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• Issue 78 »
• Comparing ragwort then with now: part two

Comparing ragwort then with now: part two

Ragwort

In the previous issue we introduced a project where we trialled a new approach that involved revisiting 71 farms on which the ragwort flea beetle (Longitarsus jacobaeae) had been released some 20–30 years ago. This project looked at the extent of ragwort (Jacobaea vulgaris) on those farms now and also asked questions about land management, including herbicide use. Farmers were also asked what they thought about the impact of the beetles and whether they had done a good job.

“The survey results have provided some incredibly useful data,” said Simon Fowler, who did the analysis. “Not surprisingly, given the time elapsed since the early releases, it was unusual for the same landowners and council staff who were involved at release time to be involved in this survey, emphasising the need for good record keeping,” Simon said. However, the records on file were sufficient to at least get close to the release points. The data show conclusively that ragwort density has declined enormously following the release of the ragwort flea beetle, particularly in drier regions. Only in very wet regions, such as the west coast of the South Island and Southland, where the beetles struggle, are the densities of ragwort still relatively high.

The survey found that the density of ragwort is lower on farms that have low herbicide use than those with high herbicide use. Sustained low weed density is most likely due to biocontrol. However, the higher incidence of ragwort on the farms with high herbicide use could indicate that herbicide interferes with the effectiveness of biocontrol agents, or that landowners are using more herbicide where they have a bigger problem.

“We found that the use of boom spraying had declined dramatically (from 21 farms down to 5), with much less intensive herbicide use or even manual control such as pulling/grubbing,” said Simon. On six of the farms, the farmers thought that control was entirely attributed to stock (sheep) grazing pressure. The data do not support this, with no significant difference between initial mean number of ragwort plants per hectare in sheep, beef or dairy farms. If sheep were contributing to suppression of ragwort on heavily infested farms, sheep farms would be expected to have lower levels of ragwort compared to beef or dairy farms when the flea beetle was released. “We have been particularly encouraged by the 16 farms where ragwort control is no longer needed,” said Simon. The most obvious explanation is that biocontrol by the ragwort flea beetle has virtually eliminated ragwort from these farms.

Spot herbicide treatments (spray or granules) and carpet rolling are the best weed control treatments to use in conjunction with the ragwort flea beetle as these generally leave smaller plants for the beetle while preventing ragwort from flowering. Only boom spraying is likely to be wholly incompatible with biocontrol, as no food resources are left intact,” explained Simon. “Since the beetles were released there have been major reductions in the use of indiscriminate boom spraying against ragwort,” said Simon. “Instead there were increases in spot spraying/prilling, pulling/grubbing or the use of stock (sheep or goats), all of which are much more compatible with biocontrol,” Simon added.

Despite the ragwort flea beetle, the results from the survey suggested that ragwort control costs were still high on a few of the farms (up to NZ$20,000/year). “Only one farm provided ragwort control costs before and after biocontrol: but on this farm control costs reduced from NZ$4000/year pre-biocontrol, to NZ$100/year post-biocontrol (a reduction of 98%),” said Simon. A majority of farms in the Auckland/Northland region and the drier eastern regions of the North Island reported spending very little on ragwort control. But some of the southerly and westerly regions are still spending a reasonable amount on control, reflecting the reduced effectiveness of the ragwort flea beetle in cooler and/or wetter areas.

A recent economic analysis found that the savings in ragwort control on dairy farms in New Zealand as a direct result of the flea beetle were predicted to be $44 million for 2015 alone. A net present value analysis of the annual benefits and costs from 1926 onwards gave a benefit-cost ratio of 14:1, i.e. every dollar invested in ragwort biocontrol New Zealand has gained $14 in reduced ragwort control costs. This figure does not include benefits to other farming types or even all benefits to dairy (e.g. reduction in cattle deaths, increased pasture production). This study found that the best results were achieved on beef and sheep farms, which suggests that the $44 million dollar figure and the NPV are extremely conservative measures of the value of ragwort biocontrol to New Zealand.

Respondents were also asked to sum up what they thought of the biocontrol programme for ragwort in just three words (see table). There were some interesting responses! Of the 52 farmers that responded, 25 were very positive about biocontrol, with 19 intermediate/uncertain views, leaving only 8 of the farmers that either thought biocontrol was useless or were unaware of biocontrol. “What I found interesting was that all eight farmers who were unaware of biocontrol or considered it useless, actually had excellent reductions in ragwort density on their farms,” exclaimed Simon. Two farmers attributed the excellent ragwort control to sheep alone, even though sheep were clearly not controlling ragwort before the release of the ragwort flea beetle. “However, we do acknowledge that sheep are likely to complement the action of the flea beetle,” Simon commented. Five farmers appeared convinced that their ragwort suppression was solely a result of repeated herbicide use. This highlights the importance of communicating which land management practices are complementary to biocontrol.

To conclude, we understand that monitoring weed biocontrol outcomes can be expensive and this presents a conundrum for funders, who need to weigh up spending funding on follow-up assessment against targeting new weed species. The survey has shown that it is possible to design a cost-effective monitoring system in which stakeholders help to collect the data. It also enabled comparisons to be made between weed abundance at the time of release and now. This yielded important information such as the level of weed density with existing management practices, current control measures used for ragwort, presence/absence of all the ragwort biocontrol agents and land manager views on biocontrol. It also provided an opportunity to reconnect with the land managers and continue dialogue about weed biocontrol, including the most complementary land practices for biocontrol agents. In 20 years’ time few will remember the problem that ragwort once posed and how a small gold beetle changed farming for many in New Zealand. Nevertheless this study completes a compelling story about a big success that happened here, and the benefits that biocontrol can provide to communities.

This project was funded and data for it was collected by the National Biocontrol Collective. A huge thanks to everyone who contributed to this survey!