Third NZ weed evolves Roundup resistance

In a new study, scientists report the resistant wild carrots for the first time, after a 2022 study found that more than half of the vineyards in Marlborough had Roundup-resistant ryegrass. The researchers say that while glyphosate-resistant wild carrots are not yet that widespread, they have the potential to be.

"Weeds resistant to herbicides are becoming more common across the world, and New Zealand is no different. While resistant weeds have been present in New Zealand for over forty-five years, only a few years ago the first field surveys for them were done to estimate their prevalence. These surveys were done in key crops at high risk of having resistant weeds, including vineyards.

"Vineyards and orchards rely on glyphosate especially, which is often used two-three times a year. As a consequence of repeated glyphosate use over several years, two species of ryegrass (Lolium multiflorum and Lolium perenne) evolved resistance. The vineyard survey revealed that more than half of the vineyards in Marlborough had glyphosate resistant ryegrass (seen at a lesser extent in other regions). Also, during the survey we were alerted to a potential new case of resistance in wild carrot (Daucus carota L.). This study presents evidence to confirm that yes, wild carrot populations are becoming resistant to glyphosate (now the third weed species to do so in New Zealand) in Marlborough vineyards.

"Wild carrot is the weedy version of the cultivated carrot and is most often observed along roadsides and disturbed areas. Wild carrot isn’t a particularly bad weed, but with glyphosate resistance becomes difficult to control. It has a white, wiry, tap root and is insect-pollinated (which can spread pollen kilometres). Seed spread can be prevented by good management but spread of resistance through pollen can be more difficult. Viticulturalists who notice new patches of wild carrot on their rows should be vigilant and use a different tactic (hand-pulling, other herbicides) to prevent the plant from establishing.

"Herbicide resistance is a repetitive phenomenon, which can be prevented and dealt with through good management (see: https://resistance.nzpps.org/). While we have surveyed some key crops, there are many that we haven’t been able to look at yet. Other fruit orchards such as kiwifruit and stonefruit will likely have similar risks as vineyards.

"We often hear about glyphosate (the active ingredient in products like RoundUp) in the media. Glyphosate has been banned (or is being phased out) in some countries due to concerns about safety and impacts on human health. However, in New Zealand, glyphosate is still approved for use. As recently as last year, New Zealand’s Environmental Protection Authority considered whether there were grounds to reassess use of glyphosate and glyphosate-containing substances, and concluded it is safe to use if the existing rules around its use are followed."