Frosted Scale

No, we are not discussing a breakfast cereal.

Frosted scale (Parthenolecanium pruinosum) is a soft scale pest of walnuts. If you have a walnut tree (and why wouldn’t you), scale insect pests can be a major problem. These sap-sucking pests also feed on stone fruits, such as apricot and peach, along with apples, pears, raspberries, grapes, pistachio, roses, laurel, birch, locust, sycamore and elm, spreading disease as they go.

Frosted scale description

Like other scale insects, adult female frosted scale are 1/4 inch, dark brown ovals, with a protective, dome-shaped covering. As the name suggests, frosted scale has a waxy, frost-like coating over its shell. This frosty coating stays in place for a while, but it eventually wears off, leaving behind a brown shell that can remain in place for a year or so.

Frosted scale lifecycle

Nymphs overwinter on twigs. In early spring, these nymphs quickly grow to adult size. By late spring, females lay many eggs, filling the space between their body and their protective shell. After the eggs are laid, the female dies. When the eggs hatch, the nymphs, or crawlers, come out from hiding and begin feeding on the underside of leaves. They will continue feeding until fall, when they molt and move back onto twigs, where they spend the winter. There is one generation each year.

Problems associated with scale feeding

​Frosted scale insects feed on the sugary plant juices found in leaves and new twigs. As they feed, these nymphs produce large amounts of honeydew (sugary bug poop), which attracts disease carrying ants, and promotes the growth of sooty mold. Small numbers of frosted scale insects are not a problem. Heavy infestations, however, can suck the vitality from your trees, reducing crop size and quality. Also, as scale insects feed, they create wounds. These wounds make it easier for infections to take hold. One such fungi, Botryosphaeria, can lead to lower limb dieback and other potentially fatal fungal diseases.

Controlling frosted scale

In the world of commercial agriculture, insecticides are recommended if 5 or more nymphs are found per foot of the previous year’s wood. This means grabbing a hand lens and looking very, very closely. In the home garden, beneficial hunters, such as parasitic wasps, will provide the best protection. You can tell that a frosted scale nymph has been parasitized because it will turn black. Parasitized adults will have perforated shells. ​

​You can increase the populations of these tiny, beneficial wasps by avoiding the use of broad spectrum insecticides and pesticides. Dormant oils can be used in winter and early spring to rid your tree of scale insects, but walnut trees are sensitive to horticultural oils and you need to use narrow-range oils to avoid harming the tree.

Scale infestations can sneak up on you. Be sure to take the time every month or so to inspect your trees for signs of infestation.