Frosted Scale – Parthenolecanium pruinosum

Frosted Scale: Appearance, Territory, Damage and Life

Latin name: Parthenolecanium pruinosum

Appearances: Large, oval, dark brown, and mature females have a characteristic powdery white wax coating covering the entire wax shell. It is quite comparable to the European Fruit Lecanium scale, which is different in that it lacks the white waxy powdered coating. A thick layer of waxy white powder covers a large brown scale. Nymphs in their second instar are light brown and hardly convex.

Host plants: This scale prefers walnut trees and is the most significant soft scale pest of walnut in California. However, it also infects a wide variety of other trees and shrubs, including stone fruits, pistachio, apple and pear, locust, grape, rose, elm, birch, laurel, sycamore, raspberry, and others.

Territory: Originally from the mountains of Mexico, but now found in California, Pennsylvania, and even further north into western Canada. Additionally, Australia has it.

Damage insect caused: Plant liquids are sucked from leaves and twigs by soft scales. Smaller nuts and lower-quality kernels result from heavy numbers, which inhibit terminal growth and vigor. Low to moderate numbers, it appears, are not harmful. Soft scales can secrete copious amounts of honeydew that can cover nuts and encourage the growth of sooty mold, increasing the risk of sunburn damage. Examples of these scales include frosted scale and European fruit lecanium.

Life cycle and habits: The most significant soft scale pest of walnuts is the frosted scale. There is one generation per year on this scale. It spends the winter on twigs and small branches as a nymph. It develops a frost-like waxy layer, turns convex, and grows quickly in the spring. Frosted scale and European fruit lecanium are only clearly distinguishable during this time. Females deposit a large number of eggs in the late spring, which completely occupy the area under their cover and then perish. Oval dark brown covers that could last a year or more replace the white waxy substance as it weathers away. From late May through June, newly born nymphs, or crawlers, emerge from beneath the scale cover and primarily dwell on the underside of leaves. For the remainder of the summer, they eat here. The nymphs molt in the fall and return to twigs. The life cycle of the European fruit lecanium is very similar. Although the adult stages do not create the thick, frost-like layer in the spring, the immature stages closely resemble those of the frosted scale.