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Broom Gall Mite Aceria genistae

Many mites can hardly be seen without a microscope, for most are very small indeed. This is especially true for gall mites. Most are less than 1mm long. Yet it is very easy to determine whether they are there are not. Because where they sit on a plant, the plant reacts by showing an abnormal growth or swelling at that place. Such a swelling or growth is called a gall. Best known are the round, reddish balls on the leaves of oak. Galls may appear on the twig of a plant as well. Sometimes galls get quite big, in particular those on twigs. These galls are made up of very small leaves and twigs growing like a mistletoe. These bigger galls on twigs are called witches' brooms. Not only mites can cause galls. There are many other creatures capable of causing galls: gall flies, gall wasps, certain beetles, some moths and barklice. Even some fungi may cause galls! In many cases one can determine which creature caused the gall by looking at the gall itself! And that is a good way of identifying many small creatures, without the need to trace them down and put them under the microscope. In the picture a typical witches' broom. It can be seen on Broom regularly and when appearing often in considerable numbers. The galls are not very big, around 2 cm, but striking. They are caused by a very small mite: the Broom Gall Mite (Aceria genistae).