Impact of the gall fly, Urophora stylata, on seed production of Cirsium vulgare in New Zealand

Abstract

The gall fly, Urophora stylata, was released in 1998 as a biocontrol agent for the thistle weed, Cirsium vulgare (spear thistle, bull thistle, Scotch thistle), in New Zealand. In the summer of 2018, a survey was conducted to assess the impact of attack by the gall fly on seed production of the weed. A stratified random selection of 20 populations of the weed within New Zealand was undertaken to ensure 10 populations on each of the North and South Islands. All selected populations were on land designated as sheep and beef pasture, where the weed is most problematic. The gall fly was found at 14 of the 20 Cirsium vulgare populations. Where the biocontrol agent was present, it reduced the number of seeds per seedhead by 47%, individual seed weight by 21%, and seed germination rate by 30%. Within populations, seed reduction was estimated to range from 11% to 61%. The impact of the gall fly significantly increased from southern to more northern latitudes of Cirsium vulgare populations in New Zealand. Attack on other thistle species present within the surveyed Cirsium vulgare populations was also assessed (1 population with Arctium sp., 2 with Carduus nutans; 13 with Cirsium arvense, and 3 with Cirsium palustre). Attack was recorded on the closely related thistle weeds, Cirsium arvense (ca. 17% of seedheads), and Cirsium palustre (ca. 1% of seedheads). This research represents the first assessment of U. stylata in New Zealand and indicates that it can have a significant impact on Cirsium vulgare, especially in northern New Zealand where seedhead attack is greatest.