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Beetle wins battle against pernicious pasture pest
Apr 4, 2005
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A tiny beetle is winning the war against one of New Zealand’s most economically damaging weeds.

Ragwort (Senecio jacobaea) replaces pasture, and contains toxins that can cause fatal liver damage to cattle, horses, goats and deer. It reduces butterfat, milk and meat production, and can taint the flavour of milk and meat, and even honey. Ragwort can also be harmful to humans if ingested in sufficient quantities, or when weeded out by hand over an extended period.

In the 1930s, a spate of exploding trousers ensued as farmers desperate to improve pasture production sprayed the “miracle” weedkiller sodium chlorate – which reacts with natural fibres like wool and cotton in work clothes. Garments detonated with the slightest heat or spark – sometimes with people inside them. But packing even more of a punch, and much more safely, is the mighty ragwort flea beetle, introduced in 1983. Its caterpillars eat ragwort roots, thereby killing the plants.

Landcare Research weed researcher Hugh Gourlay says field experiments found that the beetle achieved a 95% reduction in ragwort within two to ten years of its release in both North Island and South Island sites. “Also, we started hearing anecdotally how ragwort was disappearing, and so we asked around.

“We have had stunning reports of the flea beetle’s success from farmers and council staff around the country. We knew the flea beetle was capable of being a great biocontrol agent. The stories we heard show how truly beneficial biocontrol can be.” (See following testimonies.)

Mr Gourlay says the success of the flea beetle represents a very high cost–benefit return.

“A United States study has shown that for every $1 invested by the government in biocontrol $13 was returned to the taxpayer, and that is likely to be similar here. We get the benefits of higher pasture, meat and milk production, and improvement in the quality of milk and meat that do NOT contain toxins from ragwort.

“Also, the benefits go beyond the purely economic. We cannot put a price on an improved environment where ragwort has been removed and native species allowed to regenerate.”

The flea beetle has succeeded around the country except for the West Coast and Southland, and to a lesser extent Northland, for reasons not yet fully understood.

Landcare Research is trialling two new insects that feed in similar ways to the flea beetle, but are better adapted to these wetter climates.

Mr Gourlay says ragwort will never be eradicated from New Zealand entirely. “However, with vigilance it can be reduced to a minor nuisance pest plant.”

Ragwort is at its most obvious now when its masses of bright yellow flowers can be seen on the roadsides and on farms. Some district and regional councils send reminders at this time of year for local farmers to be particularly alert to the need for ragwort control.

In the meantime, ragwort flea beetles are now coming out of their summer hibernation – ready to munch on any remaining ragwort near you.

The Beetles’ Greatest Hits – testimonials (from North to South):

“Ragwort flea beetle numbers have been at good levels… and have provided good ragwort control on most properties.”

Greg Hoskins, Biosecurity Officer, Auckland Regional Council (South-western Rodney)

“Like so many others in my line of work, I have always dreaded the holiday period because of the numerous complaints and nasty phone calls that lovely little yellow flower caused. This year there haven’t been any, because there is no ragwort in the Western Bay of Plenty.”

Walter Stahel, Plant Pest Officer, Environment Bay of Plenty.