Question
Oak tree leaf
I have these growths on my white oak tree leaves. I can't tell if they are some type of leaf bug or what. They have spines on the exterior and are very hard. Could you tell me what they are and are they harmful to the leaves? What do I use to get rid of them? Thanks.
AnswerSounds like a leaf gall more than likely Wool Sower Galls, Some insects and mites cause unusual growths on plants called galls. Galls may form on any part of the plant from the flowers, leaves and stems to the roots. The abnormal growths developing in plant tissue are due to the powerful enzymes given off by the immature gall-forming insect as it grows. The plant tissue is remarkably altered even to the point of replication of chromosomes without cell division.
Most galls are formed by three kinds of insects or mites: gall wasps, gall midges, and gall mites. Other less common gall producing insects are aphids, psyllids, and gall flies. Since most galls seem to do no permanent damage to their host plants, limited research has been done on the biology or control. This note is about galls formed on oak by gall wasps.
The wool sower gall is caused by secretions of grubs of a small gall wasp, Callirhytis seminator. The wool sower gall is specific to white oak and only occurs in the spring. The galls contain seed-like structures. The gall wasps develop inside these structures.
(This gall is also called the oak seed gall.) Fortunately, wool sower galls are usually never abundant so that the health of infested trees is rarely threatened. These wasps probably lay their eggs in midwinter and the eggs hatch just as the new growth emerges in spring. By the time the galls are noticed, it is too late to effectively control the gall wasps. Wool sower gall wasps probably have an alternate generation of wasps which develops in galls in the buds, twigs or on the leaves.
The wool sower gall is a distinct and unusual plant growth induced by the secretions of the grubs of a tiny gall wasp, Callirhytis seminator. If a fresh wool sower gall is held in a plastic bag out of the sun (so it will not become too hot), within one to three weeks the tiny, harmless gall wasps will emerge. The wool sower gall is specific to white oak and only occurs in the spring. Pulling the gall apart exposes small seed-like structures. The gall wasp grubs develop inside these structures (this gall is also called the oak seed gall). Fortunately, wool sower galls are rarely abundant enough to cause harm to white oaks. If the galls are actually damaging the trees, the best time to control them is mid-winter when the wasps are laying their eggs or spring just as the buds are breaking. The eggs hatch just as the new growth emerges in the spring. Orthene or another contact insecticide should provide adequate suppression of these insects. However, by the time the galls are noticed, it is too late to effectively control the gall wasps. Gall wasps invariably have an alternation of generations in which one generation develops in one type of gall (leaf gall) and their offspring develop in another type of gall (stem gall). Wasps of each alternate generation are slightly different is size and the galls of each generation are enormously different from the parents. The wool sower gall is probably the leaf gall of this species because of its transient nature.
Usually by the time a gall is noticed, it is too late to carry out adequate control measures. In order to properly time spray applications for gall wasp control, collect some of the galls and store them in plastic bags outside, in the shade. When the small, dark wasps emerge, it is time to spray. If after a few weeks no wasps have emerged, then collect another sample and store them in the shade and wait. Gouty oak gall wasps emerge from the stem galls in winter. Many of the wasps from leaves and succulent stem gall emerge in late spring or early summer. When the gall wasps emerge, spray the tree thoroughly. Sevin (carbaryl) pesticide is labeled for gall wasp control on shade trees and park trees. Do not allow public use of treated areas during applications or until sprays have dried. These formulations are suitable for home use, however, chemical control is rarely effective and often not worth the expense. These galls are generally not life-threatening to the tree. Damage is mainly cosmetic to a mature tree. Damage to young trees by stem galls in a nursery setting may cause weakened branches in a tree as it matures. Attempts at protective sprays may be warranted in such a setting.
I would not worry about the galls on the leaves since thye really do not harm the tree.
