Forage Science

Ongoing thistle control

13 August 2015
Ongoing thistle control | Farm Research | AgResearch NZ

Effective management of an invasive farm weed is becoming more simple and robust thanks to ongoing work undertaken by AgResearch scientists.

Californian thistle is widespread throughout New Zealand pasture and during its seasonal peak, covers 12% and 6% of grazed land on sheep and sheep/beef farms, respectively, resulting in an annual national revenue loss of $233m. This loss increases to $685m peryear when dairy and deer farms are included.

Work by AgResearch scientists has added a number of new tools to be included in farmers’ ‘toolboxes’ to fight the aggressive weed.

The release – and subsequent success – of a biocontrol agent to help fight the thistle is showing great promise.

The Californian Thistle Action Group, based in Southland, released the green thistle beetle (Cassida rubiginosa) in 2007 following its introduction into New Zealand by Landcare Research. The beetle is now established in several parts of New Zealand.

Reports of severe and extensive feeding on Californian thistle are encouraging, and suggest great potential for the beetle, entomologist Dr Mike Cripps says.

He believes its success is likely due to “enemy-free space” experienced by the beetle here, allowing for the maintenance of high and long beetle densities and a longer duration of sustained feeding.

“In Europe, I recorded approximately 50% mortality of the beetle one week after a field release of hundreds of its larvae. In contrast, at Lincoln I noted constant densities of larvae for a month after field release,” he says.

While they do not eliminate weeds, biocontrol attack is likely to result in smaller, weaker plants that are less likely to spread, says Dr Cripps. For farmers that means the thistle can be more easily outcompeted by other plants or controlled by traditional means.

“Infestations may be reduced to a level that we can live with, or eliminated effectively and economically. Biocontrol has the greatest impact when used in conjunction with wider good land management practices.“

Dr Cripps warns that while initial progress has been impressive, any significant and prolonged impact won’t be seen for many years.

“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.”

Meanwhile, research continues into earlier work that found mowing Californian thistle during the rain improves the control of the weed.

On-farm trials found up to 30% greater reduction in thistle shoot population density after mowing in the rain as compared to mowing in during dry weather conditions. The likely cause is that wet conditions allow for greater transfer of, and infection of cut stems by, one or more disease-causing fungi, which then transfer to the root system and kill the new shoots as they emerge the following growing season.|

Now, researchers are trying to learn more about the fungal pathogens of the thistle, including the thistle rust fungus, and their potential to be manipulated by farmers to control the weed.

A population modelling study is also shedding new light on how a thistle population will respond to defoliation AgResearch weed ecologist Dr Graeme Bourdôt says.

“With this modelling we can simulate different mowing strategies and figure out how frequently and at what time of the year to mow the thistle to achieve a certain outcome.

“For example, if a farmer wanted to keep the thistle population as it is now they would have to undertake one particular strategy, but if they wanted to really knock it back then the modelling could show them other strategies.

“The model is being extended to allow the effects of defoliation by the green thistle beetle to be simulated.”

The green thistle beetle research programme is funded by investment partners Beef + Lamb New Zealand and Wools of New Zealand, with the Ministry for Primary Industries through the Sustainable Farming Fund, while the modelling study is funded by AgResearch core funds. The goal of the programme is to better understand the value of the green thistle beetle, particularly in hill-country pasture where conventional control techniques are not practical or cost-effective. Releases of the beetle were carried out in spring 2014 in hill-country pasture in Canterbury and Manawatu and will be evaluated over the next two years.

Benefit to New Zealand

Over a 15 year period, results suggest that a discounted (at 8%) net benefit of $29 million is accrued for the specific segmented extensive sheep hill country sector, representing a real benefit-cost ratio of 25 to 1 for the R&D investment.

This analysis is based on a simplified mathematical model, has evaluated the potential net benefits to New Zealand extensive and steep hill country pastoral sheep livestock systems through the adoption of biological control for thistle species. These segmented farm systems are not amenable for other conventional thistle control options due to cost of application and practicality of access. The biocontrol agent selection involved the establishment of the leaf-feeding insect beetle Cassida rubiginosa. The economic value proposition, based on net benefits at the on-farm (farm gate) revenue level, encompassed analysis of weed control impacts on pasture cover and livestock productivity; and resultant positive effects regarding operating surplus, a cost-effective scabby mouth vaccination scenario (via sheep risk management assumptions to thistle exposure), and the revenue implications of addressing vegetable matter contamination in wool arising from conjectured changes in thistle seed structure exposure. Key assumptions were estimates for peak 40% efficacy of the biocontrol agent after seven years, and biocontrol costs reducing from $20 per ha at year  one to stabilise at $10 per ha from year three.

Adoption of the biological thistle weed control is also expected to provide additional industry benefits through underpinning cost-effective environmentally sustainable hill country pastoral systems, and reducing potential health and safety risk factors in weed management on steep hill country pastures.

Project Investment

Total project investment for the extensive sheep hill country component, over three years from 2014, encompasses $579,000 from the MPI Sustainable Farming Fund (Ministry for Primary Industries with investment partners Beef + Lamb New Zealand and Wools of New Zealand) with an additional $600,000 from AgResearch Core Funds.