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Witch Hazel Gall.The raised part of this leaf, which looks like a witch's hat, was caused by a female aphid.  She bites the leaf, causing the leaf to react and cause the growth seen above. The gall grows around the insect, and she reproduces asexually to fill the gall with 50 to 70 offspring. As the gall gets crowded, the next generation develops wings and disperses, creating additional galls.  The third generation includes males and females, who mate on birch trees and lays eggs on the witch hazel branches.

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Photographs by Valerie Lykes, Janine Ngai and Laura Kroon Bush