Soft scales are relatively easy to control and the June and July period is when crawlers are active (except Magnolia and Tuliptree scales which are later). Horticultural oil sprays in the dormant season are also effictive as are systemic insecticides applied to the soil in fall or spring. The following is a good article on the subject from Rutgers University.
Compared to armored scales, the soft scales are relatively easy to suppress with either contact sprays or systemic treatments. Some of the common landscape soft scale species include Calico, Fletcher, Wax, Terrapin, Cottony Maple, Lecanium, Cottony Taxus, Pine Tortoise, Striped Pine, and Spruce Bud. 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. The best window for control when using spray treatments is toward the crawler emergence period. With only two major exceptions (Magnolia & Tuliptree scales), all other soft scale species produce crawlers during the months of June or July. Although scale crawlers are only 2 to 3 times the size of spider mites, they are usually clearly visible without magnification. Most crawlers have a yellowish or reddish coloration. 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. 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. 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.
Information from Steven K. Rettke, Ornamental IPM Program Associate, Rutgers University.