Ehrhorn's oak scale—Mycetococcus ehrhorni
This sucking insect (Asterolecaniidae) occurs on evergreen oaks and is common in southern California. Hosts include canyon live oak (Quercus chrysolepis), coast live oak (Q. agrifolia), and tanoak (Notholithocarpus densiflorus).
Identification
To observe Ehrhorn's oak scale, look underneath the pale mycelia of an associated fungus that usually covers this scale on bark. The mature female scale is bright red, rounded to pyriform-shaped, and about 1/25 inch in diameter. The nymphs are smaller and pale orangish to red.
Several species closely resemble Ehrhorn's oak scale. Kuwana oak scale, Kuwania quercus, in California occurs only on blue oak. It's feeding causes obvious roughening and flaking off of bark; the other species named here do not.
Oak eriococcin, Acanthococcus quercus, occurs primarily on the bark of young shoots and small branches. Adult females are dark reddish-purple with a lighter, lengthwise stripe. Some individuals may be covered with powdery wax. Mature females produce a cottony, white egg sac.
Oak xylococcus scale, Xylococculus quercus, adult females are 1/6 to 2/5 inch long, elongate-oval, and dull reddish brown. Females and the red nymphs occur in bark crevices, are covered with wax, and produce a single wax strand up to 1/2 inch long on bark.
Life cycle
Ehrhorn's oak scale females and nymphs occur on bark under the white or grayish fungal mycelia of Septobasidium canescens. The fungus derives nourishment from honeydew produced by the scale.
Damage
Heavy infestations of the scale-associated fungus give oak bark a whitewashed appearance. When abundant for a prolonged period, the scale and fungus can severely stress oaks and slow their growth.
Solutions
Horticultural oil applied to the underside of limbs with a high-pressure sprayer to penetrate growth of the associated fungus can reduce the scale's abundance. Spraying Bordeaux mixture or certain other copper fungicides may help to reduce scale populations indirectly by controlling the fungus, which may help to protect the scales from natural enemies and adverse environmental conditions. To avoid potential phytotoxicity (chemical damage to plants), do not apply Bordeaux during times of the year when temperatures may exceed about 85°F or when succulent, newly flushed leaves are present.
For more information, consult the Pest Notes: Scales and The Scale Insects of California Part 2: The Minor Families.
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