Thistle
Californian thistle (Cirsium arvense) is a widespread and problematic weed in New Zealand. Early attempts using biocontrol in the 1980s and 1990s to reduce the impact of Californian thistle were not very successful. However, renewed efforts by Landcare Research and the Californian Thistle Action Group (CalTAG) may now provide us with the right insects to get the job done.
Green thistle beetle (Cassida rubiginosa)
This beetle is a great all-rounder because both the adults and larvae feed on a range of thistle species. Adults can live for up to 80 weeks and a female beetle can lay 1000 eggs over her lifetime. This agent reduces the vigour of thistles by feeding on the leaves; when beetles are very numerous they can completely defoliate plants. Green thistle beetles have established at most release sites in Southland. We hope to collect some beetles from the first site in spring 2011 to make more releases throughout Southland.
Californian thistle stem miner (Ceratapion onopordi)
This is a world first for Southland, as this tiny weevil has not been used as a biocontrol agent anywhere else. It was first released here in 2009, so it is still too soon to know how much damage it will cause. The adult weevils lay eggs inside thistles stems, and after the larvae hatch they either burrow up through the stems or down into the roots. This type of feeding damage complements that of the green thistle beetle as both agents work together to undermine the health of thistle plants. Given this weevil hasn't previously been used as a biocontrol agent, we are still learning about how effective this agent will be in the battle against Californian thistle.
Environment Southland is the brand name of Southland Regional Council.