Most people are familiar with what an insect looks like: six legs, antennae and wings. But what about scale insects? They don’t exactly fit in most insect categories. But they can still wreak havoc on your woody plants.

Scale insects are found in two major groups: soft scales and armored or hard scales. While they are all considered scales, there are some important differences between the two groups.

Soft scales. Soft scales are generally helmet-shaped, produce a clear sticky liquid and have one generation per year. The first nymphal life stage after hatching from the egg are pretty mobile. Examples of soft scales include the lecanium scale complex, cottony maple and magnolia scales.

Armored scales. Armored scales are more flat in cross section, resemble a target with a point in the center, do not produce honeydew and have two or more generations per year. The first post-hatch life stage are much less mobile than their soft scale cousins. Because of their reduced mobility, generations of armored scales will pile up on stems and branches. They might resemble bark and be easy to overlook. Armored scales include pine needle, euonymus, obscure and oyster shell scales.

Because scales do not look like a typical insect, here are a few helpful clues for proper field diagnosis and identification:

1. Females. The scale you see on your plant is typically female. Most adult male scales are very small, winged insects and rarely observed.

2. Host. Most scale insects are fairly host specific, and their common name usually coincides with their primary host. For example, euonymus scale is only found on euonymus plants and magnolia scale on magnolia species. Like all things in biology, there are exceptions. Oyster shell can be found on many different woody plant and tree hosts, and the lecanium scale complex is common on many fruit trees and ornamental plants.

3. Location. Where the scale resides is important. Obscure scale of pin oak likes to live on the trunk and the undersides of the main branches, and pine needle scale is typically found on the needles of host pines. In contrast, euonymus scale can be found on the woody portions of the plant (twigs and stems) as well as the leaves. The same goes for oyster shell scale, but it is more common on the trunks, stems and branches of woody plants.

5. Trail. Look for the absence or presence of honeydew and sooty mold (black fungus lives on the honeydew) on the leaves and stems. If you do not see honeydew or sooty mold, you know it is an armored scale and not a soft scale.

How scales affect trees

So, why are scales important pests of our woody plants? Scales are sap-feeders and can rob the plant of vital nutrients and water that are important in photosynthesis and food production. While low levels of scales on a plant may not be very harmful, heavy scale populations can kill twigs and branches, and weaken the plant to the point where it is susceptible to lethal secondary wood-boring insects, pathogens and/or abiotic factors.

An infested plant will look sickly and may not provide the desired benefits of a healthy plant. The honeydew that is produced by soft scales provides a nice food source for the sooty mold fungus, which will cover the stems and leaves with a black coating that is very unsightly and interferes with photosynthesis.

With the exception of the magnolia scale, most soft and armored scale crawlers are active in early to midspring, but are weather dependent. The young scale crawlers are very vulnerable to any kind of a contact insecticide, but once they molt to the next stage, they are pretty well protected from any contact spray due to the waxy, waterproof covering they secrete.

If you choose to use a contact spray, be sure to thoroughly cover the plant, but at the same time, be very careful not to harm beneficial insects like predators and pollinators. Spraying very early in the morning or late in the evening, when honeybees are not as active, can help minimize bee kill. Also, be aware of honeybee hives in your area. Be a good neighbor and notify the local beekeeper well in advance that you will be spraying. Systemic insecticides can also be used but are only effective against soft scales.

For more information on scale insects, contact your local Extension office for details on specific scale pests, scale biology, host plants, and timing and selection of chemical treatments.

Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at [email protected].