SCIENCE IN FARMING

New Zealand Herald, Volume LXXII, Issue 22195, 23 August 1935, Page 17

SCIENCE IN FARMING

ENTOMOLOGISTS DISAGREE DR. MILLER'S CLAIMS VALUABLE RESULTS ACHIEVED Mr. L. Morrison, entomologist to Canterbury University College was recently reported in the Dominion's newspapers attacking the biological control of insect pests and noxious weeds. Dr. D. Miller, chief entomologist and assistant director at the Cawthron Institute, Nelson, in the following article, replies to this attack and instances many notable successes which have been achieved through biological control. "The control of insect pests and weeds," said Dr. Miller, " that have become well established aims at their reduction —not eradication, as mentioned in the report under discussion. The ideal control is to render conditions as permanently untenable as possible for the insects and weeds in question, and is based on the use of natural enemies and sound agricultural management. The two must go hand in hand as a rule. "Speaking generally, natural enemies are but auxiliaries in the war against pests, but powerful auxiliaries, withal, tho use of which cannot be neglected, especially under such an environment as one finds in New Zealand where there are rare opportunities for some of these auxiliaries becoming primary controlling factors even without the aid of agricultural management. This becomes especially the case in regard to insect pests of the forests or pests of extensive areas of grass-lands. It applies equally to noxious weeds which in many parts are beyond tho scope of agricultural management. "Wearewell awareof our failures; but these failures are not a condemnation of the principles of biological control. One has but to review the outstanding successes, the resultant benefits from which are beyond assessment. Notable Successes " Apart from the successful biological control of the woolly aphis and oak scale mentioned by Mr. Morrison there are other successes in New Zealand. The pear-midge, for example, which, after it secured a hold, put a period to tho establishment of young pear orchards in the vicinity of Auckland and elsewhere. After tho parasite was introduced and established that pest was overcome and to-day as high as 83 per cent control can be effected by tho parasite, as shown by recent researches. Again take tho cabbage white-butterfly, tho spread of which constituted one of tho most spectacular entomological events in the history of this Dominion. Though the parasitic control of this pest is yet in its infancy, Mr .T. Muggeridgc, Government Entomologist, Palmerston North, has succeeded during the first year of his fight against the insect to check it in Hawke's Bay by as much as 02 per cent and in tho Manawatu 9S per cent. Prior to this, it should bo remembered, the butterfly so abundant that tho radiators of motorcars became blocked with it. Ono has confidence that the parasite for this pest, as soon as Mr. Muggeridgo has the supplies to liberate, will give a, control in all parts of the North Island. In regard to forest trees, let us take the example of the gum-tree weevil This beetle and its grub is established throughout the country and in many places is responsible for the dwarfing of eucalypts. An egg parasite discovered in Australia was secured and liberated in New Zealand. Though this parasite, apparently for climatic reasons, cannot establish much further south than Palmerston North, it has proved so effective in the Auckland Province that the weevil is now there a rare insect. "We have considered some examples of -rfhat has been already accomplished in the field. Let us look at what is being attempted with great promise of success. There is the parasite against the mealy-bugs, which was secured from California, where it has given an absolute control of its hosts. Though too earlv to assert that it has done the same in New Zealand there is every reason to believe that it will do so. It readily attacks and destroys our mealy-bugs, develops rapidly, and already considerable liberations have been made. Again, there is the parasite of the horn-tail borer of pine trees. Against all prophecies of failure we have succeeded in introducing this imI portant parasite and have reared it successfully for tho past three years. Weed Control "In regard to weeds, it is assorted in Mr. Morrison's statement that wherever biological methods of control have been attempted against weeds they have been quite unsuccessful. Such an assertion cannot pass unnoticed when one remembers the classical example of prickly pear control in Australia, which must rank as one of tho greatest achievements in entomological science. At the time when control by insects was commenced fully 6,000,000 acres of land were invaded bv impenetrable thickets of pear. Though tho progress of control was at first slow the attempt was not abandoned. At last, in 1925, the caterpillars of tho cactoblastis moth were established, resulting in a collapse of the weed over extensive areas, until by 1932 (within a period of seven years) most of the pear throughout Queensland had been levelled to the ground, arid tho land opened for settlement. "There is also tho case of the lantana weed in Hawaii, tho seeding of which has been successfully controlled by means of insects introduced for tho purpose. "As to the biological control of weeds in New Zealand no ono imagines that tho weeds of New Zealand are at present being kept in control by introduced insects, but thero is every indication that certain weeds will be so controlled. Our researches have been centred on tho insect control of blackberry, ragwort, gorse and piri-piri, and of these only one, that of blackberry, has been definitely abandoned. Promising Prospects "The cinnabar moth has certainly been a disappointment in New Zealand in the control of ragwort, mainly becauso it produces only one brood of caterpillars each year, and these damage the ragwort for only a very brief period in tho spring. However, tho seed-fly, upon which we are now concentrating, and hope soon to lilierate, offers much better opportunities of success, since its maggots, we find, totally destroy the seeds in every infested head. Further, researches in Great Britain show that this fly, even where subjected to its parasites, is capable of destroying fully 70 per cent of ragwort seeds. "Of tho four weeds already mentioned the greatest advanco has been mado against gorse; the gorse seed weevil is n6w being gradually liberated and last season, under field conditions, where established, it destroyed over 5)0 per cent of tho gorse seeds. In tho case of piri-piri, our researches show that the Chilean saw-fly readily adapts itself to and attacks tho New Zealand weed, and there is every indication that it will give as good, if not better, results in New Zealand us it does in Chile, where it destroys up to 50 per cent of the piri-piri burrs. It is to secure sufficient supplies of this sawfly for liberation that I am proceeding to South America."