Broom Control in the Wilderness
In November last year, Simon Fowler and Quentin Paynter ventured out into the wilderness (well, the Wilderness Reserve in Te Anau!) to set up permanent plots to measure the impact of two biocontrol agents: the broom psyllid (Arytainilla spartiophila) and the broom twig miner (Leucoptera spartifoliella), which appear to be having a noticeable impact on broom (Cytisus scoparius) there.
Biocontrol impact assessment data tend to be in short supply for most biocontrol projects, so this is an important step in the right direction.
The project is a joint initiative between Environment Southland and Landcare Research and aims to quantify the amount of damage these two agents are doing to broom in the region. To do this it is necessary to look at recovery of broom when feeding pressure from the two biocontrol agents is removed. Ten of the twenty plots will have the insects removed by spraying insecticide and the remaining ten plots will not be sprayed, so that the biocontrol agents can keep doing what they do best – damaging broom.
“A previous study in North Canterbury showed that the broom twig miner stunted plants early in the year, but the effect was not sustained and plants recovered over the growing season. This experiment will monitor the impact of both the broom twig miner and the broom psyllid,” said Quent. In addition to this project, we have also set up long-term plots near Hanmer, in North Canterbury and at Waiouru on the North Island where, respectively, the broom gall mite (Aceria genistae) and the broom leaf beetle (Gonioctena olivacea) are establishing well.
It is difficult to predict in advance what the combined impact of all the broom agents will be. “We try to select agents that complement each other and maximise the cumulative impact on the target weed. For example, given enough summer rainfall we know that broom can shrug off even quite heavy early spring attack by the broom twig miner. This is where the broom leaf beetle should step in attacking the compensatory growth of the plant later in the season. However, in practice it can be hard to tell how agents will combine in new environments, which is why we need to test this experimentally,” said Simon.
The Wilderness Reserve is the best surviving remnant of the strangely stunted podocarp, bog pine (Halocarpus bidwillii). This plant grows not only in bogs (as its name suggests) but also on the well-drained, stony substrates common on river beds. Bog pine hosts the well camouflaged native caterpillar Dasyuris callicrena. Also at the site are threatened plants such as Hebe armstrongii, Coprosma intertexta, Senecio dunedinensis and Carmichaelia crassicaulis. Also resident at the reserve is the threatened moth Ericodesma cuneata. The reserve is an example of a ‘frost flat’ ecosystem, characterised by leached terraces with low fertility and extreme ranges between winter and summer temperatures.
The exclusion experiment in the Wilderness Reserve in Te Anau will compare the rate of growth of broom plants, the percentage of broom cover within the plots, seed production, and recruitment of new broom plants in plots with and without the biocontrol agents. The survival of indigenous plant species, including bog pine, is also being measured. The data will be used to determine what level of control the agents are providing and whether the levels of attack reported at the Wilderness Reserve will be likely to control broom populations at other sites.
Other broom biocontrol agents are being established in Southland. The broom seed beetle (Bruchidius villosus) is now widespread. Establishment of the broom shoot moth (Agonopterix assimilella) is uncertain, but the broom leaf beetle (Gonioctena olivacea) and the broom gall mite (Aceria genistae) appear to be doing well, and it is hoped they will add even more pressure to the broom in due course.
This project is funded by the Ministry of Science and Innovation through the Beating Weeds Programme.