Spill-over attack by the gall fly, Urophora stylata, on congeners of its target weed, Cirsium vulgare

The gall fly, Urophora stylata, was released in New Zealand in 1998 as a biocontrol agent against the thistle weed, Cirsium vulgare (Scotch thistle). In the summer of 2018, a survey was conducted to assess the field host range of the biocontrol agent in New Zealand. A random selection of 18 pasture populations under sheep and/or beef production, where C. vulgare was present, was surveyed to quantify the attack intensity (gall size relative to seedhead size) on C. vulgare, and the attack rate on other thistle weeds within the same population. At each location, seedheads were collected from C. vulgare and all other thistle species (Cardueae) present, which included Cirsium arvense (Californian thistle), Cirsium palustre (marsh thistle), Carduus nutans (nodding thistle), and an Arctium species (burdock). In addition to Cirsium vulgare, the gall fly was recorded on C. arvense (six locations) and C. palustre (one location). The probability of attack on C. arvense was positively correlated with attack intensity on C. vulgare, suggesting that attack on C. arvense is a ‘spill-over effect’ occurring where seedheads of C. vulgare are in limited supply.