BYGL Industry Insight May 8, 2008 GALL TIME
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 Dave Shetlar reported receiving a phone call from a garden center asking for a recommendation for controlling galls on oak … a tall order. Over 700 different kinds of galls can occur on oaks; however, little is known of the life-cycles for the vast majority of the gall-makers. Making control recommendations without knowing the life-cycle would literally be a shot in the dark. The good news is that most oak galls simply affect the tree's appearance. Only a handful cause sufficient harm to the tree to support the possible need for control recommendations. Galls may detract from the aesthetic appeal of an oak; however, beauty is in the eye of the beholder. Their development could teach us much about host-plant relationships and plant physiology. Galls are plant structures. Their entire development is orchestrated by a gall-maker that has hi-jacked the plant's normal growth directives. On oaks, the most frequent gall-makers are wasps belonging to the family Cynipidae or midge flies belonging to the family Cecidomyiidae. The female wasp or fly initiates gall growth by injecting chemicals when she inserts her egg(s) into developing plant tissue. Once the eggs hatch, the larval gall-maker continues to direct gall growth by exuding chemicals that turn plant genes on-and-off in just the right order to guide gall growth. These chemicals are often plant hormones or plant hormone analogues … but they are produced by an insect! The close relationship between the growth of the plant gall structure and the gall-maker's timely release of specific chemicals explains why gall-makers have a very specific relationship with their host plant. The host-plant relationship is so specific that gall-makers can be identified to species based on the gall structure alone. Since gall-makers must use plant tissue that can be changed by their chemicals, spring is a prime time to find galls developing on newly expanding leaves. Once leaves stop expanding, galls cannot form.
Oak-apple galls look like … apples. They range in size from 1/4-2" in diameter when they mature, and are also produced by several species of wasps. The galls are usually green in their early stages of development, but as they mature many become light brown in color. Some are conspicuously spotted. Surfaces range from smooth to hairy. Oak-apple galls each hold a single wasp larva housed in a seed-like cell at the center of the gall; however, they are generally divided into two groups based on the tissue surrounding the larval cell. A spongy tissue closely resembling the flesh of an apple surrounds the central cell in "succulent" oak-apple galls. The central cell of "filamentous" oak-apples is surrounded and supported by very distinct, radiating filaments of tissue. For more information, see:
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| Last Updated ( Thursday, 08 May 2008 20:09 ) |
