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Posts tagged “Gall wasps

Blackberry Knot Gall

12-28-10      Blackberry Knot Gall

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Winter is a good time to look for galls (abnormal plant growths caused by different agents, including insects, fungi, mites and bacteria) such as the blackberry knot gall, which is much more noticeable when there are no leaves to hide it. Whereas many galls are inhabited by a solitary insect, the blackberry knot gall contains many individual chambers, each containing the larva of the tiny wasp Diastrophus nebulosus. During the spring and summer months, this little wasp deposits eggs into the ridged stems of blackberry which stimulates the plant’s tissue into abnormal growth along the stem. This particular colonial gall can be six inches in length, although two or three inches is more typical (the more eggs that are laid, the larger the gall).  The eggs hatch and the larvae overwinter inside the gall. Adult wasps emerge in the spring and chew their way out of the gall, leaving tiny holes along the gall’s lumpy ridges. In the first photograph you can see where a hungry predator has worked its way into two of the larval chambers.  In the second, multiple chambers and larvae are exposed (sacrificed for the sake of knowledge, but popular food for chickadees on a very cold morning).


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.