This soft scale (family Coccidae) generally does not threaten the health of plants it infests, but its copious honeydew and the resulting blackish sooty mold can be annoying. Brown soft scale prefers broadleaf evergreens, but it can occur on the leaves and twigs of almost any broadleaved plant.
Identification
Brown soft scale females are oval, slightly convex in profile, and yellowish to dark or light brown. Mature females are 1/10 to 1/6 inch long. The scale occurs on leaves or twigs and rarely on fruit.
Citricola scale resembles brown soft scale and both species can occur on citrus and hackberry (Celtis species). However, citricola scale has only one generation per year so its individuals are uniform in size unlike the differing age (size) in colonies of brown soft scale. Citricola scale occurs on leaves only during the summer and fall and on twigs in the late fall, winter, and spring. Brown soft scale is present on twigs throughout the year and on leaves when present, such as overwinter on evergreen hosts.
Life cycle
Female brown soft scales lay a few eggs at a time over an extended period, mostly throughout the summer. Eggs hatch almost immediately, and the crawlers (mobile first instars) soon settle and feed. Nymphs molt twice and mature into females. No males have been observed in California.
Several different-sized life stages usually occur together at the same time. Populations are usually highest from midsummer to early fall. Brown soft scale has three to five generations per year.
Damage
Brown soft scales suck phloem sap and excrete sticky honeydew on which grows blackish sooty mold. However even when very abundant this scale appears to have no significant adverse effect on its host. Hosts are numerous and include aspen, avocado, bay laurel, citrus, cottonwood, holly, manzanita, palm, poplar, strawberry tree, and willow. See the U.S. Department of Agriculture's ScaleNet database for a complete list of the plants this scale can infest.
Solutions
Predators and especially tiny, parasitic wasps can provide effective biological control of brown soft scale if conservation of natural enemies is practiced. To improve the effectiveness of natural enemies, control ants, minimize dustiness (e.g., periodically hose off shrubs), and avoid the application of broad-spectrum, persistent insecticides. See Protecting Natural Enemies and Pollinators for more information.
Parasitic wasps attacking brown soft scale include Metaphycus angustifrons, M. luteolus, M. stanleyi, and Coccophagus species. The female parasites primarily lay their eggs in the early instars, before scales mature and can produce offspring. Because brown soft scale generations overlap, it provides parasites with the nymphal stages susceptible to parasitism throughout the year. Parasitism of brown soft scale is very effective unless disrupted by ants, dust, or broad-spectrum, persistent pesticides. Evidence of parasitism includes exit holes of adult parasites in nymphs or mature females. When parasitized, nymphs may turn black. The immature Metaphycus parasites themselves (larvae and pupae) can sometimes be observed through the cuticle (outer covering) of brown soft scale.
The Australian lady beetle (Rhyzobius lophanthae), brown lacewings, green lacewings, and twicestabbed lady beetle prey on all stages of brown soft scale. During the scale's adult stage, early instars of the lady beetles can feed beneath their host so they are easily overlooked.
In locations with hot summers, pruning to open plant canopies can increase scale mortality from heat exposure and improve parasite access to this pest. When direct control is warranted, after employing the natural enemy conservation practices, infested terminals can be thoroughly sprayed with horticultural oil during the dormant season or when monitoring indicates that crawlers are active in the spring. See the section "Monitoring" in Pest Notes: Scales for how to effectively time oil application.
Adapted from Integrated Pest Management for Citrus and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
