Bark scale refers to a category of sap-sucking insect pests that target the woody parts of various trees and shrubs. These insects use needle-like mouthparts to pierce the bark and feed on the plant’s internal fluid. The name describes both the insect and the physical, protective covering it creates on the host plant. The stationary adult stage is concealed beneath a waxy or felt-like shell, making the pest appear like a growth or abnormality on the bark surface.
Identification and Common Hosts
The adult female bark scale is typically immobile and appears as a small, oval structure, often white, gray, or pinkish in color, depending on the species. The protective covering is a waxy secretion that shields the insect from predators and many topical treatments. Due to its sedentary nature and unusual appearance, the pest is frequently mistaken for a disease or fungal growth.
Crape Myrtles are a common host for the Crape Myrtle Bark Scale (CMBS). Other species infest a wide variety of trees, including maples (gloomy scale) and elms (Japanese maple scale). These pests primarily concentrate on the trunk, branches, and twigs rather than the leaves.
The Scale Life Cycle
Understanding the life cycle is important for determining the most effective management timing. The pests typically spend the winter months overwintering as nymphs, tucked into bark crevices or under the protective covering of adult females. As temperatures warm in the spring, eggs hatch, releasing the mobile stage known as “crawlers.”
The crawler is the newly hatched nymph, which possesses legs and is the only stage capable of actively dispersing to new feeding sites or being carried by wind. This mobile stage is the most vulnerable to insecticide applications because it lacks the hard, waxy protective shell. Depending on the climate, bark scales can complete a life cycle in two to three months, often leading to two to four generations per year in warmer regions.
Recognizable Signs of Infestation
One of the first visual clues of infestation is the presence of honeydew, a sticky, sugary waste product excreted by the feeding insects. This material adheres to the bark, leaves, and surfaces beneath the infested plant. The sugary residue provides an ideal environment for the growth of sooty mold, a black fungus.
Sooty mold is a secondary symptom that darkens the bark and leaves, giving the plant a scorched or dirty appearance. While the mold itself does not directly harm the plant tissue, heavy coverage can interfere with photosynthesis. Infestations are often most noticeable where the insects congregate, such as in branch crotches, along pruning scars, and in the natural furrows of the bark.
Control and Eradication Methods
Management often begins with cultural control methods, such as the physical removal of the pests. For accessible areas, a soft brush and a solution of mild soap and water can be used to scrub the bark, dislodging the scale bodies and removing honeydew. Maintaining tree health through proper pruning and avoiding excessive nitrogen fertilizer is also beneficial, as new growth is favored by the pests.
Targeted chemical control focuses on the crawler stage, which lacks the protective waxy coating. Horticultural oils and insecticidal soaps are topical treatments that smother the insects upon contact. These should be applied when crawlers are active, typically in the spring and mid-summer, to achieve the highest rate of control.
For severe or hard-to-reach infestations, systemic insecticides are applied to the soil or trunk and absorbed by the plant. These chemicals move through the plant’s vascular system, poisoning the scale insects as they feed. Systemic applications provide longer-lasting control, but careful timing is necessary to minimize potential impact on beneficial insects, as natural enemies like lady beetles are often ineffective against heavy infestations.