Galls are abnormal structures on plants induced by chemical cues from certain arthropods, many of which are insects. This golf-ball-sized brown gall on creosote was induced by the maggots of a tiny fly. It is a non-detachable stem gall - the gall occurs on the stem, in this case completely encircling it, and cannot be removed without breaking the stem. The gall midges have left but the old gall will remain for a long time, perhaps years.
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