Spotted knapweed may bolt and produce seed heads for as many as three years after the rosette stage. If all seeds were destroyed, eradication of diffuse and spotted knapweed infestations would take over three years because both species can grow vegetatively for two or more years as rosettes before bolting. Thinning trials on seedlings showed that density-dependent competition between seedlings regulated knapweed population densities. Experiments in seeding showed that habitat and weather conditions determine knapweed germination and seedling survival in new locations more than the density of seeds sown. Although the gall flies reduce the number of seeds, they will have a negligible effect in slowing the rate of spread of diffuse and spotted knapweed because of the extent of the infestations and the ease of spread by seed. quadrifasciata were an 80% reduction in seed numbers on both diffuse and spotted knapweed. There was some loss in viability of spotted knapweed seeds, but the major effect on this species was a 33% reduction in average seed weight. Viability of diffuse knapweed seed decreased by 30% accompanied by a loss of the waxy seed coat and dark-brown pigmentation. Gall-fly larvae not only reduce seed numbers in attacked heads, but gall formation pulls nutrients from other parts of the plant, causing a further decrease in the number of seeds in unattacked heads and an increase in the percentage of heads that remain undeveloped. quadrifasciata can survive in large-headed plants during the first generation and capitalize on the heads produced by small-headed plants during the second generation when U. quadrifasciata emerges one week earlier than U. These species are not ecological homologues and U. quadrifasciata, saturate oviposition sites before they reach the stage of maturity required by U. affinis, which oviposits in younger heads than U. affinis first-instar larval densities exceed 0.5 per head, U. The two gall-fly species attack the same part of the plant, and when U. Density-dependent mortality of first-instar larvae in the pre-gall stage probably restricts numbers of U. Proximal heads that would normally abort developed in knapweed that had many superparasitized distal heads. affinis caused distal head abortion by superparasitization of the heads. Normally distal heads do not abort, but proximal heads may abort at anthesis: It appeared that U. Average larval densities above 0.5 and 1.0 per diffuse and spotted knapweed head, respectively, caused distal head abortion. Larval densities per head peaked in 1977, five and six years after the flies were introduced against diffuse and spotted knapweed, respectively. The gall-forming fly larvae feed in immature knapweed heads. (Compositae diffuse and spotted knapweed, respectively) in British Columbia, Canada. quadrifasciata Meigen (Diptera: Tephritidae) on the rangeland weeds, Centaurea diffusa Lam. The aim of this study was to determine the effects of the seed-reducing gall flies, Urophora affinis Frauenfeld and U.
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