Distribution of Urophora galls in flower heads of diffuse and spotted knapweed in British Columbia.
Abstract
The distribution of Urophora affinis (Frnf.) and U. quadrifasciata (Wied.) among the flower heads of diffuse (Centaurea diffusa) and spotted (C. maculosa) knapweed was determined during a population explosion in British Columbia following the introduction of the gall flies into Canada in 1970-72 as biological control agents. The distribution of U. quadrifasciata was more clumped than that of U. affinis and the galls of both flies had more clumped distributions in the seed heads of spotted knapweed. Diffuse knapweed has smaller heads and less synchronous bud development than spotted knapweed. Similar patterns are observed in European populations of the flies. On diffuse knapweed, U. quadrifasciata galls were more randomly distributed in periods of population peaks and U. affinis galls were found to be more contagiously distributed since the post-introduction peak. Apparently, the equilibrium density of flies is not maintained through adjustment of ovipositional behaviour, but is more likely to be mediated through the response of the plant to fly attack, and by environmental factors that influence the phenology of bud development and fly emergence. It was thought that the introduction of these 2 closely related flies had led to greater seed destruction than would have been achieved by either species alone.