Scale insects occur on a wide variety of plants in general. They are inconspicuous because they are immobile most of their lives and do not resemble other insects. Heavy scale infestations will weaken a plant and cause it to grow slowly. Parts of a plant that remain heavily infested may eventually be killed. Scales attach themselves to leaves and stems of plants and feed by sucking plant sap. Some scales produce honeydew, which gives plants a sticky appearance. Black sooty mold commonly grows on honeydew covered leaves.
Scales are divided into two different groups according to the type of body covering. These are soft and armored scales. Most soft scales found on herbaceous plants and herbs range in size from a few millimeters to about ¼ of an inch. Adult females are usually semicircular in outline. Eggs are deposited in soft cottony sacs or beneath the scale body. The young nymphs are called crawlers. They are very tiny, have legs and antennae and are very active. Examples of scales found on herbaceous plants and herbs include brown soft scale and hemispherical scale.
Armored scales are 1-2mm long and have a strong waxy covering under which the females lay their eggs. Examples include the fern scale, Florida red scale, and bamboo scale.
Hemispherical scale on fern
Fern scale. White male cover on left, female cover on right. Photo: John. A. Davidson, Univ. Md, College Park, Bugwood.org
Various techniques can control scales. They can be carefully scraped from wood portions of stems (such as rosemary). Insecticidal soap and horticultural oil sprays can be applied at any time. Good coverage is essential. Often, beneficial insects such as parasitic wasps and ladybird beetles will control scale infestations. When using any insecticide, including soaps and oils, always read the label carefully and follow all directions and precautions.