Armored Scale Insects & Control

Many armored scales are serious pests of ornamental shrubs, trees, groundcovers, and turfgrasses in South Carolina. Almost 40% of the Clemson Plant Problem Clinic sample submissions for home landscapes during 2012 and 2013 were scale insect pests, and of these, almost 90% of the species were the more difficult to control armored scales. Twenty-four different armored scales were identified on residential landscape plants. As winters have become warmer in recent years, additional insect pests may have extended their range more northward into South Carolina from Florida and coastal Georgia. More armored scale samples on ornamentals were submitted from the coastal areas of South Carolina than from the rest of the state, probably due to the milder winter weather there (see Table 1 for scale insects identified).

Table 1. Armored Scale Insects Identified on Landscape Plants by the Clemson University Plant Problem Clinic in 2012 & 2013

Some armored scales damage only branches, while others infest foliage or fruits. A severe infestation of armored scales may weaken or kill a tree or shrub.

Tea scale injury on camellia upper leaf surface
Clemson University – USDA Cooperative Extension Slide Series

Adult tea scales on lower camellia leaf surface.
Clemson University – USDA Cooperative Extension Slide Series

Scale adults are the most noticeable stage on plants, and these may be white, gray, or brown. Adult scales may be round, pear-shaped, or oyster-shell shaped but vary somewhat depending on the species. They secrete a waxy protective covering over their body, which makes control difficult. Some or all life stages of the scale may be found throughout the year (eggs, crawlers or immatures, nymphs, and adults).

Armored scales do not produce honeydew as do soft scales. The test (hard covering over the adult armored scales) will often have concentric rings or overlapping layers. Some soft scales may also have a hard covering present, but it will be smooth or with ridges but no overlapping layers. Flip an adult scale over, and if there is a separate soft body beneath the hard shell, it is an armored scale.

Identification of the scale is important as it may aid in better control. A sample of the infested plant material may be taken to the local Clemson Extension Service county office. From there, it will be sent to the Plant & Pest Diagnostic Clinic at Clemson University for accurate insect identification.

Euonymus scale on foliage.
John A. Weidhass, Virginia Tech University

Cultural Control on Ornamentals

Plants should be kept as healthy as possible to reduce the chance of scale infestation. Plants under stress are more susceptible to armored scale infestations. Maintain plant vigor, but do not over-fertilize trees and shrubs, as this can lead to increased scale problems. Fertilize trees and shrubs approximately mid-March along the coast or April 1^st in the upstate with a slow-release tree & shrub fertilizer. Use an azalea and camellia fertilizer for plants that require acidic soil. Follow fertilizer label directions for rate.

Water trees and shrubs as needed during periods of no rainfall, which is usually no more than weekly during the growing season and monthly during the winter. The rate of irrigation water should be 1” per application. Mulch plants out as far as the drip line of the branches at 3” deep to conserve soil moisture. Do not use weed killers, such as weed and feed products, beneath the canopy of trees and shrubs, as this will add another stress factor to the plants.

For new plantings, plant trees and shrubs in the proper amount of sunlight for the species, plant at the correct depth, and prepare the soil for best growth. For more information on planting, see HGIC 1050, Choosing a Planting Location, HGIC 1052, Planting Shrubs Correctly, and HGIC 1001, Planting Trees Correctly.

If only a portion of the shrub is infested, prune out heavily infested shoots or limbs and promptly dispose of prunings.

Chemical Control on Ornamentals

In general, avoid using contact insecticides as much as possible as they will often kill the naturally occurring enemies of scale insects. Most contact insecticides cannot penetrate the waxy covering on scale nymphs and adults, so the crawler stage is the only life stage that these insecticides control. Failure of contact sprays to work often results from not timing the applications to coincide with crawler activity.

Crawler activity often coincides with the flush of new plant growth in the spring. However, with some scale insects, there may be overlapping generations with an extended crawler emergence period. The crawler emergence can be monitored with double-faced tape wrapped around a branch or by putting an infested shoot or leaf into a baggie and watch for crawler movement. The presence of crawlers can sometimes be determined by sharply tapping an infested twig on a piece of white paper. Crawlers are very small and will appear as moving specks of dust.

Horticultural Oils: Horticultural oils are safe to use and are especially good choices for sensitive areas, such as where people are present soon after treatment. Because of their short residual, they help to conserve beneficial insect species. If possible, time spray applications to coincide with the scale crawler stage, which is most susceptible to all insecticides.

With good spray coverage, horticultural oil sprays may kill all stages of scales that are present at the time of application and often give good control as they kill by suffocation. However, as with all pesticides, multiple applications may be necessary depending upon the scale species and the degree of infestation. It may take multiple applications to control the armored scales because of the layers of adult scales protecting each other, but the horticultural oil is the most efficient, the safest, and the least harmful to beneficial insects.

Horticultural oils are of the highest grade and may be used on tolerant plants during the growing season but at reduced spray concentrations. Tall trees are difficult to treat, but smaller landscape trees and shrubs can be sprayed during the growing season with 1 to 2% horticultural oil. This rate would be 2½ to 5 tablespoons of oil per gallon of water in a pump-up sprayer. If the spring foliage is not out yet on a deciduous plant, spray with a horticultural oil spray so that better coverage of the trunk and limbs is possible. For application during the dormant season, late fall through early spring, apply horticultural oil sprays at 2 to 4% mixture (5 to 10 tablespoons of oil per gallon of water).

Most trees and shrubs can tolerate an application of horticultural oil even during the summer months. However, during high heat and humidity, coupled with drought, some trees, such as maples, are sensitive to oil applications. New needle growth on Eastern hemlocks is sensitive to horticultural oil sprays until the needles mature. Refer to the product label for guidelines on plant sensitivity and any temperature restrictions. In general, sprays should be applied when temperatures are between 45 and 85 °F, and no rainfall is forecast for 24 hours. To reduce the chance of injury due to drought stress, water the trees or shrubs well a couple of days before spraying. If any phytotoxicity (damage) occurs on foliage with oil sprays, wait until leaf drop for additional spray applications (in the fall). A more dilute spray is applied when foliage is present on some sensitive plants to avoid causing injury.

For most small landscape trees and shrubs, apply these spray applications when new leaves start to expand in the spring. At least three applications are needed at five- to six-week intervals. Even when sprays are properly timed, repeated applications may be needed if crawler activity extends over time. Spray the plants thoroughly so that the oils drip or “run-off” from the upper and undersides of leaves, twigs, and plant stems. See Table 2 for examples of horticultural oils for scale control.

Contact Insecticides: Contact insecticides can be sprayed if timed correctly, but only for control of scale crawlers; however, for safety reasons, one must be very careful when spraying contact insecticides upward onto tall shrubs and trees. See the product label for protective clothing and protective equipment that should be used. To reduce the chance of killing pollinating insects, make pesticide applications when shrubs and trees are not in bloom, and always spray in the early evening. Insecticides that may be applied for crawler control are acephate, bifenthrin, cyfluthrin, cyhalothrin, malathion, permethrin, or insecticidal soap. See Table 2 for examples of contact insecticides for scale crawler control.

Soil Applied Systemic Insecticides: Although soil treatments with common insecticide products that contain imidacloprid will control some tree and shrub pests, such as aphids, lace bugs, and white flies, these products do little to control both soft and armored scales. In fact, treatments with imidacloprid may very well increase populations of spider mites on the plants during the summer.

However, systemic products containing dinotefuran will aid in soft and armored scale control. The granular products are applied around the base of the tree or shrub and watered into the soil. The liquid products are mixed with water and slowly poured around the base of the plant. If additional control is needed (beyond the use of horticultural oil sprays), apply one of these dinotefuran products in the spring as new growth appears. However, evergreen plants can also be treated in the fall. The majority (approximately three-fourths) of the ornamental plant species infested with armored scales that were identified in the last two years were evergreen plants. Dinotefuran soil treatment can be used in combination with horticultural oil sprays for improved control. These products may be purchased at landscaper supply stores or ordered on-line. See Table 2 for examples of soil-applied products for armored scale control, as well as soft scale control.

Table 2. Insecticide Active Ingredients & Products Labeled for Control of Armored Scale Insects in Residential Landscapes

Pesticides are updated annually. Last updates were done on 8/24 by N. Jordan Franklin.

Originally published 11/13