Landscape - How Well do You Understand Scale Insects?

One on the most misunderstood insect pest groups are the scale insects. Because of this, control measures are often ineffective. The following is an article that details the differences between soft and armored scales and the biology of each.

A Large Scale Dilemma

Undoubtedly many arborists, landscapers, nurserymen, and golf course superintendents would agree that effectively controlling scale insects is one of the more frustrating pest management challenges encountered. Of the half-dozen major families of scale insects common in the urban landscape, the armored scales are the most troublesome. With their protective waxy covering, armored scales are considerably less susceptible to various insecticide spray treatments. Many pesticide spray applicators fail to achieve satisfactory controls because they do not have the time or inclination to apply sprays during the scale crawler emergence periods. To complicate matters, the crawler periods for the various armored scale species are quite variable. Furthermore, the improper timing of long residual pyrethroid insecticides can virtually eliminate important parasitoid bio-control activity and hence, often encourage scale infestations. The intention of this article is to stress the importance of properly timed treatments in order to achieve better management of scale insects. This is especially true when attempting to control armored scales.

Scale vs. Scale: Contrasting Armored and Soft Scales Signs and Symptoms:

Without question the two most common families of scale insects found in the urban landscape are armored and soft scales. When physically removed from bark or leaf tissue both soft and armored scale species will leave a white ring of their outer margins. These white rings are waxy adhesives used to closely attach themselves to the plant host. This is an important sign that can be used to avoid making unnecessary treatments against non-scale look-a-likes sometimes seen in the landscape. Soft scales are vascular feeders and secrete large quantities of honeydew that typically becomes associated with black sooty mold. Although the sooty mold is a fungus, it is not pathogenic and is mostly only an aesthetic concern. However, it can be a problem if large areas of leaf tissue become covered, resulting in reduced photosynthesis. No leaf stippling is produced from soft scale feeders since they do not remove green chlorophyll or cause individual plant cell death. As a result, soft scales are less damaging to plant hosts and healthy trees and shrubs can tolerate moderate infestations with little affect. Alternatively, armored scales feed in mesophyll cells of plants and do create leaf stippling and typically cause more damage. They are more likely to generate individual branch dieback and can even kill trees. Since armored scales no not feed in vascular tissues they do not secrete honeydew and give rise to the corresponding black sooty mold.

Feeding Locations:

Most of armored scale species with broadleaf hosts will feed either on bark or leaf tissues, but usually not both (Euonymus scales are a notable exception). The crawlers have only 24 to 48 hours after hatching before they must insert their mouthparts into plant cells and begin feeding. Once armored scale crawlers settle they do not move again for the remainder of their lives (winged adult male scales do change positions, but cannot feed and only live for 24 hours). On the other hand, most soft scale species feed both on bark and leaf tissues during different stages of their life cycles (there are exceptions, especially with coniferous hosts). Unlike armored scales, the soft scales do not lose their legs after the crawlers settle and therefore can move to different feeding locations. On deciduous hosts, the 1st instar crawlers of most species will emerge in June or July from under adult females located on bark. They then move to settle and feed on leaves for the remainder of the summer. Before the leaves drop in autumn the nymphs will move back to bark tissue where they will over winter as 2nd instar nymphs.

Physical Attributes:

Unlike armored scales, soft scales do not produce a detachable protective covering. As a result, soft scale nymphs are exposed to not only sprays, but are easy prey for many predators and parasites. They also suffer high mortality from harsh environmental conditions. Even without insecticide treatments, mortality rates exceeding 98% are typical for soft scale nymphs. To help compensate for these huge losses, soft scale species will lay between 300 to 2000 eggs per female.

The armored scales protective covering is produced by waxy filaments and glue exuded through ducts and pores on the body. Without the protective cover the soft “jelly-like” body of an armored scale would be extremely susceptible to desiccation and predation. This protective covering gives armored scales higher natural survival rates throughout the various life stages and allows for more conservative egg laying. As a result the average armored scale species only produces between 10 to 80 eggs per female.

There are 15 families of scales that have been classified by entomologists. A total of approximately 7,000 species of scales have been identified within these families. More than half of the species (4,000) are included within the armored scale family. It can be said that the family of armored scales have genetically figured out how to best survive and proliferate. Armored scales have reached the evolutionary pinnacle of success.

Reprinted from "Effectively Managing Scale Insect Pests in the Urban Landscape" by Steven K. Rettke, RCRE Ornamental IPM Program Associate, in the March 16, 2006 edition of the Plant and Pest Advisory, Landscape, Nursery, and Turf edition, Rutgers University