ABSTRACT
Nodding thistle, Carduus nutans, is a serious pasture weed in tableland regions of Eastern Australia. It originates from Europe, but was accidentally introduced from New Zealand in the 1950s. The seed-fly, Urophora solstitialis, undergoes a partial second generation and, hence, is active over a longer period than the weevil. U. solstitialis larva induces the plant to form a woody unilocular gall. Such galls invariably coalesce and act as powerful metabolic sinks, sequestering metabolites away from developing seed in the same capitula. Flies began to emerge from the overwintering stage in late October. Female flies do not need a period after emergence to complete ovarian development but, upon mating, begin to oviposit if suitable green capitula are available. A seed reduction of 45% is an encouraging result so soon after release of the fly. However, it does appear that the insect is out of synchrony with its host plant.
