Tutsan Agents Imminent
Leaf-tying moth larva attacking a tutsan seed pod.
South Islanders might not be familiar with tutsan (Hypericum androsaemum), but it is now a highly visible and well-known weed in the central North Island. Tutsan grows to about 1.5 m in height and has bright yellow flowers that appear from November until February. Round black berries are produced that are attractive to birds (and probably possums), who distribute the seed around the landscape. Like many other weeds, tutsan was originally introduced to New Zealand as a garden ornamental but soon naturalised into the wider environment, repeating the invasion patterns seen in a number of other countries including Australia. It was noted as a ‘weed of significance’ back in 1955 and since then has become a major issue in the hill country of the North Island, particularly around Ruapehu. In the last 50 years tutsan has not only affected productive landscapes but also conservation land.
As mentioned in Issue 66, there appears to be two different genetic origins of this weed in New Zealand. Tutsan growing in the North Island is more genetically similar to tutsan from Wales and Ireland, whereas the plants growing the South Island are more genetically similar to tutsan from England and France. Both genetic groups have a specialist rust disease (Melampsora hypericorum) associated with them, which may be holding South Island populations in check but, unfortunately, not North Island populations, which are continuing to expand.
“Since 2013, we have been investigating some new biocontrol agents on behalf of the Tutsan Action Group, a farmer-led group supported by Horizons Regional Council,” explained Hugh Gourlay, who has led this project. “One of the challenges with finding suitable candidates for this plant has been the level of host specificity required. There are 19 Hypericum species in New Zealand including another invasive weed, St John’s wort (H. perforatum). Four of these species are native to New Zealand and we need to be sure that any insects that we introduce will not harm them,” said Hugh.
Two specialist insect species were imported from Europe, a leaf-tying moth (Lathronympha strigana), which attacks the stems, shoot tips and seed pods of the plant, and a small leaf beetle (Chrysolina abchasica), which attacks the foliage. Both have been in containment at Lincoln since 2014, and appear to be highly damaging. Host testing has recently been completed. The leaf-tying moth laid eggs only on Hypericum plants, with a preference for tutsan (H. androsaemum). The resulting larvae survived only on tutsan and St John’s wort, leading us to conclude that there is no significant risk of non-target attack on other Hypericum species, including the native species.
The leaf beetle testing results were not quite so clear-cut. Testing showed that two of the native Hypericum species (H. pusillum and H. rubicundulum) can be considered fundamental hosts i.e., the beetle was able to complete its lifecycle on them in the laboratory in an unnatural environment where there is no choice of host plant. However, most of the adults that were produced on these two plant species died soon after emergence. Not all plants that are fundamental hosts prove to be actual hosts in the field, and testing in cages can overestimate the risks. “In reality the risk of the leaf beetles attacking the two native Hypericum species in the field is low. These new beetles are likely to be less of a threat than the existing St John’s wort beetles (Chrysolina spp.), which were released to control St John’s wort over 50 years ago,” said Simon Fowler. “To reach this conclusion, we used recent research in the Beating Weeds programme (see Issue 68), which shows that the relative performance of agents in laboratory trials on a test plant versus the target weed is a good predictor of whether a fundamental host will actually be attacked in the field,” added Simon. In this case, the relative performance scores suggest the tutsan leaf beetle is a safer bet than the St John’s wort beetles. Notably, the St John’s wort beetles have proven to be well-behaved in the field and do not do any significant damage to indigenous Hypericum species.
An application to release the leaf-tying moth and the leaf beetle is currently being prepared and will be submitted to the Environmental Protection Authority before Christmas.
This project is funded mainly by the Ministry for Primary Industries’ Sustainable Farming Fund (401451), with co-funding provided by a range of other organisations.