Developmental morphology of the gall of Urophora cardui (Diptera, Tephritidae) in the stems of Canada thistle (Cirsium arvense)

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Abstract
Urophora cardui (L.) induces a large multichambered gall in stem tissues of Cirsium arvense (L.) Scop. Four phases in gall development are identified: initiation, growth, maturation, and dehiscence. Initiation begins when larvae tunnel from a shoot tip into the developing stem and change the polarity of pith and procambial tissues. These cells become gall parenchyma and proliferate during the growth phase. Procambial strands and gall laticifers appear among the gall parenchyma and the cells nearest the larvae differentiate into primary nutritive cells. In the maturation phase, gall tissues cease proliferating, procambial strands near the larvae give rise to secondary nutritive tissue, and all remaining gall parenchyma lignifies. Dehiscence occurs when callus tissues at the top of the gall degrade. Developmental studies utilizing insect-induced galls may increase our understanding of normal plant growth and development.