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How to identify it
- Adults and nymphs are oblong or pear-shaped, tapered toward the rear, and about 1/25 inch (1 mm) long or less, so you need a hand lens to see them clearly.[1]
- Most California species, including Phylloxera davidsoni, P. querceti, P. reticulata and P. stanfordiana, are yellow to orange. P. rileyi is dark brown with blackish bumps on its back.[1]
- They lack cornicles, usually look spiny and gather in clusters. Winged forms have reduced wing veins.[2]
- Eggs are yellow, oblong and about 1/50 inch (0.5 mm) long or less.[1]
- The telltale sign is a roundish spot about 1/16 inch (1.5 mm) across at each feeding site, bright yellow at first and then brown, visible from both sides of the leaf. Some species cluster their spots along the veins; others scatter them.[1]
Where it lives
Several species feed on oaks in California, and the Pacific Northwest handbook covers oak leaf phylloxera on red and white oaks in that region. Phylloxera glabra, introduced from Europe, occurs in British Columbia but has not been reported in California.[1][2]
Biology and life cycle
They develop from egg to nymph to adult. Most eggs are laid by wingless adult females, and at least some species also produce winged adults.[1]
They are most abundant in late summer and fall, spend the winter as eggs in bark crevices and perhaps as young nymphs on twigs, and have several generations a year.[1]
In California, hosts include blue oak, Engelmann oak, Oregon white oak and valley oak. In the Pacific Northwest they attack buds and the undersides of young leaves at the branch ends of red and white oaks of all sizes.[1][2]
Their biology has not been well studied in California, and the true identity of some of the species is uncertain.[1]
Damage and signs
- Feeding causes small yellow and brown spots, and in the Pacific Northwest also leaf distortion on young growth.[1][2]
- Heavily infested leaves may drop early, late in the growing season, and the PNW handbook notes that significant damage and defoliation sometimes occur.[1][2]
- In British Columbia, repeated early leaf drop caused by Phylloxera glabra sometimes kills certain oaks.[1]
- The California species appear to be of minor importance, with no apparent harm to tree health.[1]
Health risk
The sources describe them only as oak-leaf feeders; neither mentions any risk to people or pets.[1][2]
Control
Prevention
Oaks appear to tolerate this feeding, and UC IPM says no control is warranted to protect plant health. Spotting the insects early on the foliage leaves time to decide whether anything is worth doing.[1][2]
Biological control
Natural controls usually keep damage low; ten species of predators have been recorded feeding on oak phylloxera. They include minute pirate bugs (Anthocoris), brown and green lacewings, and lady beetles such as Axion plagiatum and the multicolored Asian lady beetle.[2][1]
Pesticides
Where the spotting cannot be tolerated on a smaller oak, thoroughly spray the undersides of the leaves with horticultural oil or insecticidal soap to reduce the insects and later damage.[1]
Common questions
- What are the tiny yellow spots under my oak leaves?
- They may be feeding spots of oak leaf phylloxera. Look under the leaf with a hand lens for tiny yellow to orange, pear-shaped insects and their yellow eggs.[1]
- Will oak leaf phylloxera kill my oak?
- In California the species involved have no apparent effect on tree health. In British Columbia, repeated defoliation by the introduced Phylloxera glabra sometimes kills certain oaks.[1]
- Should I spray my oak?
- Usually not. If the spotting bothers you on a small oak, horticultural oil or insecticidal soap sprayed thoroughly on the leaf undersides can reduce the insects.[1]
Sources
- University of California Statewide IPM Program, “Pest Notes: Oak Leaf Phylloxera (Home and Landscape).” ipm.ucanr.edu/PMG/GARDEN/PLANTS/INVERT/oaklfphyll.html (accessed 2026-11-09)
- Pacific Northwest Pest Management Handbooks (Oregon State University, Washington State University and University of Idaho Extension), “Oak (Quercus)-Oak leaf phylloxera, Pacific Northwest Insect Management Handbook.” pnwhandbooks.org/insect/hort/landscape/hosts-pests-landscape-plants/oak-quercus-oak-leaf-phylloxera (accessed 2026-11-09)
- iNaturalist, “Phylloxera taxon page (order and family).” www.inaturalist.org/taxa/319317 (accessed 2026-11-09)