Mites lead in gorse attack

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Gorse is on the DSIR’s hit list.

Since March of this year, the Entomology Division of the Department of Scien­tific and Industrial Re­search has been waging biological warfare on the plant that Dr Richard Hill, the scientist in charge of the project, calls “New Zea­land’s commonest and most costly weed”.

Imported gorse spider mites have been released throughout the country in a control programme that aims to reduce the abun­dance, growth rate and regeneration of gorse. Local authorities in 76 counties are contributing around $2,000 each in a co­operative five-year opera­tion to bring not just gorse under control, but also ragwort, Californian and nodding thistle, broom and alligator weed.

Gorse spider mites are a brick-red colour and are no bigger than a grain of sand. They live in colonies of several thousand inside webs which they spin on to gorse branches, and feed exclusively on gorse, piercing the plant’s cells and extracting the chloro­phyll.

The mites live for about a month and adults lay one to two eggs per day.

Although they are capable of killing off whole plants, their expected effect on the country’s gorse problem will be more of a thinning action, rather than total eradication. At present gorse covers almost 10,000 square kilometres of land, and 1985 estimates of annual control costs ran to $17 million. If the gorse mite can establish itself successfully, it should be able to keep gorse at low to medium levels and stop it from taking over produc­tive pasture and forestry land.

Says Judy Grindell, extension manager for the weeds programme, “Gorse will still be a common plant, but hopefully it will be unable to form the huge, impenetrable blocks of ‘old man gorse’ which have been such a problem to farmers and foresters.”

The gorse mite pro­gramme suffered a tempo­rary setback late last year, just after the first shipment of mites was brought in from England. While the mites were being held in quarantine, it was noticed that some were infected with a fungal disease. The mites were successfully treated with a fungicide, but with an unexpected outcome: all the offspring were male.

A fresh shipment of mites was required, and now a mass-rearing pro­gramme is in full swing at DSIR Lincoln. The first releases were made in South Otago in early March, and by the end of autumn the mites will have been released as far north as Mangonui.

Each release consists of five colonies of mites, each containing 50 – 100 adults and 200 – 300 offspring. Each colony is housed on a single gorse shoot which is tied to the host bush. Left to themselves, the mites spread by wind or spun threads, but to speed up the process DSIR staff and noxious weeds officers will “harvest” new colonies and relocate them to suitable sites.

The DSIR does not expect biological control of gorse to replace conven­tional herbicidal control, but there are advantages.

“Bio-control is environ­mentally more acceptable than the wide-scale use of herbicides,” says Grindell. It is also cheaper and self-sustaining.”

One previous attempt has been made in New Zealand to curb the spread of gorse with an intro­duced insect. In 1931, the gorse seed weevil began ef­ficiently destroying gorse seeds each spring, and can still be found in gorse pods. Unfortunately, New Zealand gorse, unlike the UK variety, blossoms in autumn as well as spring, and the weevil is not active during the autumn bloom.

The DSIR is presently looking at three other gorse control agents: the shoot moth, pod moth and gorse thrip.

Between all five, gorse in New Zealand shouldn’t stand a chance.

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