Introduction

A new insect parasite has been released into the Mackenzie Basin and Upper Waitaki for the first time, to help control the spread of broom.

A new insect parasite has been released into the Mackenzie basin and upper Waitaki for the first time, to help control the spread of broom.

The Department of Conservation oversaw the release of the gall mites at three sites last month – Jollie Stream on the eastern side of Lake Pukaki, Mt Ostler near Twizel and Otematata in the Waitaki valley – all with dense infestations of broom. The aim is to establish healthy populations of the mites, which can then be moved to other sites as needed. 

Gall on broom plant created by gall mite. 
Gall on broom plant from gall mite

Department of Conservation ranger Peter Willemse says the broom gall mite attacks broom plants in autumn and is expected to work in concert with three other broom-eating insects already established in the area, which target the plant in spring and summer. 

“It is hoped that together the insects will weaken broom plants and reduce the spread of this introduced invasive weed,” he says.

The other broom-eating insects were also introduced into the area to help control broom. The broom psyllid, also released by DOC, targets the plant’s spring growth. The caterpillar of the twig miner moth, which probably self-introduced about 50 years ago, browses on the plant’s stems. While the broom seed beetle, released by Environment Canterbury over ten years ago, has larvae that eats the soft green broom seeds. 

"These insects are already affecting the vigour of their host plants, in particular the twig miner, although a widespread reduction is some time off", says Willemse.

“This is a long term strategic move – it’s not a quick fix.

"In five years we should start to see a noticeable knock-back of broom as the pysllids and gall mites join forces with the other insects.”

The broom gall mite is native to Western Europe and was first brought into New Zealand by Landcare Research in 2006, with releases taking place two years later.  The mite is host-specific and is considered very unlikely to attack other plant species.  

The adult mites are so small you cannot see them with the naked eye and the best way to detect their presence is to look for the galls that form as the mites feed.

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