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The Homeowners Column
Plant Galls – the perfect baby nursery
State Master Gardener Coordinator
Late summer brings an array of
odd-looking bumps and lumps on tree leaves or twigs of maples, oaks,
hackberries, cherries and many other plants. The bumps enjoy lively names such
as maple bladder gall, roly-poly oak gall, succulent oak gall and hackberry
nipple gall (pictured below). Galls are growths on leaves, flowers, stems and twigs of many woody
and non-woody plants. Galls can be created by insects, mites, bacteria, fungi,
virus and nematodes. The ones most often brought into our Extension office are
caused by insects and mites.
Galls are created by the plants in response to feeding or egg-laying by certain species of insects and mites. As gall-making insects feed or lay eggs, they create mechanical damage. The damage and/or the insect saliva initiate the production of plant growth hormones. These hormones produce abnormal plant cell growth that results in the development of the galls around the egg or young insect. For gall formation to occur the feeding or egg laying typically occurs while the leaves or plant part is growing rapidly in early spring. When late summer rolls around the galls are larger, often change color and are more visible.
For gall-making insects and mites it's the perfect baby nursery: the classic adaptation of "baby in a box with its lunch". The larva develops safely enclosed in the gall and feeds on the surrounding plant tissue. As the young develops it eventually pupates or molts into an adult that makes its way out of the gall to restart the cycle. Gall-making insects are very picky to the species of plants they attack. Their resulting galls are so ubiquitous to certain plant species some early botanical drawings show galls as part of the normal plant.
The common maple bladder gall is a bladder or pouch-like growth on the leaves of silver maples caused by an eriophyid mite. The galls form on the tops of leaves and change in color from yellow-green to a vivid red and eventually to black as the season progresses. Maple spindle gall is similar, but found on sugar maple.
Another gall-making eriophyid mite
is ash flower gall (pictured below). The hard brown clusters at the ends of ash twigs are
obvious in late summer into winter. Feeding by ash flower gall mite in spring
causes a proliferation of flower buds as it disfigures the male flowers. Mites
overwinter as adults in the bud scales. Ash flower galls may persist for two
years after development.
Oaks can have numerous types of galls causing deformities of various sizes, shapes, and colors on leaves, twigs, flowers, acorns and buds. An estimated 80% of these are caused by tiny gall-making wasps.
Jumping oak gall can actually provide plenty of family entertainment. Early in the season the galls resemble green pimples on white and bur oak leaves. As the galls and the enclosed gall wasp mature in summer, the galls frequently break loose from the leaves and fall to the ground. As the galls drop to a patio or other hard surface they quiver and jump a bit, hence their name.
The good news about galls is the majority do little or no damage to trees. The trees are just well accessorized. Generally there is enough unaffected foliage for the trees to remain vigorous. The exception would be gouty oak gall and horned oak gall that forms on oak twigs. Heavily infested young trees can experience branch dieback as the gall girdles the stems. If possible, pruning and destroying oak twig galls on young trees may help to lessen populations next year. Once galls form, little else can be done to manage them.
Chemical control of most galls is difficult to impossible and seldom needed. Timing of any sprays is challenging since sprays must be initiated before gall formation begins. Galls offer a fascinating window into the wonderful world of adaptation.