Scale insects on Minnesota trees and shrubs
Table of contents
Scale insects feed on many plants and appear as small bumps on the foliage, twigs and branches of many landscape trees and shrubs in Minnesota. Because they are immobile and blend in with the bark and leaves, they are often overlooked. The waxy shell, the scale cover that gives them their name, is secreted by the insects to protect them from the environment and predators. This covering is also a barrier to many pesticides used for insect management. Management of scale insects requires knowledge of scale biology and its susceptible life stages to apply treatments effectively.
Scale insects are categorized as being a soft scale or an armored scale. Both types of scale insects spend most of their lives under the protection of the coverings that they produce. The scale is not a part of the insect, but rather a protective barrier that they construct.
Soft scales are convex and about 1/8 to 1/4 inch long. They feed on the phloem tissue of trees and shrubs. They excrete excess sap from the phloem as a by-product called honeydew (Figure 1). Honeydew is usually the first sign that a soft scale is feeding on the tree. Honeydew will stick leaves together and attract many insects (e.g. ants, wasps, and flies) that will feed on the excretions (Figure 2). If honeydew is sighted, look closely for signs of soft scales. Keep in mind that other insects, especially aphids and woolly aphids can also produce honeydew.
Joe Boggs, The Ohio State University Extension
Figure 1. Honeydew produced by soft scales.
Brian Kunkel, University of Delaware Extension
Figure 2. Many insects feed on the honeydew produced by soft scales.
Armored scales are smaller than soft scales (about 1/16 to 1/8 inch long) and usually oval or somewhat elongate. They feed on individual plant cells (instead of the phloem) and do not produce honeydew. Many armored scales have shells that match the color of the bark of their host tree making them difficult to spot until damage to the tree becomes obvious (Figure 3). The exception to this is the pine needle scale which has a bright white shell in contrast to the green needles on which it feeds (Figure 4).
University of Wisconsin Extension
Figure 3. Oystershell scales. Armored scales can blend in with the color of branches and stems.
Jeffrey Hahn, University of Minnesota Extension
Figure 4. Pine needle scales are easy to identify on infested plants due to their white coloration.
Scale insects begin their lives as eggs laid under the shell of the females (Figure 5). The eggs hatch into nymphs, commonly called crawlers, that move from under the cover of the scales to colonize other sites on the plants (Figure 6). There are two instars (growth stages) of nymphs.
Lyle J. Buss, University of Florida
Figure 5. Lecanium scale eggs laid under the shell of the female scale
Utah State University Landscape IPM Advisory
Figure 6. Lecanium scale crawlers on the foliage of the tree before they migrate back to the stem.
The crawlers of soft scales typically move out to feed on foliage and the second instar (growth stage) nymphs return to twigs and branches to mature and build their own protective shells. The crawler stage of armored scales will travel to new tissues and settle down to feed and create new shell coverings near their parent scale. The crawler stage of scale insects is the primary method of dispersal. The crawlers are small enough to be blown by wind to other plants and are also able to be transferred on feet of birds and other animals that frequent the trees on which they feed.
When scale crawlers settle on plants to feed they create a waxy shell covering. Armored scales lose their legs and become non-mobile, remaining in the same location for the remainder of their life. Soft scales retain their tiny legs and are able to move but very slowly. At full maturity, female scales mate with winged males (rarely seen) and lay eggs under their protective shell cover and die.
Soft scales overwinter as nymphs or adults whereas many armored scales overwinter as eggs under the hard shell of their mother. This difference in overwintering habits as well as timing of the crawler stages of each of the scale species is very important to determine the best method of management as well as timing of management applications.