NOXIOUS WEEDS

Timaru Herald, Volume CXLII, Issue 20610, 26 December 1936, Page 15

NOXIOUS WEEDS

PARASITE CONTROL THE CAWTHRON INSTITUTE The parasitic control of noxious weeds is a problem of vast importance to New Zealand and is one to which the Cawthron Institute in Nelson devotes much time. The chief entomologist there, Dr D. Miller, recently outlined the work which was being done in this direction and gave some indications as to how the problem was being tackled.

A noxious weed, he said, could be defined as an unwanted plant occupying too much valuable land. His job was a biological one—the use of Insects not only to control other insect pests, but to control weeds. The basis of weed control was sound farm management, but the trouble was that farmers could not afford to apply sound farm management, and all country was not suitable to sound farm management. The use of biological methods could be brought in and used in the fight against weeds. Prickly pear in Australia had been levelled to the ground by the use of insects. Today farms were being established, and he understood the ground was invaluable for the growing of oats. The control of gorse, ragwort, and bidi-bidi were the jobs which were occupying their attention at the institute. They had started out with another weed, blackberry, but had had to give that up. He had spent seven years on that particular research, combing the whole world for likely insects, but it was a difficult matter to find an insect which would confine its attention only to blackberry. They decided upon a flat-headed borer from the South of France. The grub of the insect did extensive damage to the blackberry, but the trouble was that when the beetle developed from that grub it had a weakness for defoliating apple trees. So far they had been unable to locate any other Insect. They had to be very careful indeed that the insects were so specialised that they would confine themselves to that weed and nothing else. Gorse Control In gorse they had a legume. It was a risky business undertaking control of gorse by insects on account of the danger of the parasites attacking other legumes of which there were a number in New Zealand. They had discovered that there was a weevil existing in Great Britain, the grubs of which attacked seeds of gorse. From studies in England and New Zealand (under strict quarantine) they had found that these Weevils were so highly specialised that they could not lay eggs on any other legume. When the pods were soft the female bored a hole in the pod, put In the eggs, and blocked up the hole. When the pods burst the weevils were ejected, not the seeds. The first liberations were made near Alexandra, and in the area where these weevils were liberated they would not find a pod that was not infested with these insects. It would take a long time to get the weevils spread right throughout the country. His only regret was that this weevil did not destroy the flower. Gorse was an important plant from a hedge point of view, and before they started the above work they decided that they could not very well attack the plant. However, North Island farmers wanted to know if they could get an insect which would destroy the plant, and they were Investigating the matter. The Ragwort Menace The institute had been working on ragwort for some years. It liberated the cinnabar moth about six years ago. Unfortunately, the moth produced only one generation of caterpillars a year, which worked only six weeks before Christmas doing damage to the ragwort. A parasite turned its attention to this moth and destroyed it, so that it had doed out after a few years. The institute had been blamed for introducing different insect pests, including

the white butterfly, but as the cinnabar moth was black and the other was white, he did not know how they could be confused. The seeds of the ragwort were windborne, and they had now a fly which was attacking the seeds. Before the flower of the ragwort opened the fly was attracted by the smell. The fly would lay an egg in the disc of each flower, and i maggot was hatched, eating out every seed, and then boring out into the head of the flower, causing the whole flower to wither. Experiments had gone so far and they were so satisfied with it that they had commenced liberation in a small way. The position Was very acute in the

Auckland Province and in Taranaki. Public bodies, certain banks, freezing companies, insurance companies, harbour boards (all those Interested in land investment) had offered to put up finance to secure enough insects, the only stipulation being that they should be liberated in the Auckland Province. The result had been that this year there were 490,000 of these insects on the way to New Zealand. When this supply had been established on a large scale they would draw supplies from the Auckland Province for other places. The Bidi-Bidi Pest The bidi-bidi pest was a very serious matter With sheep farmers, particularly in western parts. Some time ago he Went to a wool store and estimated at that time that the value of the wool in that store was depreciated 33 per cent, through bidi-bidi. It was restricted in its range to New Zealand and one or two areas in Australia, but its main home W'as in Chile. In 1930 he went to Chile and found an insect which he thought would be ideal. He brought it to New Zealand, and carried out experiments which were so satisfactory that he went back last year and got a very large supply. The first liberations were made about three months ago, and the work done was most spectacular. Already they could find the grubs from these insects working in great style. He hoped it would do here what it did in chile.

St. John’s wort was very prevalent in certain districts. It Was not a weed of national importance, but it was of great importance locally. In 1928 the institute commenced working on it, but when they found that it was occupying the attention of the Australians because it was a weed of national importance, they gave up working on it. Later on they would utilise the researches of Australia. In the North Island the variegated thistle was becoming a pest, and he had been asked to look into the matter.