Feature

Biological Weed Control

Increasing concern about sprays has meant a boom in biological control research.

By Lynley Hayes

As more and more concern is expressed about the spread of noxious plants throughout New Zealand, researchers are turning to biological methods to provide a safe, effective approach to control.

Introduced weeds often grow extremely well in New Zealand because the insects and diseases which help keep them in check overseas get left at home. The biological control approach aims to restore the natural balance between a weed and the environment by introducing insects and diseases which attack the weed in its native country.

Biological control agents are selected very carefully. A study is carried out overseas to identify all the organisms found attacking a weed. From this list some species will be chosen for further testing. Any insect imported for biological control must be host specific, that is, it will not attack other plants. Stringent testing is carried out on close relatives of the weed, native plants and economically important plants.

To date, more than 600 insects have been introduced for weed control worldwide and the safety procedures, when properly conducted, have never yet failed.

Once permission has been granted by MAF Qual, the insects are held in quarantine, where they remain until they have received a disease and parasite clearance. This step is necessary to ensure that no unwanted companion species are imported with the control agents. After release from quarantine, large numbers are reared and liberated as widely as possible to ensure the best chance of survival and establishment.

Setting A Balance

There is no guarantee that an insect will establish in New Zealand, or that it will control the weed. Experience overseas has shown that several control agents are usually required to have a significant impact on a weed.

Biological control does not aim to eliminate weeds, because the agents always need some surviving plants to complete their life cycle on. Rather, a successful biological control programme reduces the vigour and abundance of a weed to a level that we can live with, or that allows us to eliminate the weed effectively and economically by other means.

The approach is initially expensive and slow but, once in place, the on-going costs are nil. Another advantage is that biological control agents damage only the weed -- a result difficult to achieve by mechanical or chemical control.

Gorse Control

Gorse is considered to be New Zealand's most common and costly weed. It was estimated in 1986 that at least $18 million was spent each year controlling the spread of gorse in farming and forestry.

The first attempt at biological control of gorse was made in 1931 with the introduction of the gorse seed weevil, Apion ulicis. Female weevils lay their eggs in green gorse seed pods in the spring. Larvae hatching from the eggs feed on and destroy developing seeds. Studies have shown that up to 96% of seed produced during spring and early summer can be destroyed in this manner.

The gorse seed weevil has successfully established itself, and is one of New Zealand's commonest insects. However, the weevil is unable to control gorse on its own, since in New Zealand seed pods are also formed in the autumn and winter, when the weevil is not reproductively active. Efforts are being made to find species which will attack the gorse seed which escapes predation by the gorse seed weevil.

More recently, the gorse spider mite, Tetranychus lintearius, has been imported and released. The tiny red mites live in colonies and spin sheets of webbing over the gorse. The mites' sucking mouth parts pierce the gorse foliage and extract the cell contents.

Last year, two new insects were released to attack gorse. Tiny, black, cigar-shaped gorse thrips, Sericothrips staphylinus, which also feed by sucking on gorse foliage, were released at a few sites. This summer, large numbers of thrips will be released throughout the country.

First releases of the soft shoot moth, Agonopterix ulicetella, were also made. As the name suggests, caterpillars of this moth attack soft new growth produced in the spring. A hard shoot moth, Scythris grandipennis, is still being tested in quarantine at Lincoln. Hard shoot moth caterpillars feed on old gorse foliage throughout the winter.

Nodding Thistle Threat

Nodding thistle is an aggressive weed which has become a serious agricultural problem in the last 40 years, infesting thousands of hectares of farmland throughout New Zealand.

The receptacle weevil was imported from Germany in 1973, and is now found in most nodding thistle infestations. Studies have shown that receptacle weevil populations destroy more than 80% of seed produced by early flowers.

To complement the receptacle weevil, a gall fly is being released. Female gall flies lay eggs on mid to late season flowers. The larvae destroy seed produced later in the flowering season than the receptacle weevil. A third insect, the crown weevil, was introduced to New Zealand during the 1980s.

At Lincoln, biological control programmes for a number of other weed species are underway. Insects have been released, or will be released shortly ,for ragwort, broom, Californian thistle, alligator weed and St John's wort. Suitable biological control agents are currently being sought for old man's beard, hieracium, heather and sweet briar.

Lynley Hayes works for the biological control of weeds section, DSIR Plant Protection, Lincoln.