The Ecology of the White Oak

Marie Mellinger

White Oak gall

The White Oak (Quercus alba) is the best-known and most common tree of eastern North America. It can attain great size, and reach an age of 800 years or more. Donald Culross Peattie said, “a hundred years is brief in the life of an oak.” We can trace its growth from an acorn, through sapling to maturity, and finally, decay. All along its life-way the White Oak has many intricate and varied relationships with other plants and animals, many of which are still not fully understood.

The common name, White Oak, comes from the pale gray, shallowly fissured bark, a good means of determination. The tree grows in a symmetrical manner with sturdy limbs reaching upwards, and greenish, slightly downy twigs. Leaves are regularly lobed, but can vary greatly in size. They are blue-green in summer, and paler on the underside. In late autumn they turn rusty red, then brown, and persist on the trees over winter.

Flowers appear before the leaves and are of two types: long hanging, pollen-rich aments, and short, stubby pistillate blooms. Acorns are enclosed in gray-green basket weave cups, and there is great variation in their size. The nuts have sweet kernels, and are rich in oil. They feed squirrels, deer, Quail, Ruffed Grouse, wild turkeys, bear, raccoons, Blue Jays and woodpeckers. Lee Gibbs tells us that the White Oak is a favorite nesting site of the Ruby-throated Hummingbird, for the varied gray-green lichens on the oak limbs offer perfect camouflage for their tiny nests. White Oaks are also preferred nesting places for the Blue-Gray Gnatcatcher.

White Oak trunks are often covered with patches of Blue-green algae (Gleocapsa) that brighten after every rain. The trees frequently host Mistletoe (Phoradendron), several species of lichens such as Parmelia and Lecanora, as well as the Old Man’s Beard lichen (Usnea). Gerardia flava, the False foxglove, and several allied species, are partially saprophytic on the roots of White Oak. This is a relationship not very distinctly understood.

In the litter of the forest floor and growing under White Oaks are such mushrooms as the Russula vesca, with a brown top, firm white flesh and gills, and the stem spotted with brown. Lactarius quietus, with a reddish-brown top and brown concentric ringed stem, also grows under oaks. When broken, this fungus shows whitish milk and emits a sweet, oily smell. Three species of Cortinarius grow under oaks: the dark tan C. hinnuleus, the clay colored C. anomalis, and the lilac-blue, C. albo-violaceus. A puffball, Hmenogaster, also grows in oak litter.

Various kinds of cup fungi grow in association with oak. Chrysoplenium has blue-green cups on oak wood, and its mycelium can turn oak heartwood green or yellow. The Sclerotina makes brown stemmed, funnel shaped cups on acorns. Helotium makes bright yellow cups on decaying branches or on acorns. A bark fungus, Corticium, makes the bark-flaking on White Oak. The white rot of oaks, Hydnum erinaceus, attacks oak wood with its wet, light floccose mycelium. The sporophores that extend from the bark are white, spiny growths. The piped oak rot, Corticium, attacks oak heartwood. Visible fruiting bodies are irregular, pocket-like patches of white fiber on the bark. Oak wilts, Nectria and Strumella, attack White Oaks and cause cankers on the bark.

Many sorts of shelf and bracket fungi use the oak as host, but usually only after the tree is dying or dead. Polyporus frondosus and Polyporus sulphureus both put out large and vivid growths at the base of oak trees. Bulgaria makes clustered brackets on oak bark. The elfin Mycena appears in tiny tufts in knotholes.

Insects of many species utilize the White Oak in one way or another. The blue-dotted caterpillars of Thecla calnus and the Long-tailed Hind-wing feed on oak foliage. So to do the larvae of the White Hairstreak, Thecla m-album. Other butterflies whose larva feed on oak foliage include the Northern Hairstreak, the Southern Hairstreak, Edward’s Hairstreak, the Banded Hairstreak and the Striped Hairstreak. The spined yellow and black stripe caterpillars of the Checkerspot also feed on oak leaves. The segmented caterpillars of the Dreamy Dusky-winged, and the waxy green larvae of the juvenal Dusky-wing, are also found on oak. Caterpillars of the Red-spotted purple butterfly (Basilarchia) also feed on oak foliage. Lace bugs (Corythuca) and an oak aphid, Phylloxera quercus, are also frequently found on White Oaks.

Acorns have their own collection of insect life. Moth larvae of Vellisopus live in the fallen nuts, as do the larvae of Valentina, a moth miner. Balaninus, the acorn weevils, drill holes in acorns. Squirrels are especially fond of acorns containing these weevils, according to Lutz, but other authorities say squirrels will discard all but the good nut meats.

Some four hundred species of gall-making insects choose the White Oak as their host. Many of these have a very complex life cycle, spending part of their existence in one form and in one part of the oak, and another in a different shape in another part of the tree. Most galls are slightly disfiguring but seemingly not at all harmful to the oak. The theory has been that the gall-making insect deposits an acidic secretion in plant tissue that causes the gall to form. Other studies show that there is some bacteria association, and that the bacteria may be the real cause of the gall growth. This is an area that still needs much study.

Saw flies of the genus Callirhytis make innumerable galls on oaks. C. capsulis makes stemmed galls on the underside of white oak leaves. C. similis makes pink, warty galls on the leaf petioles, and C. clavula makes a fuzzy gall, also on the petiole. C. papillatus, the nipple gall, makes nipple shaped projections on both sides of the leaves, each surrounded by a reddish areola. C. futllis makes flat galls on the leaf veins, with an alternate generation that makes woody galls on oak roots.

Entomologists cannot agree on the taxonomic names of many of the gall-makers, or even as to whether they are gall wasps, gall flies, or saw flies in certain of the complicated genera. Cynips, for example, which makes the common oak Hedgehog gall, is listed in some books as a saw fly. In other books this same gall-making insect is called Ascraspis and is listed as a wasp. So? The alternate generation of the Hedgehog gall appears on the leaf buds in early spring, and for many years was believed to be a separate species. Spiny Hedgehog galls can be colonial, and they remind one of a curled up wooly-bear caterpillar. They are eaten by Red Squirrels and by White-footed Mice.

The spotted oak apple, Cynips centricola, appears on the underside of the leaves. It is conspicuous in the fall months and persists over winter. Each is a round, thin-shelled sphere, with a thread-supported hard center. They grow to 4/5 inch in diameter and are edible when young. Cynips pezomachoides makes small galls on leaf veins, with an alternate generation on leaf buds. Cynips prinoides makes shiny, single celled galls on the under leaf surface.

A selection of showy galls is made by the gall midge, Cecidomyia poculum, and look like scattered sequins on the leaf. They also form small saucer-shaped galls on the under leaf surface, between the veins. These are often brightly colored. The gall wasp, Taphrina, also makes brightly colored leaf blister galls, often blue and yellow, that are raised circular spots on the oak leaf surface. Dishlocaspis makes bullet galls, hard round growths, on leaves or twigs. A saw fly, Andricus flocci, makes galls that look like tufts of white wool on the leaves. Another saw fly, A. petiolicola, makes many celled galls where the leaf joins the petiole. Still another Andricus makes a white gall on oak aments.

Gallflies, or wasps, of the genus Neuroterus make many forms of oak galls. Best known are the blister galls found on White Oak leaf veins. These appear on both sides of the leaves. Two species of Neuroterus make nipple galls on leaves and petioles, and two other species make corky galls on petioles. Alternate generations of Neuroterus are more typical of gallflies than of wasps. Neuroterus vesicula makes thin-shelled twig galls that remain on the twigs over winter, and are eaten by field sparrows and Goldfinches.

Best known of the oak galls is the oak apple, made by Amphibolips, but these are more common on Red Oak species than on oaks of the White Oak group. Another Amphibolips makes strange projections on acorn cups. Amphibolips is definitely a wasp. Biorhiza, also known as Xanthoceras, is a wasp that makes oak fig galls. The alternate generation appears as fleshy root galls.