Frosted Scale and European Fruit Lecanium

The frosted scale is the most important soft scale pest of walnuts. This scale has one generation per year. It overwinters as a nymph on twigs and small branches. In spring it grows rapidly, becomes convex, forms a frostlike waxy cover. It is only during this period that frosted scale can be easily distinguished from European fruit lecanium. In late spring females lay many eggs, which fill the entire space beneath their cover, and die after egg production. The white waxy substance weathers away, leaving oval dark brown covers that may be present for a year or more.

Newly hatched nymphs, or crawlers, emerge from beneath the scale cover from late May through June and settle mostly on the underside of leaves. Here they feed for the rest of the summer. In fall, the nymphs molt and move back to twigs.

The European fruit lecanium has essentially the same life cycle. The immature stages closely resemble those of the frosted scale, but the adults do not form the thick, frostlike cover in spring. Instead, the cover is domed, shiny brown, and about 0.25 inch (6 mm) in diameter with several ridges along the back.

Soft scales suck plant juices from leaves and twigs. Low to moderate numbers apparently are not damaging, but heavy numbers reduce terminal growth and vigor, resulting in smaller nuts and poor kernel quality. Soft scales, such as frosted scale and European fruit lecanium, can secrete large amounts of honeydew that cover nuts and favor the growth of sooty mold, increasing the chances for sunburn damage.

Parasites play an important role in controlling these soft scales. If this natural control is disrupted by adverse weather or by insecticides applied for other pests, a pesticide application may be required.

The most important of the parasitic wasps that attack these soft scales are Coccophagus, Encyrtus, and Metaphycus spp. The Metaphycus wasps produce several generations a year, compared with one generation of the scale, and they parasitize all stages of the frosted scale except the eggs. Parasitized nymphs are almost black and have convex covers; unparasitized nymphs are flat and opaque. Several parasites commonly emerge from a single parasitized adult scale, leaving a perforated cover.

Biological control and sprays of narrow range oils can be used in an organically certified crop. Caution should be taken when applying oils in walnuts to avoid injuring trees (see narrow range oil comments in the table below).

Monitor scales during the dormant period. For details on how to monitor these scales with other pests see DORMANT MONITORING. High numbers of soft scales often result from the use of pesticides that are disruptive to parasites and predators. If a high degree of parasitization is observed, either do not apply insecticides or delay insecticide applications until after crawlers are detected in late spring.

During the dormant period, examine the previous season's growth on randomly selected trees throughout the orchard. If you find more than five nymphs per foot of last year's wood throughout the orchard and parasitism is not significant, apply an insecticide. Apply during the delayed-dormant period before rapid scale growth begins in early spring. Insecticides applied to crawlers (spring) or first-instar scales on leaves (summer) are also effective. Place double-sided sticky tape near adult scales in early spring (mid- to late April) to monitor for crawler emergence and time treatments.

• Photos of scale