Never ending battle against plant pests

If you think invasive pest plants are on the decline, think again. Richard Woodd reports.

"Our weed ecologists tell us that the weed invasion of New Zealand has only just begun," says Hugh Gourlay, a biological control expert with Landcare Research.

He says New Zealand is host to at least 25,000 and possibly up to 40,000 exotic plant species and only 2500 indigenous species.

"A new plant is being naturalised every 30 days in New Zealand. We have an exponential problem developing," he said in a talk to the Taranaki Regional Council, one source of funds for his work.

The councillors and staff were staggered by what's required to mount multi-fronted attacks on the worst invaders.

What he does, in essence, is identify and use existing predators. If they're not effective in controlling spread or survival of the host plant, Mr Gourlay may seek approval to have a new agent tested in another country, and eventually brought into New Zealand and released. This is a lengthy, very costly process, as is the whole business of controlling pest plants.

"We can't eradicate weeds, but we can reduce their numbers and spread," he says. "We have fantastic successes and we have many total failures, like where we release agents to control a weed and they may establish but appear to have no major impact. That's one of the main downsides.

"There is some risk associated with biological control, but it's extremely low. We are highly targeted and risk averse in our processes."

An economic study looking at biological controls in Australia over 104 years, of 29 different weed programmes, 15 had given a return of $23 for every $1 spent.

"That's $4.5 million a year to run those programmes and they are producing on average a $95 million return. But $15 million was spent on programmes that did not achieve any success."

Mr Gourlay has been working on gorse since 1992. He says gorse has been here for 150 years and it will probably be another 150 before we see any reduction in its presence.

He believes broom is going to replace gorse as our No 1 plant pest because "gorse can act as a nursery for native species, but nothing will grow through broom."

It distresses him to find nurseries ignoring the problem and continuing to sell broom under various names.

He says the leakage of pest plants is mainly from residential gardens and the nursery industry. "Most invasive plants are from someone planting something that looks nice and it starts invading their garden so they toss it over the fence or over a bank somewhere. It costs $30,000-60,000 to go through the Environmental Risk Management Authority assessment and approval process so I suspect they are from clandestine seed imports. Once a plant is approved for import, anyone can import it."

He was challenged by the TRC's operations director Rob Phillips who said: "I'd be very surprised if there is any broom being sold in Taranaki. Under the National Plant Pest Accord, there are 105 species which can't be sold and we annually inspect nurseries to ensure this is not happening. There are quite stringent controls and fines so I'd suggest most of the banned species are already here."

Biological control is funded by the National Biocontrol Collective, funded by regional councils and the Department of Conservation. Landcare Research also works closely with community groups who identify a weed problem in their community, and it often turns out to be a national problem.

Mr Gourlay is working with farmers in the Taumarunui area on a pest plant called tutsan, a close relative of St John's Wort. It is estimated to comprise 60 per cent of ground cover over 1500ha there.

"We formed the Tutsan Action Group, and have just applied to the Sustainable Farming Fund for $600,000 to advance that project. Tutsan was a serious pasture weed in 1955, it's known in at least seven other regions and is an early stage invader." There are 14 biological control insects working on pest plants in Taranaki. The most successful is the ragwort flea beetle, which is now widespread throughout New Zealand.

"This cost about $1.5 million to introduce and the current return is $17 million a year in reduced control costs. The beetle was released on 10 Taranaki farms, in areas where ragwort was widespread and dense, and where chemical control was not being used.

It's now very hard to measure because in some areas there is so little ragwort left.

"We've also brought in a ragwort plume moth which is better adapted to the wet West Coast climate and it's working well there."

Other biological success and failures from Mr Gourlay's presentation:

Gorse Biocontrol started with the seed weevil in the 1930s, now widely established, but it attacks spring seed crop only. Introduced the pod moth, which eats all seeds in the pod both spring and autumn, and those are removing about 60 per cent of the annual seed crop. A kill of need 75-80 per cent is needed to get on top of it.

The gorse spider mite was the first of a new line of agents, widely distributed, but its effectiveness is limited by a predatory mite which was introduced as a beneficial insect for horticultural crops against a spider mite and red mite in orchards.

Gorse thrips are well established in Taranaki; an enormous infestation has been found along a 10km strip in the Waitotara valley. I personally think this one is the dark horse; it's very small, it feeds on and kills just germinated seedlings and has potential for a major impact on regrowth.

Soft shoot moth: Feeds on new shoots. Released six years ago and now found in good numbers in Canterbury. Only one site release Taranaki and does not look like it will establish here.

Thistles New Zealand has no related native thistles so we are able to introduce biological controls with some confidence.

Nodding: Weevil larvae feeds on the crown and fly attacks seed heads. Both established in Taranaki; the weevil is credited with a significant reduction in nodding thistle in the Akama Rd area, Strathmore where it was released and monitored, but it has had a major impact on the thistle in the wider eastern hill country.

Scotch thistle: Seed fly was released at one site and has not been seen. Planning to release more collected from Canterbury.

Californian thistle: Green thistle beetle, released in many areas. The adult and larvae defoliate above ground and we think it has great potential. The stem weevil feeds down into Californian thistle roots and at this stage is too soon to tell but we are hopeful.

Old Man's Beard The leaf miner fly causes blackening of leaves from larvae feeding; widely established but impact limited because of attack by native parasite insect.

OMB fungus was very effective but has disappeared, possibly out-competed by another clematis fungus. We are looking at this as a potential inundation type of control agent which would need to be sprayed on annually. But we first need to understand more about why it disappeared.

OMB sawfly: Foliage feeding caterpillar, released in many areas and has never been found anywhere since.

Bark beetle: Offers the greatest potential through damage from feeding in stems from ground up and will kill plants. We are having trouble trying to host test this insect. We need large plants in the native range in the UK. We are 2-5 years away from being able to import this insect.

Woolly nightshade: ERMA has just approved import of a foliar sap-sucking lace bug from South Africa, for the 2010 summer. Also host-testing a flower feeding weevil in South Africa.

Darwin's barberry Seed feeding weevil being tested in Chile. Also looking at three promising pathogens which attack this barberry.

Pampas We're currently surveying to find what's already attacking this plant in New Zealand, before we look at importing anything else. The National Pampas Biocontrol Initiative has been formed around forestry companies to look at potential agents overseas.

Wild ginger Currently funding through a UK sister organisation, a survey of effective pathogens and insects in the Himalayas.

Giant bind weed

An environmental study has been done looking at options for control of this weed in riparian areas.

Biocontrol is being attempted in USA with the release of a moth and fungus and results will be out soon.

- © Fairfax NZ News