Biology of insect galls, ecology of inhabitants, gall anatomy and development

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Galls are atypical, organ-like growths on plants caused by specialized insects that have evolved the ability to control the growth of plants to the insect's advantage. Only the insects gain in the association as they are surrounded by thick layers of cells that provide food and shelter. Several kinds of insects induce plant galls as part of their life history strategies, but the most common and diverse are midges and gall wasps. Most gall wasps attack oaks and wild roses and each species induces a structurally distinct gall. Wasps of the genus Diplolepis are restricted to inducing galls on wild roses. There are about 42 species of Diplolepis in the world and 13 in Canada with galls that vary in size from small pea-like structures on leaves to large swellings the size of golf balls on stems. Studying the anatomy and development of galls provides a novel tool for understanding how plants control their own development. Galls represent discrete microhabitats that support closed communities of specialist inhabitants and the ease by which galls can be collected, their occupants observed and counted, and the interactions among them inferred, make them useful study systems in subject areas ranging from population dynamics to the evolution and zoogeography of animal communities. Studying gall communities from southern Canada to the tree line in the NWT and northern Ontario provides insight into how flora and fauna moved into Canada following glaciation. They also provide an opportunity to study how global warming is allowing both roses and the gall insects associated with them to move further northward. Some domestic roses, such as Rosa rugosa, have become weeds in eastern Canada, the USA, and Europe, and two of the Diplolepis gallers found in Ontario are being considered as agents for their biological control.