Insects imported to wage war on giant invasive reed

Giant reed, or Arundo donax, a problem plant in Northland, is about to meet its muncher - a double-pincer attack by an introduced galling wasp and scale insect.

Two insects common around the Mediterranean will soon be chewing their way through a big problem in Northland.

The Environmental Protection Authority (EPA) has approved an application by Northland Regional Council (NRC) to introduce a wasp and a scale insect to New Zealand to control the giant reed, Arundo donax.

The reed, sometimes known as elephant grass or bamboo grass, is mainly found in Northland but could potentially establish throughout New Zealand.

It grows up to eight metres tall and inhabits streamsides, estuaries, disturbed lowland and coastal forest margins, river systems, and gullies.

Giant reed smothers, shades out and displaces all other plant species and can block streams and drains, causing flooding.

It expands via a strong rhizome network which generates large colonies of plants, growing prolifically and harming the biodiversity of insect and plant life.

The plant, which is native to eastern and southern Asia and possibly parts of North Africa, has displaced endemic and endangered species in the United States, Mexico and Canary Islands.

"The arundo galling wasp and arundo scale insect have been successful in curbing the vigour and abundance of giant reed stands in those habitats," EPA scientist Dr Clark Ehlers said.

"The galling wasp lays its eggs inside giant reed shoot tips, which swell as gall tissue builds up. Growing wasp larvae feed on this tissue. A single wasp can produce 20 to 30 offspring."

The arundo scale insect attacks the giant reed by puncturing plant tissue and sucking out nutrients, reducing plant growth.

Because the two insects' feeding niches do not overlap, their combined impact on the giant reed can be significant.

The EPA's assessment of the NRC application considered potential risks and costs, including to the environment, human health, the economy, Maori culture and the Treaty of Waitangi.

There are no native plants closely related to the giant reed so the introduced insects are unlikely to choose another food source, nor are they likely to hybridise with native insects.