The biological control of Centaurea diffusa lam. and C. maculosa lam. by Urophora affinis frauenfeld and U. quadrifasciata meigen (diptera:tephritidae)
Creators: name: Roze, Liga Dace
Publisher: name: University of British Columbia
Year: 2010

description: The aim of this study was to determine the effects of the seed-reducing gall flies, Urophora affinis Frauenfeld and U. quadrifasciata Meigen (Diptera: Tephritidae) on the rangeland weeds, Centaurea diffusa Lam. and C. maculosa Lam. (Compositae; diffuse and spotted knapweed, respectively) in British Columbia, Canada. The gall-forming fly larvae feed in immature knapweed heads. Larval densities per head peaked in 1977, five and six years after the flies were introduced against diffuse and spotted knapweed, respectively. Average larval densities above 0.5 and 1.0 per diffuse and spotted knapweed head, respectively, caused distal head abortion. Normally distal heads do not abort, but proximal heads may abort at anthesis: It appeared that U. affinis caused distal head abortion by superparasitization of the heads. Proximal heads that would normally abort developed in knapweed that had many superparasitized distal heads. Density-dependent mortality of first-instar larvae in the pre-gall stage probably restricts numbers of U. affinis. The two gall-fly species attack the same part of the plant, and when U. affinis first-instar larval densities exceed 0.5 per head, U. quadrifasciata increase is suppressed. U. affinis, which oviposits in younger heads than U. quadrifasciata, saturate oviposition sites before they reach the stage of maturity required by U. quadrifasciata. These species are not ecological homologues and U. affinis will not displace U. quadrifasciata because U. quadrifasciata emerges one week earlier than U. affinis, and when U. affinis densities are high, U. 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. affinis densities are low. 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. Viability of diffuse knapweed seed decreased by 30% accompanied by a loss of the waxy seed coat and dark-brown pigmentation. 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. The combined effects of U. affinis and U. quadrifasciata were an 80% reduction in seed numbers on both diffuse and spotted knapweed. Most of this was caused by U. affinis. - 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. 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. Thinning trials on seedlings showed that density-dependent competition between seedlings regulated knapweed population densities. 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. Spotted knapweed may bolt and produce seed heads for as many as three years after the rosette stage. Life table studies indicate that the gall flies attack knapweed too late in the life cycle to be effective.