Scale

Overview 

Scale insects are common pests of many plants. There are two general types of scale– armored scale and soft scales. They are piercing-sucking insects closely related to aphids, mealybugs, and whiteflies. Like their counterparts, their mouthparts are fused into a slender tube, or stylet, that is used to pierce the plant surface. After hatching, the young scales wander over the plant searching for a spot in which to settle and begin producing their distinctive shell coverings. As adults, they are sedentary; only a few species have the ability to move. 

Symptoms and Diagnosis 

The most common symptom of a scale infestation is that the leaves of the affected plant turn yellow and may drop; rose leaves may wilt and turn dark. In addition to leaf drop, scales may cause reduced growth and stunted plants. Some species also produce a shiny or sticky material (honeydew) that will cover the leaves of infested plants. A sticky floor beneath a susceptible indoor plant is a telltale sign that infestation is underway. Trees heavily infested with armored scale often look water-stressed. Leaves turn yellow and drop, twigs and limbs may die, and the bark cracks and gums. Armored scales can kill trees and must be controlled when their population gets too high. 

Soft scales also reduce plant vigor, but usually not enough to kill the plant. The main problem soft scales cause is the large amounts of honeydew they produce, which can cover leaves and fruit and act as a growth medium for black, sooty mold. Honeydew also attracts ants, flies, wasps, and bees which can become a nuisance. Contrary to what one might logically presuppose, the shells of the soft scale are, in fact, as hard or harder than those of armored scales. At maturity, their shells are generally larger and darker as well. 

Closer inspection will reveal small cottony or dome-shaped shells attached to stems and leaves.  Scale are identified by their body covering, which normally appears as a colored raised area on the leaf or stem of the plant that can be flicked off with the point of a knife or a fingernail. They vary in size but are generally only about 1/8 inch long. 

Life Cycle 

The life cycle of each species of scale can vary depending on the season and species. Armored scales usually have several generations a year, while soft scales have a single generation. 

Scale overwinters on leaves, needles, stems, or twigs of the infested plant under the covering (shell) it creates. Under the scale shell, the female lays eggs. In the spring, the eggs hatch, and the unprotected larvae (crawlers) emerge from under the protective shell and begin feeding on their own. The actual date of emergence varies with each type of scale insect and the weather. New plants can be infested when crawlers walk or are blown by the wind to a nearby plant. After a few days, the crawlers of armored scales settle down permanently, molt (and lose their legs), and begin to form their covering. Soon winged adult males crawl out from beneath their cover and mate with covered females. Adult male scales are tiny, winged, and seldom seen. Soft-scale crawlers move about the plant a little longer after hatching before they settle down at a permanent feeding site. Some soft-scale species move around during their immature stages and retain their legs for life. Their covers may be smooth or cottony, but they are firmly attached to their bodies. Soft scale tends to be larger than armored scale. Mature scales never move once they firmly attach themselves to the plant.  

Indoors, eggs can be laid at any time under the protective shell. The eggs hatch and the crawlers seek out new feeding sites. This crawler stage is when scale is easiest to control. Indoor scales can go through many generations a year. 

Integrated Pest Management Strategies 

1. Maintain plant vigor. Maintaining a healthy plant is the best way to prevent infection by diseases or attack by insects. This can be accomplished by proper care including watering, fertilizing, providing a well-draining soil medium, adequate lighting, and air movement. 

2. Pruning. Prune out heavily infested shoots or limbs. This will reduce the population and foster the growth of new un-infested shoots. 

3. Remove insects by hand. If caught early, scale insects can be rubbed off by hand, with a discarded toothbrush, or with a cotton swab soaked in isopropyl alcohol. Prune out heavily infested portions of the plant. 

4. Biological (outdoors). Natural enemies, such as the ladybird beetle (ladybug) and small parasitic wasps, help control many scale species. Inspect the scale for round exit holes made by the emerging adult parasites. Before applying any insecticides, check the plant for the presence of natural enemies and make sure that they are not already reducing scale numbers. 

5. Use insecticidal soap. Monitor plants frequently. When crawlers are noticed, use insecticidal soaps that are formulated to kill insects and not damage plants. Frequent sprays may be needed until the problem is under control. 

6. Chemical control. Many scale species on deciduous trees can be effectively controlled with either a dormant spray of refined horticultural, superior-type oil or summer oil spray, or both. Oils work by smothering the insects, both adults and crawlers. Other insecticides registered for use include malathion, bifenthrin, imidacloprid, and pyrethrins. 

7. Timing. Broad-spectrum insecticidal treatments are most effective when applied in the crawler stage. Monitor by using double-sided sticky tape around some branches of the tree. Crawlers appear as tiny, pale yellow specks on the tape. 

8. Repeat. In the case of overlapping generations, where every stage of development occurs, two to four applications at ten-day intervals will be needed. 

Organic Strategies 

Strategies 2, 3, and 4 are strictly organic approaches. Strategy 1 could be considered an organic approach if an organic fertilizer is used. Consult the Organic Materials Review Institute (OMRI™) for appropriate chemical products.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against scale. Not following the pesticide label before usage is a violation of federal law.