Galls on trees
Click for larger image This gall on an oak leaf (Quercus) looks like an oak flake gall caused by a wasp (Hymenoptera) but dissecting the gall is the only sure way to tell what caused the gall

Galls are abnormal growths that occur on leaves, twigs, or branches. They may be simple lumps or complicated structures, plain brown or brightly colored. There are 1500 species of gall producers, the majority of which are insects and mites. Some galls form where insects or mites feed or lay eggs. They may also develop as a response to infections by several kinds of fungi, bacteria, and viruses. Galls affecting leaves are seldom if ever a serious problem. Galls affecting twigs, such as the gouty oak gall and horned oak gall, can be more serious.

Symptoms and Diagnosis

In most cases, galls are unsightly but not damaging to the tree. Small plants may be stunted because the water and nutrient circulatory system of the plant may be damaged. Infested leaves, which can be twisted and curled, are usually able to carry out photosynthesis at near normal levels. Less striking and non-apparent galls that can occur on twigs, small branches, and roots can, over time, kill and weaken portions of a tree or cause a general decline in plant vigor. Galls can be confused with normal parts of the tree, for example, seed-bearing structures or insects such as scale.

Life Cycle

Galls result from the interaction between a chemical stimuli produced by the pest organism and the plant's hormones. These pest organisms are masters in the art of compelling the host plant to provide food and shelter for the larvae which resides inside. The walls of the gall are structurally strong, rich in protein, and provide the larvae inside with protection and food.

A specific gall producer can often be identified by the unique color, shape, and structure of the gall that develops around it or by the host plant it is on. In our area most gall-causing insects have only one generation a year making control after symptoms appear of little or no value. Many gall producers have not been extensively studied and their life histories are poorly understood. In the absence of such basic information few protocols for control exist. Since the vast majority of galls produce only cosmetic blemishes, economic incentives for funding further study are lacking.

Integrated Pest Management Strategies

1. Do nothing; live with the problem. Despite the unattractive appearance of galls, their presence is usually not harmful to the host plant. Gall producers are usually kept in control by their natural enemies. The only galls that may warrant control in St. Louis are gouty oak gall and horned oak gall.

2. Use cultural controls. Some gall producers overwinter in leaf litter beneath the tree. Therefore, some control may be achieved by raking and destroying fallen leaves. If needed, control can be helped by pruning off and destroying gall-infested twigs and branches. Prevent stress on the plant by watering during dry periods and fertilizing if needed.

3. If necessary, use chemical sprays. If a tree is small, in poor health, or defoliated several years in a row, using chemical sprays may be warranted. Sprays will also kill beneficial insects that usually keep galls and other insect pests under control so spraying may make your tree vulnerable to other pests. Also, spraying will not reduce the number of galls this year. To prevent galls, sprays must be used as a preventive before the galls form. In the rare cases where control of galls is necessary, a qualified arborist will be needed to spray the tree. Effective control is difficult, but an arborist would be best able to identify the gall-producing organism and recommend and carry out control measures.

Organic Strategies

Strategies 1 and 2 are strictly organic approaches.

More images:

Click for larger image
Leaf gall, cause unknown, on hackberry (Celtis)
Click for larger image
Leaf gall, cause unknown, on hackberry (Celtis)
Click for larger image
Woolly vein gall on hickory (Carya); cause unknown
Click for larger image
Leaf gall on Scotch elm (Ulmus) caused by aphids (Hemiptera)
Click for larger image
Sideview of leaf gall on Scotch elm (Ulmus) caused by aphids (Hemiptera)
Click for larger image
Dissected gall from leaf of a Scotch elm (Ulmus) revealing the aphids (Hemiptera) that caused the gall
Click for larger image
Hackberry nipple gall, caused by psyllids (Hemiptera), protruding from the lower leaf surfaces on hackberry (Celtis)
Click for larger image
Hackberry nipple gall, a mammiform gall caused by psyllids (Hemiptera), protruding from the lower leaf surfaces on hackberry (Celtis)
Click for larger image
On the upper leaf surface of hackberry (Celtis), the hackberry nipple galls (Hemiptera) appear as leaf spots
Click for larger image
Cone gall on Ozark witch hazel (Hamamelis vernalis); note, cone-shaped protuberances on upper leaf surfaces. Gall is caused by aphids (Hemiptera).
Click for larger image
Two cone gall, caused by aphids (Hemiptera), on Ozark witch hazel leaf (Hamamelis vernalis)
Click for larger image
Exit holes of aphids (Hemiptera) from cone gall on Ozark witch hazel (Hamamelis vernalis)
Click for larger image
These winged aphids (Hemiptera) were clustered on the underside of a witch hazel leaf (Hamamelis) on June 5, 2008. They may be the aphids that cause cone galls on witch hazel.
Click for larger image
Oak flake gall on bur oak (Quercus macrocarpa) caused by a gall wasp (Hymenopera)
Click for larger image
Oak flake gall on bur oak (Quercus macrocarpa) caused by a gall wasp (Hymenopera)
Click for larger image
Oak flake gall on bur oak (Quercus macrocarpa) caused by a gall wasp (Hymenopera)
Click for larger image
Oak flake gall on bur oak (Quercus macrocarpa) caused by a gall wasp (Hymenopera)
Click for larger image
Spiney witch hazel aphid galls (Hemiptera) on Heritage birch (Betula nigra 'Cully' HERITAGE). The gall begins on witch hazel (Hamamelis) and moves to birch in a complex 2-year cycle.
Click for larger image
Leaf galls on bald cypress (Taxodium) probably caused by a midge (Diptera)
Click for larger image
Wool sower gall on white oak (Quercus alba) caused by a wasp (Hymenoptera)
Click for larger image
"Seed-like" structures of the wool sower gall on white oak (Quercus alba) each contain a gall-forming insect, a wasp (Hymenoptera)
Click for larger image
Maple vein gall on maple (Acer) caused by a midge (Diptera)
Click for larger image
Gouty vein gall on maple caused by a midge (Diptera)
Click for larger image

 

Eastern spruce galls, here on spruce (Picea), resemble tiny pineapples. The gall is caused by the feeding of adelgids (Hemiptera)

Click for larger image
Interior of Eastern spruce gall on spruce (Picea), revealing the adelgids (Hemiptera) that cause the gall.
Click for larger image
Vein galls and blistered puckered leaves on this pin oak (Quercus palustris) possibly caused by a midge (Diptera)
Click for larger image
Bald cypress (Taxodium) do get galls caused by midges (Diptera) but, although round, this is a cone, not a gall, and is normal on a mature bald cypress, a conifer
Click for larger image
Witch-hazel cone gall on witch hazel (Hamamelis) caused by aphids (Hemiptera)
Click for larger image
Close-up of witch-hazel cone gall on witch hazel (Hamamelis) caused by aphids (Hemiptera)
Click for larger image
Leaf gall on pin oak (Quercus) showing exit holes of gall-forming insect
Click for larger image
Witch-hazel cone gall on witch hazel (Hamamelis) caused by aphids (Hemiptera)
Click for larger image
Large oak-apple gall on oak leaf (Quercus) caused by a cynipid wasp (Hymenoptera). J. Solomon, USFS, Bugwood.org
Click for larger image
Dried galls of the cooley spruce gall on Colorado blue spruce (Picea) caused by an adelgid (Hemiptera). W. Cranshaw, CSU, Bugwood.org


Pests and Problems by Category

Click a link in the site map below to see other "Pests and Problems" pages.