Galls, abnormal plant growths caused by a variety of agents including insects, come in myriad sizes, shapes and colors. One of the most distinct is the Elm Cockscomb Gall which is caused by an aphid (Colopha compressa). These galls, named for their striking resemblance to a rooster’s comb, are maturing and turning red this time of year.
For much of the summer the aphids responsible for these galls live underground sucking sap from grass roots. In the fall a new (sexual) generation is born, takes to the air, mates and heads for an American Elm (Ulmus americana) tree where each female aphid lays a single egg under the bark. In the spring the emerging nymphs seek young American Elm leaves on which to feed. As they do so, the aphid nymphs emit compounds that result in the formation of galls. Each nymph matures inside a gall and then reproduces asexually, giving birth to hundreds of young within the gall. The mature, reproductive adult aphid dies and the young aphids develop into winged adults that exit the cockscomb gall through a slit on the undersurface of the leaf. These aphids then go down into the soil to feed on the sap of roots until the next generation is born.
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