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Oak-galls and Other Growths on Plants and Trees.

Updated on August 7, 2015

Notes from a Lancashire Countryman

As a plant enthusiast I pride myself at being able to look at a subject as opposed to seeing a subject. For example those of us that admire flora {whether wild or cultivated} will be in one or the other camps. Many will see a plant and admire its beauty, the flowers, foliage, colour and many will know the name of the plant they are admiring. To most this is enough and there is nothing wrong with that. Then there are those of us that look more deeply at the subject. For instance how many petals does the flower have? How many sepals?, stamens, anthers and so on? Are the leaves simple , divided, lobed ? etc etc.

Those of us that take this closer observation stance,, also observe many more subjects that occur in the natural world, such as insects, not only the conspicuous ones such as bees and butterflies, but the tiny invertebrates that need to be looked for on the foliage and flowers of flora including trees. During the course of my ramblings I encounter many weird and wonderful growths on various species of plants and trees and the tell tale signs of their work.

I suppose the most familiar of these growths are the galls which include Robin's pincushion. Galls on plants and trees are caused by insects and other organisms invading the plants tissues. So, what are galls? They are usually defined as being unusual or abnormal growths that provide food and security for the invading organisms. These parasites vary and may be fungi, bacteria insects and mites. A great majority {especially the common galls } are caused by insects. All parts of the subject invaded can be affected, the leaves, flowers, stems, roots and even seeds. However, each part that is affected is usually by one specialised invader.

The study of galls is called Cecidology from the Greek kekis meaning a gall. There are over 2,000 species in Britain, and more are discovered regularly. The Robins pincushion is probably the most conspicuous of the plant galls its colour attracting attention of the observant.

Robins pincushion

The Robin's pincushion is a wonderful structure
The Robin's pincushion is a wonderful structure | Source

Caused by a Gall wasp

They are usually encountered on wild roses and are formed by a Gall wasp, Diplolepsis rosoe, which lays eggs in the flower buds. Inside the weird structure there will be several tiny white grub. The normal development of the flower bud is changed by the chemicals secreted by the female laying her eggs and by the grubs themselves.

gall wasps are only tiny and only distantly related to the familiar yellow and black wasps so familiar to all of us. They are more ant sized than conventional wasp size and of a dark colour. Most of them are winged but some are wing-less.

Oak trees play host to many types of gall . There are two types that are more conspicuous than the others. One of these the Marble oak gall is produced by a gall wasp known as Andricus kollari. These are common and wide spread

Oak apple galls

oak apple galls  on the flowers of oak.
oak apple galls on the flowers of oak. | Source

Comparitively large galls

They are comparatively large galls, round and smooth, green or brown and woody up to 25mm across which develop from the buds after the female wasp lay her eggs in the spring. Each of these galls contain a single grub which feeds inside. The adults emerge through holes made in the gall during September and October. They may remain on the tree for many years.

Marble gall

Marble gall on oak.
Marble gall on oak. | Source

Oak apple gall

The oak apple gall caused by the wasp Biorhzza pallida is also common and wide spread and is often confused with the above species.These are slightly irregular, smooth galls that develop in the buds during spring. At first they are of a pale colour but quickly turn to red taking on the appearance of tiny apples. Within the galls interior there are several chambers that are occupied by grubs. The adults emerge in June and July.

Studies have revealed that females that emerge crawl down the trunk to lay their eggs on the roots. gall then occur on or in the roots. Within these galls larvae feed and over winter. Adults that emerge from these the following spring then make their back up to the buds to lat their eggs and the whole cycle continues.

Top Artichoke Gall. below. Red pea Gall

Artichoke gall is another species that may be found on oak and other trees such as the yew.
Artichoke gall is another species that may be found on oak and other trees such as the yew. | Source
Red pea gall is another species found on oak.
Red pea gall is another species found on oak. | Source

Leaf miner

Gardeners will be familiar with the classic signs of the Leaf Miner, discoloured markings on the surface of the leaf that portrays the movement of the larva inside the tissues as it munches its along. Chrysanthemums in particular and tomato foliage is often host to such larvae.

Tell tale signs of the Leaf Miner

Leaf miner larvae leave the tell tale signs of their presence.
Leaf miner larvae leave the tell tale signs of their presence. | Source

Sawfly galls

Sawflies can also contribute to the production of galls such as those found on the willow foliage. many species of saw fly of the genus Potania cause various leaf galls. These can be recognised as conspicuous hard red or green galls up to 2 cm long which develop on the foliage during the summer months. The larvae within the galls munch out cavities. A second generation emerge in October and over winter in cocoons on the bark or in the soil.

The most noticeable growth , particularly on birch trees, is commonly called Witches Broom. they consist of large clusters of growing twigs which give the impression of a large bird's nest or as the common name implies the base of a witches broom stick. These close growing twigs often have disfigured foliage. They can become extremely large and may eventually take up much of the crown. it is not fully understood what causes this growth but it is thought that fungal attacks are the main cause.

Witches Broom

The witches broom can appear as though there are large birds nests in the tree.
The witches broom can appear as though there are large birds nests in the tree. | Source

Gall flies

Gall Flies or gall midges of the family Cecidomyiidae are the most numerous of the gall causing flies. many of them have orange bodies and tiny wings. they may be observed swarming around street lights at night.

The lime gall mite Eriophyes tilliae produces a gall known as lime nail galls. they are small conical and red and appear on the upper surface of lime leaves. usually from May till June. A single leaf may bear hundreds of galls and their colour contrasts sharply with the green colour of the foliage. Sycamores and maples also endure attacks of nail gall mites. Readers might like to click on photographs to see larger images.

Gallery . Symptoms and signs of damage

Maple leaf gall is distinctive.
Maple leaf gall is distinctive. | Source
The damage done by the leaf miners can be mistaken for autumn colour on horse chestnut.
The damage done by the leaf miners can be mistaken for autumn colour on horse chestnut. | Source
The adult of the horse chestnut leaf miner.
The adult of the horse chestnut leaf miner. | Source
Close up of the knapper gall that disfigures acorns.
Close up of the knapper gall that disfigures acorns. | Source


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    • D.A.L. profile imageAUTHOR


      8 years ago from Lancashire north west England

      Hi Nell, your welcome. Nice to see you here. Best wishes to you.

    • Nell Rose profile image

      Nell Rose 

      8 years ago from England

      Hi, the Robins pincushion! what a great name! fascinating, I had never really looked before, but I will keep my eyes open now,and I never knew they were called galls, thanks nell

    • D.A.L. profile imageAUTHOR


      8 years ago from Lancashire north west England

      Hi, Carol, they are fascinating things. Over 2,000 are known to exist in Britain and it is thought there is many yet to be discovered. Thank you for visiting . Best wishes to you.

    • reddog1027 profile image


      8 years ago from Atlanta, GA

      I always learn something new from your hubs D.A.L. I didn't know there were so many different and interesting types of galls.

    • D.A.L. profile imageAUTHOR


      8 years ago from Lancashire north west England

      Hi thoughtforce, nice to meet you. Thank you for reading glad you found the hub useful. Your kind comments are appreciated. Best wishes to you.

      Hi, DREAM ON,glad to have been of hel;p. Thank you for reading and for your appreciated comments. Best wishes to you.

    • DREAM ON profile image

      DREAM ON 

      8 years ago

      I never looked at plants so close before.You opened my eyes.

    • thougtforce profile image

      Christina Lornemark 

      8 years ago from Sweden

      I think the pictures are beautiful even if it is galls! It is amazing that even such a thing can look so different! Escpecially the knapper gall is remarkable! And I found this hub useful to, now I know what I see if I find some of them!


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