Biological control winning slowly

The ragwort flea beetle loves eating ragwort but doesn’t like getting its feet wet. The tiny insect’s huge appetite is saving New Zealand dairy farmers $44 million a year in control costs, a Landcare Research study has shown.

“Until now we have only been able to speculate on the financial benefits of the ragwort flea beetle to farmers,” Landcare Research scientist Simon Fowler said.

“We had no hard data. We had amazing before and after photos of the flea beetle’s work.”

Fowler used data from a 2005 study on the West Coast where the flea beetle failed to thrive because of the wet climate. The amount of money West Coast farmers spent killing the weed was extrapolated across NZ’s dairy sector, taking into account inflation and national dairy herd size. Ragwort, which invades pastoral land and is toxic to cattle and horses, is thought to cost NZ dairy farmers $20 million a year in control costs.

The flea beetle has wiped out ragwort on 50% of NZ farms, but it remains a “moderate” problem on about a quarter and the remaining farms – many in wet regions on the West Coast, Taranaki and Northland – are still fighting the weed. A second insect, a plume moth that thrives in wet conditions, is being deployed to tackle remaining ragwort strongholds and looks set to reduce the threat of ragwort further.

Ragwort flea beetle was shortlisted in NZ as a potential control agent in the 1930s but dismissed – a decision with no scientific basis that cost the country a staggering $8 billion dollars Fowler said.

“That’s how much farmers spent on controlling ragwort up until 1983, when the DSIR imported and released the flea beetle.”

“Farmers can get on a continual treadmill of spraying herbicides to control the weed but that doesn’t give the flea beetle a chance, and only provides temporary relief. It takes a bit of nerve but sometimes they need to take a deep big breath and do nothing. Be patient and let the ragwort flea beetle do the work for you.”

Spraying tends to disturb the pasture, which then dies, and weed seed in the soil has a chance to establish new plants. The spray also kills the beetles.

“Biocontrol agents are not always as successful and many fail to have any material effect. However, the flea beetle is a success story and our quantitative analysis shows how important these tools can be when we assess control agents. The $44 million saving is ongoing and free, and we can be confident that this little golden beetle will not do anything other than munch through this particular weed.”

Farmers can read more on how to find the flea beetle, transfer it and manage release sites here.

A range of biological agents have been employed against thistles and while some are working well, others are not, Lynley Hayes, Landcare programme leader for biocontrol of weeds, said.

“Some of the problem could be management, where farmers are mowing at the wrong time and the biocontrol agents are killed by accident,” she said.

The green thistle beetle is working well on California thistles in many areas, with good spread from the original release sites of Southland Otago and later into the Greater Wellington and Horizons regions. 

Herbicide use can be difficult because of pasture kill across heavy thistle infestations and the persistent root systems on the weed that makes eradication difficult. Evidence has shown beetles established on 90% of the original 2008 release sites and many regions have been able to spread beetles to a number of new sites.

Anecdotally, researchers are encouraged by the heavy defoliation of thistles in some regions, and a Sustainable Farming Fund project is underway to collect quantitative data to demonstrate their true impact and determine how much the above-ground defoliation by the green thistle beetle limits the growth of thistles in the following season.

For more information go to www.landcareresearch.co.nz/publications/newsletters/biological-control-of-weeds/issue-72/green-thistle-beetle

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