Tree scale refers to small, sap-sucking insects that commonly infest trees and ornamental shrubs. These pests are often overlooked because their immobile, shell-like appearance makes them look more like natural bumps on the bark or leaves. Scale insects attach themselves to the host plant and use specialized, thread-like mouthparts to extract vital fluids from the tree’s vascular system. A large infestation can weaken a tree over time, leading to significant decline and, in severe cases, the eventual death of the plant.
The Unique Biology of Scale Insects
Scale insects belong to the order Hemiptera, which also includes aphids and whiteflies. The two primary categories of scale that affect trees are armored (or hard) scale and soft scale, and their differences affect how they are managed. Armored scales create a hard, waxy covering, called a test, that is not physically attached to the insect’s body beneath it. This protective shield is composed of shed skins and wax, making this group highly resistant to most contact insecticides.
Soft scales, in contrast, secrete a waxy or cottony layer that remains firmly attached to their body. Unlike armored scales, which feed by bursting plant cells, soft scales directly tap into the phloem. This feeding method causes soft scales to produce a sticky, sugary waste product, while armored scales generally do not. Both types share a similar life cycle, with the mobile “crawler” stage being the most important for control.
The crawler is the first nymphal stage after eggs hatch, and it is the only time the insect is mobile, searching for a new feeding site on the host tree. Crawlers are extremely small and lack the waxy armor that protects the adults. Once a crawler settles, it inserts its mouthparts and becomes sessile, or stationary, for the rest of its life. Armored scale species may have several generations per year, while soft scales typically produce one or two.
Identifying Signs of Scale Damage on Trees
A general sign of a scale infestation is a decline in overall tree health, often visible before the insects themselves are noticed. This decline may include stunted growth, branch dieback, and a general thinning of the canopy. The constant removal of sap or plant cell contents weakens the tree, leading to the yellowing of leaves and premature leaf drop. Severe infestations concentrated on a limb can cause that particular branch to die entirely.
Soft scale infestations are frequently identified by the presence of a sticky liquid known as “honeydew.” This honeydew drips onto leaves, branches, and surfaces beneath the tree. The sweet residue attracts ants and can lead to the growth of sooty mold, a black, non-parasitic fungus that feeds on the sugary excretion. While sooty mold does not directly infect the plant tissue, a heavy coating can block sunlight, reducing the tree’s ability to perform photosynthesis.
Armored scale damage tends to manifest more directly as localized tissue death. Their cell-feeding causes purpling or yellow mottling on foliage. To confirm the presence of either type, inspect the bark and leaf undersides for small, shell-like bumps, which are the scale covers. Scraping a bump can reveal the soft body of the insect underneath, confirming a live infestation.
Practical Strategies for Scale Management
Effective scale management begins with cultural control, maintaining the tree’s health through proper watering and pruning, as vigorous trees are better able to withstand pest pressure. For small infestations on accessible plants, heavily infested branches can be pruned and removed. Alternatively, the scales can be manually scrubbed off with a soft brush and soapy water.
Biological control methods focus on encouraging natural predators and parasites, such as lady beetles, parasitic wasps, and lacewings. Identifying tiny exit holes in the scale cover indicates the presence of parasitic wasps. The use of broad-spectrum chemical treatments should be minimized to avoid harming these beneficial insects.
Chemical and horticultural treatments are most effective when applied during the mobile crawler stage when the insects lack their protective armor. Horticultural oils, including dormant and summer oils, work by coating the pests and suffocating them, offering a low-toxicity option.
Horticultural Oils
Dormant oil applications use higher concentrations and are applied in late winter or early spring when deciduous trees are leafless and temperatures are between 40°F and 70°F. These target overwintering eggs and nymphs. Summer oils, applied at lower concentrations, target the newly hatched crawlers during the growing season. Insecticidal soaps are also effective contact treatments against exposed crawlers, working by disrupting the insect’s cell membranes.
For severe infestations, especially of soft scale, a systemic insecticide can be applied to the soil or trunk. The tree absorbs the chemical, which then moves through the system to poison the feeding insects.