins again. Most species of scale that attack outdoor trees and shrubs in our region have only one generation a year. However, scale insects found on indoor and greenhouse plants can produce several generations a year.
It is in the crawler stage that scales move from plant to plant, but the tiny crawlers can’t fly and aren’t able to crawl or walk to different plants. They are moved about in different ways. Their mode of transportation can be the wind, other insects, birds, animals, or even man.
Managing a Scale Infestations
Managing a scale infestation is not an easy task. Because of their sneaky nature, scale populations tend to build up to large numbers before they’re noticed. It’s also especially difficult to control these insects because their scale protects them from most insecticides. Some scales can be controlled with dormant oils applied in late winter. The oils work by suffocation. However, it’s in the unprotected crawler stage that scales are most vulnerable to control with insecticide applications. The problem is that the crawlers are so small it’s difficult to detect their presence and there is a relatively short time before they start covering themselves with a scale.
A magnifying glass or hand lens will assist you in seeing them. You can also trap crawlers with double-sided sticky tape wrapped around twigs and branches. About the time when the crawlers are expected to emerge, place the tape near adult scales and trap the crawlers as they move out to feed on new growth. You may need to renew the tape from time to time, when it loses its stickiness. Keep an eye on the tape and apply the appropriate sprays when they emerge.
Scale Insects Troublesome to Trees & Shrubs in Our Region
Pine Needle Scale: This scale is a often problem on stressed pines, especially older trees or those located in dusty areas. Pine needle scale is white, pear-shaped and about 1/8 inch long. Its feeding weakens and slows growth. Pine needle scale can be controlled with dormant oil applied in the spring before new growth begins and with sprays at the crawler stage about April.
Spruce Bud Scale: More and more of this sneaky scale is showing up in local spruces. It’s often found on the lower branches of spruce trees, but it’s not easily noticed because the scales resemble spruce buds and are located at the base of new twig growth. Gardeners sometime notice the excessive amounts of honeydew produced by this scale; the bees and wasps attracted to this copious sweet residue; or the sooty mold growing on the honeydew. However, it may not be noticed until dieback starts occurring. Spruce bud scale can be controlled with delayed dormant oil in the spring and with sprays when the crawlers appear about June. (Be aware that oil sprays will turn a “blue” spruce to green.)
Juniper Scale: Here’s a little round scale that feeds on the needles of juniper, arborvitae, chamaecyparis, cypress, and red cedar, but it’s most often seen on juniper in this region. The scale is off-white with a central yellow dot. It tends to be a problem on older, less vigorous junipers. It can be controlled with dormant oil in late March to early April before new growth begins and with sprays when the crawlers appear about June.
San Jose Scale: This is also a very small scale (1/16 inch in diameter) round scale. It attacks a wide variety of hosts including apple, cherry, pear, poplar, willow, maple, birch, and many other deciduous trees and shrubs. It’s black and tends to blend in with the bark of many trees and shrubs.
This is a common problem on fruit trees and is moved about by the wind, birds, and humans. Predators sometimes keep small populations of San Jose scale under control, but rampant infestations can be quite damaging. Control can be achieved with late winter dormant oil and with sprays when the crawlers appear in June and again in September.
Other types of scale sometimes found in this region include cottony maple scale on maples, oystershell scale on various deciduous trees and shrubs, elm scale on elm, and hemlock scale on spruce.
If you should notice some scale on one of your trees and shrubs, don’t panic. Many scale infestations are kept under control by natural enemies. Sprays may not be warranted if predators or parasites have kept the population in check. There are also cultural controls that gardeners can employ to assist with prevention and management. First and foremost, keep your plants happy and healthy. A tree or shrub that’s growing vigorously and not stressed will be less prone to attack.