Galls of hardwoods
Damage, symptoms and biology
Most galls of deciduous trees are caused by mites so small that they are barely visible to the naked eye. Other galls are caused by some wasps, midges, and aphids. The mites insert their slender mouthparts into the leaf or flower tissue and suck out the plant juices. When feeding, they apparently inject a growth-promoting substance into the tissues which results in the formation of swellings or galls on the surface of the leaves or flowers.
The effects of feeding and gall formation on growth are not well known but it is probable that they reduce the ability of leaves to produce foods and thus weaken trees to some extent. Severe infestations make the foliage unsightly.
Most of these mites overwinter as adults, hibernating in the crevices of the bark and beneath bud scales. When the buds begin to swell, the mites crawl to the opening buds where they feed and lay eggs. The eggs hatch in 7 to 10 days and the nymphs settle down on the young leaves. Galls begin to form around the nymphs shortly after they begin feeding, and within 2 weeks they are completely entombed within the galls. By early August, when full grown, they emerge from the galls and go into hibernation.
Ash flower gall: A very small mite, Eriophyes fraxiniflora Felt, sometimes causes the flowers of ash to become enlarged and distorted. This distortion results in unsightly masses on the twigs. Such galls turn black and remain on the trees throughout the winter. Trees are not likely to be killed but some dieback of the current year's growth may occur, resulting in deformed trees.
Maple bladder gall: This gall occurs on silver and red maples and is caused by the mite Vasates quadripes (Shimer). The galls are globular, green when first formed, but later turn to red or black.
Maple spindle gall: The maple spindle gall occurs mostly on sugar maple but is also found on silver and red maple. The maple spindle gall is caused by the mite Vasates aceris-crumena(Riley) and occurs on the upper surface of the leaves. This gall is green, somewhat filiform or spindle-shaped, and about 5 mm high.
Leaf pile: Leaf pile is found most commonly on sugar maple, red maple and beech. Mites, Eriophyesspp., cause a "pile" or velvet-like green-red growth on the upper surface of the leaves. The piles or velvet-like deformations are the result of the excessive development of plant hairs.
Leaf blister galls: Blister galls may be found on several tree species including mountain-ash, oak, linden, maple, poplar, elm, and apple. They are caused by mites, midges, and wasps. The mites burrow into the leaf tissue and thereby cause blister-like galls on the leaves. The individual galls are minute, yellowish or red, depending on the host species, but later black on some species. Several generations occur in a relatively short time which results in an abundant population, thus causing many blisters that ultimately deform the leaves.
Ocellate galls: The maple leaf spot is a circular, eye-like blister gall that occurs on the leaves of red maple. It is about 4 mm in diameter, yellow, with a red border. It is caused by a small fly, Cecidomyia ocellaris O.S. This gall does not appear to cause any serious damage to the trees.
Large oak apple gall: This striking globular swelling (25 - 35 mm) occurs mostly on red oak and is usually fastened to a vein or the petiole of a leaf. The gall is caused by a small cynipid wasp and is filled with a spongy mass made up of many long fibres radiating from a central cell containing the insect larva. Although sometimes numerous, galls of this kind rarely affect the health of the trees.
Gouty oak gall: This gouty oak gall and others similar to it cause twig swelling by a massive distortion of growth produced by a cynipid wasp. The galls, which measure up to 50 mm in diameter, are woody inside with many small larval cells. They completely encircle twigs and small branches and often grow together to form a long swollen mass that may extend along the entire length of small branches. These galls are occasionally found in large numbers on certain trees; unlike foliage galls, they can be quite injurious and can kill large branches and even trees.