Galls, abnormal plants growths caused by many agents including insects, are formed during the growing season on the buds, leaves, roots and branches of plants as a response to chemicals or physical irritation. Many of these galls serve as shelters and a source of food for their developing inhabitants.
Blackberry is host to numerous gall-making insects, including mites, midges and gall wasps, and their temporary homes (galls) are most obvious in the winter. The Blackberry Seed Gall is caused by a tiny cynipid gall wasp, Diastrophus cuscutaeformis. A cluster of small, globular, seed-like galls within which the gall wasp larvae live are pressed together in a lump surrounding the cane. These galls derive their species name from their resemblance to dodder (Cuscuta) fruits. Each of these 1/10th-inch diameter chambers bears a spine, and together they create a reddish-brown hairy mass.
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