How Good Are Beetles at Finding St John’s Wort?
Biocontrol of St John’s wort (Hypericum perforatum) is undoubtedly a success story. Where there were once vast areas of the weed infesting agricultural land, the plant is much less common thanks to two small beetles, Chrysolina hyperici and Chrysolina quadrigemina, which were introduced into New Zealand in the 1940s and 1960s, respectively.
Over the past year intern student Anna Pittam from the University of Birmingham has been looking at how good the beetles are at locating the now much rarer St John’s wort plants in the landscape, and the implications for biocontrol and for growing the plant for its medicinal properties. “I was interested to find out how good the St John’s wort beetles are at detecting small, isolated patches of their host plant, and how this relates to keeping the weed under control.”
The ability of the beetles to detect relatively isolated plants is dependent on how far they have to travel and factors such as weather patterns, terrain and geographical isolation. “In the same way we think of offshore islands, patches of St John’s wort occur now as ‘islands’ in the rural landscape,” said Ronny Groenteman, who has been guiding Anna. “We were interested in seeing whether there was a relationship between the size of St John’s wort infestations and the presence of the beetles,” Ronny explained. A positive relationship would support the well-known island biogeography theory (suggested by MacArthur and Wilson in 1967), which suggests that there is a link between the size of islands and the likelihood of fauna finding them.
Anna surveyed some councils and Department of Conservation offices to try to get a handle on beetle densities and whether there was subsequently less reliance on manual or chemical control of the plant, repeating a similar survey conducted in 1987. “Five out of six of the councils reported the presence of the beetles and indicated that they had observed a decline in St John’s wort as an agricultural weed, but that it still occurred in isolated patches along the roadside,” said Anna. There was an expansion in the geographical area where the beetles were found compared with the 1987 survey, with higher beetle densities in the lower-rainfall eastern side of the South Island. “The beetles don’t like the wet weather on the West Coast,” said Ronny, but St John’s wort is not common in these parts of the country either – the plant and its natural enemies fortunately favouring similar conditions in this instance.”
In 1987 there were 10 control programmes being undertaken, mainly in the eastern parts of New Zealand, whereas in 2016 there were none, and some council staff weren’t even familiar with the weed! “Overall, St John’s wort beetles appear to be continuing to control the plant well, keeping it much less prevalent than before,” said Anna. Although the beetles appear to be effective at finding and controlling small, isolated patches of the weed, their migration between patches can be slow.
Anna’s work also included a survey of herb growers cultivating St John’s wort to ask whether they had noticed any damage to the foliage. Although there wasn’t a strong response rate, the indication was that crops on the eastern side of New Zealand were more commonly defoliated, as were crops that have been grown continuously for longer periods of time. “Unfortunately, most of the growers were not able to comment on the proximity of nearby St John’s wort infestations and whether they may be acting as reservoirs for the beetles. But as the weed declines in the landscape, there will be fewer beetles around and less impact on plants being grown as a herbal remedy,” concluded Anna. As an example, one of the respondents to the survey had been growing St John’s wort for 27 years, but the small number of plants (only 1 m2) had not suffered any significant defoliation.
Anna has returned to the UK now, where she is continuing her studies towards her environmental science degree. This includes a dissertation on horehound (Marrubium vulgare), which she also studied during her internship here.
This project was supported by the Ministry for Business, Innovation and Employment as part of Manaaki Whenua – Landcare Research’s Beating Weeds programme.
Contact
Anna Pittam – annapittam@googlemail.com
