Oak leaf phylloxera—Phylloxera spp.
These phylloxera are soft-bodied aphidlike insects that suck and feed on the underside of oak leaves. This feeding causes small, brown and yellow spots that are visible on both sides of infested leaves.
Oak leaf phylloxera adults and nymphs (immatures) are oblong or pear shaped and tapered toward the rear end. They are 1/25 inch (1 mm) or less in length and a hand lens is required to clearly distinguish the individual insects.
Adults and nymphs of most California species are yellow to orange, including Phylloxera davidsoni, P. querceti, P. reticulata, and P. stanfordiana. Phylloxera rileyi is dark brown with blackish tubercles (raised bumps) on top of the body. The yellow eggs are oblong and 1/50 inch (0.5 mm) or less in length.
The biology of oak leaf phylloxera has not been well studied in California. The true species identity of at least some of these insects is uncertain.
Phylloxera develop through three life stages: egg, nymph, and adult. Most egg laying is done by wingless adult females. At least some species produce winged adults. The nymphs develop through several increasingly larger instars.
Oak leaf phylloxera are generally most abundant in late summer and fall. They overwinter as eggs in bark crevices and perhaps as young nymphs on twigs. These phylloxera have multiple generations per year.
Oak leaf phylloxera suck and feed on the underside of the leaves of Quercus species. Some species, and the discolored, roundish spots from their feeding, are most abundant along the leaf veins. Others feed more scattered throughout the leaf underside. Where each phylloxera feeds the leaves develop a roundish spot about 1/16 inch (1.5 mm) in diameter that initially is bright yellow then darkens to brown. Heavily infested leaves may drop prematurely late in the growing season. Hosts include blue oak, Engelmann oak, Oregon white oak, and valley oak.
In British Columbia repeated premature defoliation of certain oaks by P. glabra sometimes kills the tree. But this phylloxera species, inadvertently introduced from Europe, has not been reported in California. The oak leaf phylloxera species in California appear to be of minor importance and have no apparent adverse effect on tree health.
Management has not been investigated in California. Oaks appear to tolerate phylloxera feeding on their foliage in California. No control is warranted to protect plant health.
Various predatory insects feed on oak leaf phylloxera. These include Anthocoris species minute pirate bugs, brown lacewings, green lacewings, and lady beetle (ladybug) adults and larvae including those of Axion plagiatum and multicolored Asian lady beetle.
Where their aesthetic damage cannot be tolerated, based on recommendations from the Pacific Northwest, the underside of the leaves of smaller oaks can be thoroughly sprayed with horticultural oil or insecticidal soap to reduce these pests' abundance and subsequent feeding damage.
For more information see Aphids on the World’s Plants: An Online Identification and Information Guide from the Natural History Museum of London, A Field Guide to Insects and Diseases of California Oaks from the U.S. Forest Service, and Oak Leaf Phylloxera from Washington State University.
Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).