A bit of background

Since 1769, more than 25,000 exotic plant species have been introduced in New Zealand – 90 per cent of them deliberately.

A new species naturalises about every 39 days, and in New Zealand naturalised species outnumber native species. Not all introduced plants turn into weeds but there are about 500 that are considered pest plants at present.

Introduced pest plants are expensive and damaging to the economy. It’s estimated that pastoral weeds cost about $1.1 billion a year to control (2005), while ecosystem service losses are estimated at about $2.52 billion a year (2008). The Department of Conservation and regional councils annually spend about $18 million a year to reduce biodiversity loss from weeds but are failing to contain many populations.

Biocontrol is used as a last resort, for large infestations. It is a technique used worldwide where we attempt to restore the balance between a weed and the environment by reuniting it with some of its key natural enemies from its natural origin – usually invertebrates and fungi.

The advantages of biocontrol are it only damages the target plants, it’s environmentally friendly, there are no health risks, it’s cost-effective once established, weeds are removed gradually, and the weeds become controlled everywhere.

A disadvantage is it’s a very slow and difficult process. There are no guarantees of finding suitable agents and the target weeds will never be totally eliminated.

Worldwide, about 600 agents have been released across 260 weeds in 90 different countries since the 1800s.

Of these, 33 per cent have been so successful that no other control options were required; 50 per cent have been partially successful and 17 per cent were failures – meaning they’ve had no impact on the pest plant. Four have had serious non-target impacts, all on plants in same genus as the target weed and all predictable by today’s standards.

In New Zealand, we have been doing biocontrol for about 90 years. Sixty-five agents have been released, and there have been no significant non-target attacks.

We are continuously working to improve success rate and cost effectiveness. Host-range testing has improved considerably to reduce any potential risks.