PARASITES
Oak-Galls and Oak-Wasps (50+ kinds, harmless)
See 40 photos of oak galls
Oak apple Biorrhiza paillda
The oak-apple...is formed by the gall-wasp...which pierces the terminal buds of the leafless twigs in January, laying numerous eggs...The eggs hatch at the beginning of May and on the emergence of the larvae begins to grow and completes its development about the end of the month...The wasps emerge in July...The fertilized female crawls down the tree, enters the soil and pierces young oak roots in which she lays her eggs. When these hatch into larvae, hard brown spherical galls up to half an inch in diameter are formed on the young root...In January the wasp is mature and bores through the hard wall of the root-gall, pushes through the overlying earth, crawls up the trunk of the leafless tree in search of suitable terminal buds...
Currant gall Neuterus baccarum
Formed on the male catkins when they appear in May. These are of the size, shape and colour of red currants, a bunch of which is closely imitated when an oak catkin is crowded with the galls. The wasps develop in them with great rapidity and most have emerged by the end of June. The summer galls may also be formed on the leaves, where they are larger and green, not red. The fertilized female wasp then attacks the lower surface of the leaves, causing the very common disc-shaped
spangle galls to appear in July. These are of a beutiful red colour, at first with a central umbo, and sometimes cover the undersurface of the leaf very thickly, causing it to wither prematurely. In September the spangle gallsbecome detached, fall to the ground and round off, the contained larvae continuing to grow through the winter. The wasps appear in April and attack the catkins or leaves, producing the summer currant galls.
Moths (caterpillers of)
The caterpillars of a few moths feed on various parts of oak-trees. Of these the Ghost Swift Hepialus humuli larva tunnels in the roots and stems of oak and beech seedlings in late summer and again in the spring after hibernation, forming an underground pupa in April or May. The moth emerges at midsummer, laying eggs on the soil surface, and its narrow, white or yellowish-white forewings and smoky-grey hindwings and its weak, flitting flight justify its name. The Buff Tip Pygaera bucephala has light, buff-coloured tips to its dark brown forewings, and when at rest with its wings folded looks like a half-rotten twig broken across. The caterpillar sometimes strips oak foliage. The Mottled Umber Hybernia defoliaria and the Winter Moth Cheimatobia brumata are specially destructive to the foliage of fruit trees, which they often damage severely, but are also found on oaks. The females are winglass and on emerging from the subterranean pupae in October or November crawl up the trunk of the tree to lay their eggs on the trunk or branches or near the winter buds. The caterpillars appear about the end of April, are full fed in June and descend the tree to enter the soil, where they pupate. Though theselarvae may destroy much oak foliage, by far the most destructive caterpillar is that of the Oak Roller Tortrix viridana a small moth with bright green forewings. In some years these larvae strip the foliage from whole trees and sometimes from whole oak woods, by the beginning of June. They turn over the edges of the leaves, upper side outwards, and eventually pupate under the cover of a partly eaten leaf. The hordes of larvae mainly attack mature trees, beginning at the summits. Tha attack is often followed by the mildew fungus Oidium, which completes the damage. It is not established that these attacks actually kill oaks in this country, though they certainly weaken them. Lammas shoots in large numbers commonly grow out later in the summer to renew the foliage. On the Continent the attacks of the Oak roller are so destructive that insect-destroying powder is now often showered on oak woods from aeroplanes. The pedunculate oak is nearly always the species attacked, the sessile oak being either completely immune or only slightly eaten. This difference has been attributed to the fact that the leaves unfold rather earlier and have time to toughen before the caterpillars are ready to attack them in force. During the 1930s the pedunculate oaks of the Forest of Dean were very badly defoliated by the Oak Rollers and Winter Moths for several years in succession, but the sessile oaks escaped. The Goat Moth Cossus cossus, a large bulky moth, has very voracious caterpillars which may be 3 inches long and eat out great tunnels, first between bark and wood, and then in the wood, of several kinds of tree, including oaks. They are developed from eggs in June or July at the base of the tree, and live for two, three or even four years in the interior of the trunk. Eventually they leave the trees and form pupae which lie in cocoons buried in the soil. The Leopard Moth Zeuzera aesculi belongs to the same family but is smaller. It has whitewings covered with bluish-black spots. The eggs are laid in summer on the stem and branches of the tree and the caterpillars bore like those of the Goat Moth, mostly the upper parts of the tree, and live there for two or three years. The Leopard Moth attacks many kinds of broad-leaved tres including oaks.
Beetles
Of the weevils Balanimus larvae live in acorns, those of Agroderus feed on the leaves which they roll up, and those of Orchestes mine oak leaves. The larvae of bark beetles make galleries or tunnels between the bark and the wood. Those of Scolytus inticatus are very common in the South of England, and Dryocoetes villosus breeds in dying and felled oak. The 'pinhole borers' make tunnels from the outside of the bark straight through into the wood, and in some species the tunnels are lined with 'ambrosia' fungi which the beetles carry in with them and on which they feed, the fungi subsisting on the excrement of the larvae. The walls of the tunnels are blackened by the fungi. Platypus cylindrus is one of these beetles and is a real timber pest on oaks in the south and west of England. Trypodendron also attacks oaks. 'Long horn beetles' are so called because of the extreme length of the antennae in some species. Several of them attack oak wood, either in the growing tree or in the timber-yard: others infest only rotting logs or stumps. The eggs are laid in crevices of the bark or in notches cut by the females. Of the Lucanidae the well known stag beetle Lucanus cervus is the largest British beetle, two or three inches in length, and its name derives from the great antler-like mandibles of the male. It lives in the rotting wood of logs or stumps of broad-leaved trees, including the oaks. 'Furniture beetles' are originally forest species, but the damage they do is chiefly to old timber. The most nororious of the wood-beetles is the Death Watch Xestobium rufovillosum which inhabits various decaying trees in the forest, but in buildings is chiefly associated with the oak beams of roof timber. 'Powder post' beetles are associated with damage to furniture, woodwork, etc. Oak wood is their commonest food. They are so called because the wood dust produced by the larvae is fine and powdery.
Plant Bugs
Some of the Aphids (plant-lice, to which the well known 'green-fly' belong) attack the oaks, one at least being parasitic on oak toots, while the Scale insects which lie on the bark of trees under the protection of scales,which are really the cast off skins of the insects, the so called 'golden oak-scale' asterolecanium variolosum a dark green species, is common on young oak twigs, where the insects lie in shallow pits in the bark. Unlike some of the aphids, such as the so called 'Chermes' which are seriouspests on various conifers, and the 'felted beech coccus', neither coccids nor aphids appear to do much harm to oaks.
Fungi
Most of the fungi that are found on oaks occur also on other species of deciduous tree, and few are fatal to the trees that attack. Sclerotinia candolleana forms circular yellowish spots on oak leaves toward the end of June. These spots turn brown later on and most of the leaf becomes discoloured. After the leaf-fall black 'sclerotia', 4mm in diameter, develop on the dead fallen leaves, and in the spring each sclerotium produces a stalked 'apothecium', an open fructification which bears the spore sacs or asci. Bulgaria polymorpha is another Ascomycete common on the bark of felled oaks and may attack and kill the bark of old standing trees. In the dead bark are formed large black apothecia which open and expand into disc-like structures exposing the layer of asci. Rosellinia quercina causes a serious root disease of young oaks. The fungus invades the rootlets from the soil and then passes into the mainroot. The first symptom of the disease, which is most prevalent in wet seasons, is a yellowing of the foliage. The fungus spreads to the 'collar' of the young tree (the line of demarcation between the main root and th stem) and the tree soon dies. The roots of the infected tree are often covered by root-like strands of the fungus (rhizomorphs) which grow out through the soil and thus spread the fungus to other trees. The black sclerotia which they form may contaminate the soil for years. Diaporthe taleola causes a bark canker on oaks under forty yers of age, but does not kill the tree.
Microspaera alni extens In a wet season this may be very conspicuous on the lower leaves and the stool shoots of coppiced or felled oak, covering them with a white 'mildew' growth. The fungus is common in the USA but was not found in Europe till 1907. The typical fructification, a closed 'perithecium' containing asci, has not been found in England though it has on the Continent. In this country the mildew propogates itself only by 'conidia', a spore-like 'accessory' means of reproduction occuring in a great number of fungi.... The oak mildew does not seem to do much harm to the oaks it attacks, except seedlings or fter defoliation of the tree by caterpillars, when it attacks the second growth of leaves and seriously weakens the tree, which may then succomb to the attacks of other fungi. Fistulina hepatica grows in the trunks of oaks and ash, attacking the woody tissue in which it causes a dark red discoloration, and gradually it may kill the tree. The fructifications, formed on the bark, are bracket-like, four to ten inches across, fleshy, and dark red in colour, like lumps of liver or raw beefsteak. The basidia forming the spores line the sides of tubes which have pale openings. Ganoderma applanatum The bracket is cinnamon-coloured with white margins. This is a 'wound parasite', entering the trunk through a wound, attacking the heartwood, and is only found on relatively old trees. After the spores begin to ripen they may be continuouslydischarged from the inner surfaces of the basidial tubes for as much as six months. Fomes igniarius the 'firesponge', is another very commonbracket fungus on oaks and other broad-leaved trees. Infection takes place through wounds and the wood becomes dark brown with formation of gum. After delignification the colour pales to a yellowish-white. Polyporus sulfureus is another wound parasite of both broad-leaved trees and conifers. The wood turns reddish-brown and the annual rings tend to separate after penetration by the fungus. The fruit body, four to sixteen inches across, is not a bracket but an orange-coloured, imbricated mass formed on a trunk wound, with the tube openings a sulphur yellow. Polyporus squamosus has bracket-like fruit bodies four to fourteen inches in diameter and covered with dark brown scales on the uppersurface. It causes a white heart-rot, the sapwood being attacked last. The wood finally becomes quite soft and the tree often blows down owing to the loss of rigidity. This fungus attacks a variety of deciduous trees, especially elms, and is sometimes found on oaks. It takes a long time to kill the tree. Stereum hirsutum usually grows saprophytically on dead oak branches, stumps, and posts, causing rapid delignification of the wood. It sometimes invades the wood of standing oak through a wound and causes yellow or white stripes in the timber. The yellowish fruit-body forms an irregular disc with recurved yellow margins on the surface of the wood or bark, the spores being produced on the free upper surface. Stereum purpureum ('silver leaf' disease), a destructive parasite of many deciduous trees
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