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Oak Galls May Cause Concern


Q: We have been raking up some leaves from our oak tree. As the kids looked at some of the leaves, we noticed large bumps and some were very distorted. After observing the leaves, we looked at some low branches, which were also covered in large bumps. The bumps were the size of marbles to as large as golf balls. Some of the bumps break off easily, while others are woody and seem to be part of the tree branch.

How serious is this condition to the tree's health? It seemed to be fine all summer and the leaves weren't discolored or falling off early. This tree is large and old, so we would hate to lose it. We have some smaller oaks we planted in the backyard that don't have the bumps.

A: The growths are called galls. They were most likely caused by tiny wasps, but some are caused by tiny midges. By tiny, I mean smaller than a gnat, smaller than a fruit fly, and maybe as small as a grain of pepper. These wasps are harmless to people or animals.

There are thousands of species of gall wasps that affect many kinds of plants, but oaks seem especially prone to gall wasps. The galls can form on all plant parts, including acorns, bark, buds, flowers, leaves, roots and trunks. Almost all of them cause cosmetic damage and don't do any long-term or short-term damage. Small trees or those located near oak trees in a nursery can get so many damaged twigs and leaves that they may die.

The life cycle is nearly the same for all gall-making insects. In the spring, the female lays eggs inside the leaf or twig. The larva releases chemicals that cause the plant to grow a protective structure around the insect. Eventually, the larva forms a pupa, the adult hatches and then the process starts over.

Depending on the species, the larva, pupa or adult stage is the one that is dormant over the winter, which could be in the gall, in the ground or on the tree.

Some species have more than one generation in a summer, while some take more than one year to develop in the gall. If the larva or pupa is still in the gall, raking up and destroying leaves with galls as well as pruning twigs with fresh galls may help reduce the population. Old galls can be pruned off, but they have nothing left inside.

Insecticides are only consistently harmful to the insects when they are out infecting the new plant tissues in the spring. Systemic insecticides in the plant may kill the larva after it is in the protective case if it feeds on the plant tissue, but very often the gall tissue is not directly connected to the plant's conductive tissues. Therefore, insecticides don't move easily into the gall.

Oak apple galls are aptly named as they look like an apple is growing on an oak leaf. The larva resides inside a spongy mass that is a hard "seed." They dry to a paper-thin brown ball that easily breaks open. Removing them before they dry can get rid of the wasp.

Jumping oak galls are found in small blisters on oak leaves. They fall off the leaf when they mature. The activity of the larva can make the gall jump several inches off the ground.

Leaf pocket galls come from several varieties of midges. They live inside a flat gall made from the veins of the leaf. The leaf may be distorted.

Roly-poly galls look like grapes. Wooly galls look like balls of fuzzy lint. Leaf galls look like enlarged leaves. Gouty oak galls look like oak golf balls; they may be solid wood surrounding a twig. Horned oak galls are solid wood with sharp pointed horns. I have seen galls on oak leaves that looked like pinto beans, popcorn and marshmallows. Frequently, you can leave the galls alone.

E-mail questions to Jeff Rugg, at To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at



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Jeff Rugg
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