Impact of the gall fly, Urophora stylata, on the pasture weed, Cirsium vulgare, in New Zealand

Abstract

The gall fly, Urophora stylata F. (Diptera: Tephritidae), was released in New Zealand in 1998 as a biocontrol agent against the thistle weed, Cirsium vulgare (Savi) Tenore. A survey of 20 randomly selected pasture populations of C. vulgare was conducted to assess the impact of the gall fly on seed production. Attack by the biocontrol agent reduced the number of seeds per seedhead by 47%, individual seed weight by 21%, and seed germination rate by 30%. Where the biocontrol agent was present, population seed reduction ranged from 11 to 61%. The impact of the gall fly significantly increased from southern to more northern latitudes of C. vulgare populations in New Zealand. This study represents the first post-release assessment of this biocontrol agent and indicates that U. stylata can have a significant impact on the seed production of C. vulgare, especially in northern New Zealand where seedhead attack is greatest.