Scales are common insect pests on ornamental trees and shrubs in landscapes. Scales may be a problem in urban environments due to the absence of natural enemies, and plants may be under physiological stress. Scales feed, with their tubelike mouthparts, within the vascular system, where nutrients and fluids are transported. Scales may resemble galls on plants. The best way to distinguish scales from plant galls is to use a fingernail and flip scales over. Galls, when removed with a fingernail, usually break off with bark attached.
Some scales are host specific, feeding on only certain plant species, whereas other scales feed on a wide range of plant species. Scales rarely kill a plant by themselves but may predispose plants to attack from wood-boring insects or secondary pathogens.
There are two types of scales: soft, or bark, scales and hard, or armored, scales. Characteristics of soft scales include (1) generally one generation per year; (2) produce honeydew; (3) typically overwinter as immature fertilized females; (4) appear convex in shape or resemble a helmet; (5) highly active crawlers; and (6) have a protective body wall. Characteristics of hard scales include (1) generally, two or more generations per year; (2) do not produce honeydew; (3) typically overwinter as eggs underneath the body of the dead female; (4) appear circular or rounded in shape; (5) crawlers are less active, compared to soft scale crawlers; and 6) separate protective covering.
There are several exceptions to these characteristics that are likely to be confusing. For example, obscure and euonymus scale, which are hard scales, do not overwinter as eggs. Oystershell and pine needle scale, which are hard scales, are not rounded but are more elliptical. Common scales are listed in Table 1.
Table 1. Common scales found in landscapes.
Cottony maple scale (Pulvinaria innumerabilis)
European elm scale (Gossyparia spuria)
Fletcher’s scale (Parthenolecanium fletcheri)
Magnolia scale (Neolecanium cornuparvum)
Pine tortoise scale (Toumeyella parvicornus)
Tuliptree scale (Toumeyella liriodendrii)
Spruce bud scale (Physokermes piceae)
Hemlock scale (Abragallaspis ithacae)
Obscure scale (Melanaspis obscura)
Oystershell scale (Lepidosaphes ulmi)
San Jose scale (Quadraspidiatus perniciousus)
Euonymus scale (Unaspis euonymi)
Pine needle scale (Chionaspis pinifoliae)
Scurfy scale (Chionaspis furfura)
Juniper scale (Carulaspis juniperi)
Eggs are laid underneath the female scale cover. Hard scales can reproduce either sexually or asexually, and females can lay eggs or produce live offspring. Depending on the species, up to 2,000 eggs can be produced from a single female. Eggs hatch into oval, flat active (mobile) crawlers that vary in color (orange, yellow, gray, or brown), depending on the species. Scales disperse by means of the mobile first instar or crawler stage. Crawlers are generally located on the undersides of branches and leaves and on the sides of tree trunks that provide protection from direct sunlight, wind, and rain. Crawlers eventually settle down and begin feeding by inserting their mouthparts into plant tissue and withdrawing plant fluids.
Hard scales do not produce honeydew because they feed differently than soft scales. Instead, hard scales rupture and destroy plant cells they are feeding on and oftentimes bypass the plant vascular bundles that transport nutrients through the plant. In contrast, soft scales, which produce honeydew, feed on plant fluids that move through the vascular system. These plant fluids, after passing through the scale, are the basis for honeydew. Honeydew is a clear sticky liquid that serves as a growing medium for sooty mold fungi, which produces a black coating on leaves. This coating interferes with the plant’s ability to manufacture food through photosynthesis.
Scales, when feeding, may inject salvia that can be toxic to plants. Also, feeding by scales opens up wounds that provide entry sites for plant pathogens. Males eventually molt into very small, winged, gnat-like insects that live for about 2 weeks. Their primary function is to fertilize females (for soft scales). Hard scale females continue to molt and later lose their legs and cannot move (remaining sessile for the rest of their life). In contrast, soft scale females retain their legs. Both hard and soft scale females ultimately die, and their bodies form a protective covering over the eggs and for emerging crawlers.
Systemic insecticides are effective in controlling soft scales because they feed on plant fluids containing these insecticides that are transported through the plant vascular system. However, they are less effective on hard scales because these scales do not feed exclusively on plant fluids.