Heather Beetle set for Even Greater Things
Heather killed by the heather beetle (greyish plants in the foreground).
In 2014, Paul Peterson (Landcare Research) and Paul Barrett (Massey University) went to Scotland to collect some larger, more ‘rugged’ heather beetles (Lochmaea suturalis) to supplement the existing population, which has struggled to cope with the conditions in some parts of Tongariro National Park.
Heather was introduced to the park to create habitat for grouse back in 1912 and since then it has expanded its range to cover more than 50 000 ha of mainly conservation land, where it forms a dense monoculture displacing native plant communities.
“Although this biocontrol programme has been underway for some time, and we estimate that nearly 3000 ha of heather have now been killed by the beetles, establishment of the heather beetle in some parts of the park has been frustratingly slow,” said Simon Fowler, who has been involved since the outset. Detailed investigations to explain the mixed performance of heather beetles since their release in 1996 have resulted in three hypotheses: poor climate matching, genetic bottle-necking and low foliar nitrogen in heather. “We suffered setbacks when line-rearing the original importation of beetles as some of them were infected with a microsporidian gut parasite,” said Lindsay Smith, who reared heather beetles in containment prior to the first field release.
Subsequent work has shown New Zealand beetles to be genetically compromised and physically smaller, so more beetles were collected from Scotland and mated with New Zealand beetles. “One of the main goals was to increase heather beetle body size so that they had sufficient fat reserves to survive the long winter and variable spring conditions faced in Tongariro National Park,” said Paul Peterson. “We had evidence to show that beetles from Scotland were larger and less likely to be carrying microsporidia, although we did screen all the new ones just to make sure” he added.
Last November more than 300 of the new line of beetles were released at a low altitude site (~400 m a.s.l.) near Turangi into a field cage, to prevent them from dispersing too quickly and hopefully improve their chances of establishing. “We also added fertiliser to the area because our research has suggested that the beetles survive better on heather that has higher nitrogen levels in the foliage,” said Paul. “If we can get the new population to establish, comparisons between the original and new populations can then be made to see if performance has been improved. It is hoped that larger heather beetles will perform better at higher altitudes where heather has become increasingly prevalent and is altering the composition of alpine plant communities, according to DOC technical adviser Harry Keys. Lab rearing of remaining stock is ongoing and more releases will be made this year to improve the chances of establishing the beetles.
Previously we compared the effects of using herbicide versus biocontrol. In the plots where heather was controlled using herbicide, a lot of native plants (mainly dicots) were also killed from contact with the herbicide and failed to recover afterwards, whereas in the plots controlled using biocontrol, the native plants had a new lease of life. Many native and exotic grasses also benefited from the removal of heather in both plots.
“We are closely monitoring the total area of heather being damaged by the existing population and most damage is occurring at four key sites within the park,” said Paul. At each site, the damage is roughly doubling each year, which is slow compared with how quickly populations grow in the native range. “If we can’t improve current performance, we predict it will take 18 years for the beetles to damage heather over the 50 000+ ha area currently infested,” said Paul.
As well as monitoring the progress of new ‘genetically rescued’ heather beetle populations, future research is likely to include continuing to assess the recovery of native vegetation following heather biocontrol and monitoring native insect diversity and impacts on native and exotic vertebrates such as skinks, mice and birds.
This project is funded by the Ministry of Business, Innovation and Employment as part of Landcare Research’s Beating Weeds programme.