Every two years, thousands of rural landowners take part in the Survey of Rural Decision Makers (SRDM) – a nationwide effort to understand what’s happening on the ground in Aotearoa New Zealand’s food and fibre sector. In the last three rounds of the survey (2019, 2021 & 2023) we have included questions about a particularly thorny issue: weeds of the future.
Weeds cost landowners time and money. Biocontrol can be an effective long-term option to manage weeds. However, biocontrol tends to be applied to weeds that are already widespread because of the high upfront costs of developing a biocontrol programme, which can be up to millions of dollars.
We know that controlling weeds in earlier phases of their invasion can be far more cost effective than waiting for them to become widespread. Can we reconcile this intellectual understanding with the reluctance to invest the high upfront costs needed to develop biocontrol for weeds when their distribution is relatively limited and they can be managed by other means?
Since 2019, the SRDM has included a question asking rural land managers to list weeds on their properties that aren’t a big problem yet, but that they expect might require management within the following five years. We wrote about these first results from the 2019 survey in Issue 96. Now, we wanted to examine results from 2021 and 2023 as well to study if there were repeated weeds or trends in the survey responses.
On top of acting as an early-warning system by picking up on weeds that are only starting to become apparent or are spreading into new areas, the survey serves two important purposes in the context of biocontrol: First, it helps us assess whether our current system for prioritising target weeds for biocontrol is fit to identify the weeds that landowners are concerned about. Second, it highlights opportunities for ‘repeat’ biocontrol programmes, where we could piggy-back on successful efforts overseas using agents that have already been tested and have proved their effectiveness.
One of the best predictors for the success of a biocontrol programme is previous success elsewhere, says Ronny Groenteman, who led this ‘Weeds of the Future’ study. We have shown previously (Issue 71) that such ‘repeat’ programmes are significantly cheaper to adapt for New Zealand in comparison to the cost of starting a novel programme that has not been worked on elsewhere.
Imange: oxeye daisy
We have used this knowledge previously when prioritising widespread weeds as biocontrol targets. Here we suggest that this knowledge can help justify the economics of developing biocontrol for weeds that are still a small problem, to avoid letting them get out of hand in the first place.
Interestingly, we were already aware of many of the weeds named in the survey because they have either been targeted with biocontrol already, are current targets for which biocontrol is being actively developed, or have been flagged as future priorities. This suggests the current framework we use for prioritising weed targets for biocontrol successfully identifies most of the weeds that landowners are concerned about. However, many participants continued to name long-established weeds like gorse or thistles as ‘weeds of the future’. That these weeds get mentioned repeatedly in successive iterations of the survey suggests that they may be still popping up in places where they have not previously been a problem.
While many of the weeds mentioned by survey respondents were the ‘same old’ widespread weeds, some of the named emerging weeds have not been considered for biocontrol in New Zealand yet. Dock (Rumex spp.), for instance, was the most frequently named herbaceous weed not currently on the list for biocontrol, with nearly 500 mentions across the three survey repetitions. It is a pasture weed that livestock tend to avoid, and it can quickly take over areas, especially in poorly drained soils. “It has already been successfully targeted by biocontrol in Australia,” says Luise Schulte, who analysed the data and trends from the survey, “unfortunately, this might still be a tricky target, as we have native dock species in New Zealand. It means we will have to run additional testing to confirm the agents used in Australia are safe, or we would have to start from square one to look for novel agents. Both scenarios bring up the cost of a programme”.
Another standout was prickly pear cactus (Opuntia spp.), the most frequently named ‘off the list’ woody weed. While it’s not yet a widespread weed here, and was mainly mentioned by landowners in northern regions, it has a notorious history overseas. In Australia, prickly pear cactus once spread over millions of hectares before being brought under control in the 1920s by introducing several agents, most importantly the cactus moth (Cactoblastis cactorum) – to date one of the most well-known biocontrol successes globally. “The warming climate might speed up the spread of prickly pear further south in New Zealand,” says Luise, “so it’s definitely a low hanging fruit worth looking into as a biocontrol target here.”
Image: prickly pear after attack by Cactoblastis cactorum
Wild carrot (Daucus carota), oxeye daisy (Leucanthemum vulgare), and burdock (Arctium spp.) were also mentioned as possible future concerns. Some of these may prove tricky to manage – for example, wild carrot’s close relationship to cultivated carrots might limit the biocontrol options – but others, like oxeye daisy, could be explored further. A root moth has been released against oxeye daisy in 2023 in Canada and is currently under investigation for release in Australia. With a relatively modest investment non-target species of importance to NZ could be included in testing already taking place overseas, saving us the cost of running a complete set of tests here.
While the survey data has its flaws – people might list weeds that bother them currently instead of their predicted future weeds, and common plant names can be vague or used inconsistently – the survey serves to shine a light on emerging weeds that might otherwise be missed by existing prioritisation processes. Importantly, it lets researchers track changes over time and spot early warning signs.
The team plans to continue including the ‘weeds of the future’ question in future surveys, and hopes to keep building the case for strategic, cost-effective biocontrol responses. “The number of weed species New Zealand might appear overwhelming, says Ronny, “yet, with a bit more forward-looking planning, and investment that is modest in the greater scheme of things, we can use biocontrol to nip in the bud some clear suspects that have demonstrated their potential to become big problems elsewhere. We can get ahead of the game”.
The survey of 2025 is currently under way. You can find more information about the Survey of Rural Decision Makers and past results here: www.landcareresearch.co.nz/SRDM
Further reading:
Schulte L, Groenteman R, Fowler S 2025. Identifying emerging weeds as targets for biocontrol in New Zealand: What can we learn from the Survey of Rural Decision Makers? New Zealand Journal of Agricultural Research 68(7): 2456-2475. DOI: 10.1080/00288233.2025.2527676
This study is part of MWLR’s Beating Weeds and the Environmental Preferences Programmes, both funded by the Ministry of Business, Innovation and Employment’s Strategic Science Investment Fund.
