WEED ERADICATION

Wairarapa Times-Age, 9 February 1940, Page 12

WEED ERADICATION

CONTROL BV INSECTS RESEARCH IN NEW ZEALAND. RECENT RAGWORT EXPERIMENTS The control of weeds depends mainly on pasture management, but there are circumstances where this method is not altogether practicable, so that additional methods, such as the use of insects, have been sought (states a pamphlet on the control, of weeds (blackberry, ragwort and bidi-bidi) issued by the Cawthron Institute. Nelson). In the expectation of results from insects, however, there is a tendency for optimism to get the better of reason in the case of many people whose livelihood is threatened by the invasion of weeds. This outlook is well understood by any one who is alive to the seriousness of the situation and of the losses that many farmers sustain. v But it cannot be too often stressed-— I in the face of such an outstanding 1 success as has been attained in the control of prickly-pear in Australia — that in most cases insects must be looked upon merely as auxiliaries to farm management in weed control. Again, on the liberation of an insect, immediate beneficial results must not be expected; control may be rapid, it may be slow, or it may be not at all. There are so many factors operating in the field concernihg which we know nothing, all or any one of which may be detrimental or beneficial to the establishment and elTectiveness of an insect once it is liberated. On the other hand, the information derived from our researches in Now Zealand and from observations elsewhere tells us of the possibilities of successful control by insects. In consequence, the experiment is worthy of trial in view of its small cost in comparison with what is at stake and the sums annually spent upon the suppression of weeds in other ways. I NO BLACKBERRY CONTROL. With regard to blackberry, almost every mail lately has brought specimens of the common bronze beetle for identification. It appears that this insect is particularly active this season in its depredations upon the foliage of blackberry, raspberry and ornamental and fruit trees. The beetle is a small, shining brown hard-bodied insect that has a characteristic habit of leaping when disturbed on the plant. Its method of attack is to eat holes from the leaf surface, so that in severe cases the foliage appears to have been punctured by a charge of shot. The hopes held by correspondents that this is a remedy for the blackberry problem have, unfortunately, no foundation. The beetle may at times visibly check the weed, but its period of activity is limited to a few weeks each year, and it is not always constant in the severity of its attack. Further, it could hardlj' be encouraged because of its depredations upon plants Of agricultural value. The Institute's attempts to control blackberry by means of imported insects had to be given up, as there was too great a danger of related plants of economic value being attacked. Practically the whole world was combed for insects restricted to blackberry, but the only one that was considered worth trying had to be finally abandoned. The, reason for this was mainly due to the fact that although tile insect's grub confined itself to boring in the stems of blackberry, the adult insect showed a decided trend as a defoliator of apple-trees. ATTACKS ON RAGWORT. This season, a great deal of attention has been drawn to the attacks upon, ragwort of a small leaf-mining ■ fly, aiid it would appear that this insect is causing a considerable amount of havoc with the weed. The grubs ' of the fly are commonly found tunnelling in tlie leaves of ragwort, cineraria, and sow-thistle, but during the 1 present season the insect has reached 1 epidemic force. Normally, it is of ] little importance and is heavily para- • sitised by a natural enemy. Ragwort is also frequently attacked . in New Zealand by stem-boring grubs i of another species of fly as well as by the caterpillars of a moth, but like the leafminer, it is only intermittently ] that they are able to reach a sufficient force to cause a noticeable check to < the weed. ,

In the control of ragwort our researches at the Cawthron Institute have been concerned with two insects, the cinnabar-moth and the ragwortseed fly. The caterpillars of the moth are defoliators, but although we succeeded in establishing the insect it was unable to hold its own in most parts of the country. This was due, to a great extent, to the attacks of birds and insect parasites upon the caterpillars; the insect parasites were those that commonly attack the caterpillars of the native woolly-bear which is a relative of the cinnabar-moth. LIMITED INFLUENCE.

In any case, the cinnabar-moth would have had a limited influence upon the weed, as its caterpillars are active for only a tew weeks before the close of each year. It is of interest to note, however, that Mr Fitzgerald, engineer to the Matamata County Council, has reported the establishment of the cinnabar-moth in one locality on the Waikato River during the last two or three years.

In the case of the ragwort-seed fly. work is being carried on at the present time with this insect, and we are making satisfactory progress. Although the prospects arc hopeful, it must be stressed that we are not yet in a position to state that the- insect will lie a success. It must always be borne in mind that, even if successful, this fly in the maggot stage attacks only the seeds of ragwort and must not bo depended upon to clear up the weed on land accessible to farm management. The most that can be expected of lhe fly is that it will chock the seeding and spread ol the weed from neglected localities. We are not yet ready to make general liberations of the seed-fly, but when our Held observations show that the insect is of value under New Zealand Held conditions wo shall give consideration to an extensive campaign of establishment. CONTROL OF BIDI-BIDI. Firipiri (or bidi-bidi) is another of our major weed problems. The only native insect'at tacking it Io any marked extenl is a species of meal-bug that inlet.ts the roots, but, like the bronze beetle of blackberry, or the leaf-miner and stemborers of ragwort, it is only occasionally that this mealy-bug markedly checks the weed. Our main activity in control of piri-l piri is by means of the Chilean sawky. upon which we have been working for some time. Field liberations have already been made, but it is too

soon to expect anything definite. On selected areas (in Taranaki and Poverty Bay) the insect is being concentrated in order to build up a suflicient population under field conditions from which we shall be in a position to decide whether or not general establishment should be undertaken. For the time being no supplies can be distributed to farmers.

Under experimental conditions in New Zealand and from observations made in Chile this saw-fly has considerable powers of control, but when the first liberations of the insect wore made in the Dominion circumstances demanded that the supplies bo freed in small numbers instead of being concentrated. This has led interested farmers to bo rather disappointed, in that conspicuous results tiro not yet discernible.

It should bo remembered, however, that although the sawfly is a slow breeder when compared with other insects, the piripiri is virtually a fixed unit, and that the insect requires time to breed up its population to a point where its influence upon the weed will be noticeable. The available data encourage one in the opinion that there are considerable possibilities of the saw-fly reaching Illis state, but progress will bo slow. The work of field establishment cannot be hurried beyond importing, rearing and liberating the insect in countless numbers. Apart from this, the matter lies with the insect itself and with its environment