Since 1878, the dreaded Californian thistle has caused mayhem throughout New Zealand farms! It is very problematic to the country’s pastoral farming systems, costing close to $700 million each year. The invaders have been subjected to a continuous battle with herbicides. This has been an effective, if costly, method of control to stop the weed taking over our land. However, there is a growing concern that herbicide resistance will make the weed unstoppable! This has led to the study of alternative options to reduce the development of herbicide resistance in this dreaded weed, benefiting both the environment and the farmers making it a win win situation.

Cirsium arvense, better known as Californian thistle in Australia and New Zealand, is presently one of the worst invasive pastoral weeds in New Zealand. It is a perennial herb originating from Eurasia. In 1878 the thistle was first reported to have been accidentally introduced to New Zealand and has caused problems ever since.

Once the thistle has established itself in a pasture, more thistles will grow forming a patch using lateral creeping root systems. When the shoots die off every winter, the root system remains alive and will give life to the next season’s thistles. This root system is what seems to cause the problem, making them appear invincible.

While it’s great that herbicides give us a fighting chance, they only tend to kill the shoots of the thistle, as the herbicides don’t reach the outer ends of the roots. This allows for the reestablishment of the weed which requires the application of herbicides every season. We know that herbicides are a great idea in the short term but if we apply them every year we need to deal with the possibility of herbicide resistance. An alternative option is to use biocontrol agents to control the thistle.

Several biocontrol agents have already been released in New Zealand but some appear to be failing. This includes the Californian Thistle Gall fly as it gets eaten by grazing animals and the Thistle Stem Miner, which is difficult to rear. But before we get ahead of ourselves with the idea of biocontrol agents we must first understand the thistles weaknesses.

One of the first things we need to know is how Californian thistle thrives in its native and introduced ranges and how natural enemies like insect herbivores and fungal pathogens can affect the thistle. Keep your friends close and your enemies closer. This will help determine which enemies cause the most damage to the weed and maybe the best biocontrol agents, giving a better understanding on what the next course of action should be. Michael Cripps, was a Lincoln University PhD student and is now an AgResearch scientist. Cripps wrote a paper “The influence of insects and fungal pathogens on the individual and population parameters of Californian thistle in its native and introduced ranges” which determined what influence natural enemies of Californian thistle had on the weed.

Cripps tested the ‘enemy release hypothesis‘ (ERH) by conducting experiments in two locations, New Zealand (where the thistle was introduced) and Switzerland (where the thistle is native). Cripps applied four treatments to the thistles. Some plants only got water (the control), in which both insect and fungal pathogens could attack, and three different pesticide treatments. The pesticide treatments consisted of one treatment with insecticide (minimising insect attack), one with fungicide (minimising fungal damage), and some lucky thistles were protected with both insecticide and fungicide (minimising attacks from both insects and fungal pathogens).

Mixed evidence was found on the influence of pathogens on thistle growth. In 2007, the Switzerland insect exclusion allowed the thistle to flourish but less so in 2008. The control treatment caused the thistles to be attacked and diminish but with no consistent pattern to determine which natural enemy had a greater affect. In New Zealand it was found that exclusion of insects had no effect on the plant. There was also no beneficial effect in the exclusion of fungal pathogens. So what does this mean?

This study showed that generally, natural enemies can affect individual and population dynamics of Californian thistle, which gives promise to prospects of biocontrol agents in New Zealand. There have been several biocontrol agents that have already been released in New Zealand to control Californian thistle, mainly insect pathogens. There are further studies looking at fungal pathogens. Giving hope in this war against the Californian thistle!

The rust fungus Puccinia punctiformis looks promising. Rust fungus is very host specific so it won’t attack other plants and is systemic, meaning it will attack the roots, increasing the chance of a win in this fight! There have been several studies into this biocontrol agent since Cripps’s paper was published and further research can continue to better understand this pathogen and how it affects Californian thistle. The hope is that it will provide a successful biocontrol agent that will successfully establish in New Zealand. It may not be common right away as biocontrol does take time but success doesn’t happen over night.

In an article about the ongoing thistle control, Cripps stated “That’s the nature of biocontrol. It takes many years, or even decades, for the biocontrol agent to spread and become common and be able to achieve damaging levels.”

Biocontrol agents may not be an immediate solution such as herbicides but I believe we need to start looking at the long term and what might be needed in 50 years’ time. With the growing concern of herbicide resistance something has to be done!

The author Caitlin Henderson is a postgraduate student in the Master of Applied Science taught at Lincoln University. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.

Cripps, M., Bourdôt, G., Saville, D., Hinz, H., Fowler, S., & Edwards, G. (2011). Influence of insects and fungal pathogens on individual and population parameters of Cirsium arvense in its native and introduced ranges. Biol Invasions, 13(12), 2739-2754. http://dx.doi.org/10.1007/s10530-011-9944-7