Landscape - Control of Armored and Soft Scales

Scale control on landscape plants can be a challenge. This is especially the case with hard or armored scales. The following is some information on effectively controlling scale insects.

Suppressive Treatments: Armored vs. Soft Scales

Soft Scales:

Compared to armored scales, the soft scales are relatively easy to suppress with either contact sprays or systemic treatments. Although large soft scale adult females are more difficult to control, the immature nymphs are highly vulnerable to sprays when good coverage is achieved. There are numerous windows of control opportunities when applying sprays or systemic treatments against soft scales. (1) The best window for control when using spray treatments is toward the crawler emergence period. With only two major exceptions, all soft scale species produce crawlers during the months of June or July. Although scale crawlers are only 2 or 3 times the size of spider mites, they are usually clearly visible without magnification. Most crawlers have a yellowish or reddish coloration. (2) Sprays can also be successfully targeted against the settled 1st instar nymph stage feeding on foliage or bark during the growing season. Achieving adequate coverage to foliage is the major challenge with large deciduous shade trees since the settled nymphs feed on the undersides of leaves along major veins. (3) In addition, dormant oil treatments can be applied in the late fall or early spring to the over wintering 2nd instar nymphs on deciduous hosts. These nymphs have a black or brown coloration and are considerably larger than the crawlers and 1st instar nymphs. They can be observed in clusters on the bark of twigs, branches or trunks. (4) Finally, since soft scales are vascular feeders (phloem or xylem), root absorbed systemic insecticides such as imidacloprid (Merit) or dinotefuran (Safari) have provided better than 90% control rates. Root systemic treatments can be applied as a drench or be soil injected any time during the year as long as the ground is not frozen. Fall or spring applications are most typical. Having adequate soil moisture is a key factor to ensure success when applying root systemic treatments.

Armored Scales:

The primary reason armored scales are so difficult to manage is because the best control windows are dreadfully limited. The covers produced by different armored scale species vary somewhat in their abilities to deter insecticide penetration. Generally the waxy cover greatly reduces the effectiveness of insecticide sprays. (1) The most vulnerable life stage of all armored scales is during the crawler emergence period after egg hatch. The 1st instar nymphs continue to be susceptible to spray treatments for another one or two weeks after settling. However, after this time period most scales species secrete covers to a sufficient thickness to significantly diminish spray effectiveness. (2) Unfortunately, root or trunk injected systemic insecticides provide poor results. Efficacy trials have shown less than 20 to 30% suppression rates. (3) Translaminar materials such as Orthene (acephate) can cause some mortality to armored scales feeding on foliage. They are ineffective against scales feeding on woody tissue. (4) Tragically the traditional use of dormant oil treatments to smother armored scales appears to be overrated. The attempt to achieve satisfactory control by spraying dormant oils is often disappointing since suppression rates are often less than 20%. Therefore to reliably manage armored scales, insecticide treatments must be applied to the crawlers or shortly thereafter.

Research and field observations have determined the time of the year when most landscape scale species have crawler emergence. The most efficient method to monitor for armored scale crawler periods is to use growing degree-day (GDD) emergence periods. The GDD ranges for many common armored scale species are listed in the included table. All scale crawlers on the plant host do not hatch and emerge at the same time, but can range over a period of one, two or more weeks. Consequently, the GDD range represents the span of time the crawlers of a species occur, or when peak emergence takes place.

Conservation of Scale Predators and Parasitoids:

Armored scale infestations in the urban landscape often develop through secondary pest outbreaks. Some of the most effective biological controls found in the landscape are parasitoids against armored scales. When environmental conditions are favorable, parasitoid wasps can very adequately maintain armored scale populations well below thresholds levels. Research studies have shown that cover sprays with broad-spectrum insecticides (e.g., pyrethroids) will often promote scale infestations. These cover or calendar sprays are seldom timed properly against crawler emergence and consequently the treatments are ineffective. On the other hand, these broad-spectrum treatments are death to most beneficials and especially the sensitive parasitoids. Landscape cover sprays over several years duration have shown to increase armored scale populations by a factor of 3 to 4 times compared to landscapes not receiving these types of undesirable treatments. To conserve the important beneficials, the use of horticultural oils during scale crawler emergence periods is most advantageous. Scale species having multiple generations or extended crawler periods may require several oil treatments during the season.

Reprinted from "Effectively Managing Scale Insect Pests in the Urban Landscape" by Steven K. Rettke, RCRE Ornamental IPM Program Associate, in the March 16, 2006 edition of the Plant and Pest Advisory, Landscape, Nursery, and Turf edition, Rutgers University