Nodding thistle is a annual or biennial herb that grows throughout Australia infesting around 1000 hectares of land in the tablelands of NSW, ACT, VIC and TAS. When infesting pastures, nodding thistle can quickly reduce stock and productivity as animals avoid grazing nodding thistle, allowing it to spread and outcompete desirable pasture plants. The main reproductive method of nodding thistle is through dispersal of seed, which may remain viable for up to 20 years. 3 biocontrol agents have been released to combat nodding thistle in Australia.
1. Nodding thistle receptacle weevil *Rhinocyllus conicus*
2. Nodding thistle gall fly *Urophora solstitialis*
3. Nodding thistle crown weevil *Trichosirocalus horridus*
A bicontrol program for nodding thistle was commenced in 1985 to find suitable agents capable of controlling this weed. The first release of the nodding thistle receptacle weevil occurred in 1988 in 3 regions of the NSW tablelands. However, due to the longer flowering season in Australia and poor synchronisation of adult weevil emergence with the onset of flower head production, establishment was variable. In 1991 the nodding thistle gall fly was released at multiple field sites. Subsequent observations found that the galls fly had established at all sites, however this population also struggled due to the poor synchronisation between adult emergence and flowered production, causing many flies to die before suitable buds formed. The most recent agent to be released in Australia is the nodding thistle crown weevil which was released in NSW in 1993. Unlike the two agents released previously, the crown weevil was recorded to cause significant reduction in seed production and severely reduce vigour even causing death in some nodding thistle populations. Currently the crown weevil is the most effective biocontrol agent established in Australia against nodding thistle.
![enter image description here](https://lh3.googleusercontent.com/jtdigsm1oiPLiLAIx_aibT8aEoQqA1DgeW14M5qca8sUIlfV1FHSlEpIuwC5qyEZ1tVEisRBFkl-=s0 "800px-Carduus_nutans_(5841904841).jpg")
Nodding thistle, Photo: Matt Lavin, [CC BY-SA 2.0](https://creativecommons.org/licenses/by-sa/2.0/deed.en)