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Archive for August 16, 2011

Oak Apple Gall

Galls are abnormal plant growths that are caused primarily by insects, but also by fungi, mites, nematodes and bacteria.  Each insect has a specific plant host, and each gall a distinctive shape.  Of the 2,000 gall-producing insects in the United States, 1,500 of them are gall wasps or gall gnats. Plants in the Oak, Daisy, Rose and Willow families have the greatest number of galls, with oaks having over 800 different types.  The insect typically lays an egg in a growing part of the plant (twigs, leaves or leaf bud), which reacts to a chemical secretion, the egg or the burrowing larva by forming a growth around it.  The pictured gall, an oak apple gall, is caused by a wasp, Amphibolips confluenta.  These golf ball-size galls were named for their resemblance to apples.  One larval wasp lives in the center of  each oak apple gall, where it feeds and eventually pupates and emerges as an adult wasp.  The hole in the pictured gall was chewed by the exiting wasp.