NOXIOUS WEEDS

Wanganui Chronicle, Volume LXXXIII, Issue 19846, 21 May 1927, Page 21 (Supplement)

NOXIOUS WEEDS

FIGHTING THE MENACE THE PRESENT OUTLOOK In an endeavour to obtain some means of effectively controlling noxious weeds good work is at present being done by New Zealand scientists who are experimenting with fungi and insects but ie spite of numerous experiments and tests it is apparent that for the time being the grubber and the plough are the only effective weapons. Tho question is a very vital one in the Wanganui district where much land has been rendered useless for farming purposes owing to the spread of gorse, but the prospect of cheaper labour this winter should sec many areas cleaned which previously were considered not to be worth the cost. Experiments With Insects The control of noxious weeds by means of insects is one of the problems which is the concern of the Department of Scientific and Industrial Research. At the Cawthron Institute Dr. R. J. Tillyard and his assistants have been carrying on numerous experiments and his first quarterly reports has just been issued-

The report states that for the purpose of research six new. insectaries have been built and equipped, each being 12ft. square. The insectaries are inspected regularly morning and evening, and any damage noted is at once repaired. In the course of three months’ work the only damage so far found has been the cracking of one or two panes of glass, which was the result of a very violent storm. These were at once repaired. In starvation and other tests the insects are enclosed in muslin or gauze cages placed over or around plants, or portions of plants, inside the insectaries, thus affording additional protection. There are three main methods of testing which arc applied to all insects introduced for the control of noxious weeds. These are: (1) Oviposition tests; (2) starvation tests, and (3) preference tests. In the oviposition tests fertile females of the insect to be tested are placed in muslin cages which enclose the plant or portion of the plant on which oviposition is required, fresh blossoms and honey solution being supplied for food if necessary. In all cases the fertility of the female is ensured as far as possible by actual observation of pairing before the test is put in hand. With some insects pairings can be made in special cages. With others pairing has to take place in the test cage. After the test the females are opened to determine what percentage of possible egg laying has been achieved. The following results may be obtained: (1) Free oviposition on the plant; (2) refusal to oviposit on the plant; but oviposition takes place elsewhere in the cage; (3) complete refusal . to oviposit.

Starvation Tests. Starvation tests are carried out on larvae and also on the adults. These tests consist of enclosing a number of the insects to be tested in a muslin cage or sleeve containing the plant to be tested, and nothing else. The alternatives for the insects are either to feed on the plant or starve to death. If oviposition has been secured on the plant by the oviposition test, the eggs are allowed to hatch in situ' If oviposition has not been secured by this means, then fertile eggs are placed in position on the plant so that the newly-emerged larvae can begin to feed at once on the plant. The following results may be obtained from the series of tests: (1) Free feeding on the plant; (2) slight feeding without any vigorous attack on the plant; (3) feeding followed by indigestion and death; (4) complete refusal to feed, followed by death from starvation. Preference tests are carried out only when a positive result has been obtained in the previous series. Suppose X to be the natural plant food of a given insect, and Y to be the test plant which also gives a positive result. A further batch of insects is placed on Y in a cage, but a plant of X is introduced into the same cage. The preference of the insects is shown by one of the following results: (1) They may continue to feed on Y, taking no notice of X; (2) a proportion of them may desert Y for X; (3) Y may be completely deserted and the insects all feed on X. No Finality Reached. Naturally no finality has been reached as yet in these tests, but much valuable information has already been obtained, and there is every indication ’that in time some of the insects being experimented with will be found to be powerful agents in the extermination of noxious weeds. Amongst the weeds being concentrated upon are the blackberry, the foxglove, gorse, and ragwort.

Very frequently dead gorse-plants may be noted growing among healthy ons, and dead plants of this type are among those most frequently sent to the Government Laboratory for examination and report. Up to the present only one fungus has been isolated from such material. The work was conducted by Mr J. C. Neill, Field Mycologist, who inoculated with the organism isolated (the fungus was not identified) seedling gorse-plants grown in pots, but all experiments were negative. Gorse is mentioned here (it being so frequently forwarded for examination) so that in future those interested will recognise the fact that, despite many attempts, only one fungus, and that not pathogenic, has been isolated from it. A satisfactory explanation for the frequent death of these plants is not forthcoming. Frequently examination has shown borer to be responsible; in other cases the plants have been found infested with cottony cushion scale (Icerya Purchasi), a pest known to kill this and other plants on occasions; but many other specimens have been examined in which neither borer nor scale was present. • Fungus Attacking Blackberry Owing to its economic importance, careful observations during the past

five years have been made of the fungi present on blackberry. The* most outstanding instance of fungous attack upon this host occurred in 1922. In this instance it was reported in the Press that large areas of blackberry (many hundreds of acres) had been killed outright in Auckland Province. Mr L. Paynter and the writer made a survey of the Ngawaro, Oropi, Tauranga, Waihi, To Aroha, and Morrinsvillc distrutSj in which the disease was reported to be prevalent. As is usual on such occasions, the areas when examined were found to be small, ranging in extent from a few yards to about an acre. The organism causing the death of these plants was isolated and found to be Leptosphaeria Coniothyrium, the cause of cane-wilt of raspberry. Inoculation experiments showed that, although this disease had killed numbers of plants in the field, it did but slight damage to the plants artificially infected, at best killing a few canes, but generally merely forming lesions on the latter. The following year tho disease did not appear to do any damage in those areas where it was prevalent the previous season; and from’tflat date it has not been reported as being, of any moment. It is abundant on blackberry throughout New Zealand.