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Gorse enemy gains strength

5 December 2005

Gorse enemy gains strength

A European insect that devours new growth on gorse has overcome a slow start and is now being found in greatly increased numbers, much to researchers' delight. Gorse is one of New Zealand's worst weeds.

The gorse soft shoot moth (Agonopterix umbellana) was released throughout the country in the early 1990s to attack the plant, but initially struggled to survive. However, more recently the moth has been found to be thriving in some South Island sites.

"At the best site near Blenheim, we didn't even need to get out of the car to be able to see them," says Landcare Research's Biocontrol of Weeds Programme leader, Lynley Hayes.

"We estimated that there were more than 100 caterpillars per square metre of gorse bush. "We can also confirm that as expected the caterpillars are eating nothing but gorse. In other words, they are not attacking any other plants."

The Marlborough District Council liberated the moths at this site in 1996 as part of its programme to develop biological control for serious weeds in the region. Ms Hayes says the moths are also doing well at a site in North Canterbury, and are common now around Lincoln, near Christchurch.

"As well as this, they've established at a couple of sites further south. Their progress at North Island release sites is currently being checked." The gorse soft shoot moth is one of a suite of six insects imported to New Zealand as biological control agents, to attack gorse at different growth stages. Gorse thrips and gorse spider mites suck sap from the foliage during most of the year.

The gorse soft shoot moth feeds on new growth in the spring and early summer and then colonial hard shoot moth takes over and feeds on all parts of the plant from autumn through until spring.

The gorse pod moth and gorse seed weevil eat the seeds. All six agents are now established in New Zealand, but only three of them are common.

"People who are interested in establishing gorse biocontrol agents on their property should contact their regional council, who will help to source them locally," says Ms Hayes.

Landcare Research scientist Hugh Gourlay says because of the weed's seed bank, resilience and ability to regrow, this suite of insects may take some time to decrease the amount of gorse in New Zealand.

"Predictive models suggest it may take as long as 50 years for noticeable change, so biocontrol is not a quick fix for those who need to control the plant right now. Biocontrol offers a long-term solution to the problem".

The Marlborough District Council and Landcare Research will hold a field day near Blenheim on Thursday 8 December to show the caterpillars "in action", and allow people to take some home for their own gorse. Participants should bring thick gloves, secateurs, paper bags and a chilly bin to Misty Heights, Kaituna Valley (about 2 km from the Wairau River bridge), at 11 am or 1 pm. Parking areas and directions will be signposted.


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