The sac-like galls found on Staghorn and Smooth Sumac are anywhere from marble- to ping pong ball-size, and usually become obvious in late summer when they often acquire a rosy pink blush. Inside the thin walls of these galls is one big hollow cavity, teeming with tiny orange woolly aphids (Melaphis rhois). In the spring, female aphids lay an egg on the underside of a sumac leaf, causing the plant to form an abnormal growth, or gall. A number of parthenogenic generations are produced inside the gall, and then in late summer or early fall, the winged females fly to patches of moss, where they establish asexually reproducing colonies. According to biologist D.N.Hebert, these colonies produce the males and sexual females responsible for recolonizing sumac each spring.
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