Kuno scale—Eulecanium kunoensis =E. kunoense
This sucking insect (Coccidae) sometimes damages plum and other Prunus spp., mostly in the San Francisco Bay Area and Sacramento Valley.
Kuno scale females are about 1/6 inch in diameter and almost spherical, resembling beads on stems. Mature females are dark and shiny brown. Females are yellow and orangish with black bands and blotches during a short period prior to egg production in late spring.
Unlike other species of soft scales it resembles, Kuno scales have a broad, internal 'lip' or shelf-like margin where the adult female attaches to the host; this lip is best seen by removing females from the host and examining their underside.
The nymphs are yellow or brown and flattened and during summer occur mostly along veins on the underside of leaves. The elongate, translucent, male cocoons are 1/12 inch long and can be prevalent on twigs.
Nymphs overwinter on twigs and mature into adults during March through early May. Eggs produced beneath the female hatch during spring into crawlers (mobile, first-instar nymphs) that migrate to settle and feed on leaves during summer. Nymphs return to the twigs before leaf drop in the fall. Kuno scale has one generation per year.
Kuno scales suck phloem sap from leaves and twigs. The scale is most abundant on plum trees in gardens and landscapes; it is not a pest on commercial fruit and nut crops in California. Rarely damaged hosts include cotoneaster, pyracantha, rose, walnut, and other woody Rosaceae.
Kuno scales can become abundant quickly. They produce copious amounts of honeydew, which results in blackish sooty mold growth and attracts ants. Feeding by high populations of scale causes hosts to appear water stressed. Prolonged high scale populations can result in premature leaf drop and dieback of twigs.
Control ants, which attack the natural enemies of scale insects. If the scale has been abundant, spray horticultural oil on branch terminals and twigs during the delayed dormant period (as buds begin to swell) or thoroughly drench the underside of leaves in early summer. Avoid spraying dormant plum and walnut trees with oil. Do not apply oil when temperatures will exceed 90°F or if trees have suffered from a lack of adequate soil moisture.
For more information, consult the Pest Notes: Scales and The Scale Insects of California Part 1: The Soft Scales.