Armored scales are significant pests of landscape trees and shrubs. Many blend into the bark of infested trees and shrubs, but a few are very distinct. All are difficult to control because of the waxy covering over the adults and the importance of precise timing to control the vulnerable crawler stage.
Types of Armored Scales
Pine Needle Scale
An abundance of pale pine needle scales (Figure 1) gives infested trees a gray cast. Mugho and Scots pine are commonly attacked, but nearly all needle-bearing conifers can become infested. Sap removal by these insects can cause yellow, stunted needles, and poor growth. Heavy infestations may kill branches and, in some cases, entire trees. There are two generations each year.
Euonymus scales (Figure 2) can form crusts over leaves and twigs on Euonymus, Pachysandra, and bittersweet (Celastrus). Female coverings are dark brown to gray and somewhat pear-shaped. The white coverings of males are distinctly smaller and narrower. Moderately to heavily infested plants grow very slowly, if at all. Yellow spots may appear on the foliage; heavy infestations can cause branch dieback and may even kill some plants. There are two generations each year.
Gloomy scale (Figure 3) is a particularly widespread and damaging scale on red, silver, and Freeman maples. The female is protected under a 1/8” diameter grey covering that blends into the bark and often goes unnoticed until infestations become quite heavy. There is one generation each year, but crawlers can be active over a long period of time.
It takes time and a multi-faceted approach to manage armored scales.
Follow standard cultural practices to keep landscape trees and shrubs as healthy as possible. Do not over-fertilize as this can increase scale problems. Water as needed and prune heavily infested shoots or limbs, if practical.
Tiny parasitoid wasps and other beneficial insects can provide some natural control of scales, but the reproductive capacity of scales can outpace the abilities of natural enemies. This leads to ups and downs in scale infestations from season to season. Typically, there is some lag time as beneficial insects respond to increased pest populations.
Dormant oil sprays can help to reduce overwintering scales. Thorough spray coverage is essential so this approach can be limited by tree size. Also, the crusty buildup of overlapping scales can protect some individuals from direct contact of the spray. Check dormant oil labels for rates, precautions on sensitive tree and shrub species, and temperature restrictions.
Contact insecticides can be used to control the vulnerable scale crawlers as they move from under the protective shield of the female’s waxy covering to settle on bark to feed. As with dormant oil sprays, results can be limited by tree size. Crawlers of the three scales discussed above are active in May. Careful monitoring can help to determine when to treat. Exposed sticky surfaces of black electrical tape on infested branches can be used to trap moving crawlers. Insecticidal soap or horticultural oils kill by contact so they help to conserve natural enemies. However, crawler egg hatch occurs over a week or more, so multiple applications may be needed to obtain satisfactory control.
Residual insecticides provide control that may last for a week or more. These have a greater potential impact on beneficial insects, but they may be more effective against moderate to high levels of scale infestation. Pine needle scales and euonymus scales have two generations each year and gloomy scale crawlers are active over many weeks, so multiple sprays may be necessary.
Systemic insecticides containing the active ingredient dinetofuran (Safari) can control armored scales and offer advantages where spray coverage or very high infestation levels are challenges.
By Lee Townsend, Extension Entomologist