INSECTS AND WEEDS.

Otago Daily Times, Issue 19700, 29 January 1926, Page 10

INSECTS AND WEEDS.

A GRAVE ECONOMIC PROBLEM. IMPORTATION OF USEFUL INSECTS. ADDRESS BY DR TILLYARD. “The Progress of Economic Entomology in Australia and New Zealand” was the subject of the presidential address by Dr R. J. .Tillyard (chief of the Biological Department, Cawthron Institute), at the meeting of the Biology Section of the Science Congress yesterday. The speaker dealt with his subject from two aspects — namely, the control of injurious insects, and the control of noxious weeds by iuTHE CONTROL OF INJURIOUS INSECTS. The outstanding feature of the past few years he said, had been the marked increase in the application and success of the biological method of control of injurious insects by means of their natural enemies, both parasites and predators. Only six years ago, at the First Imperial Entomological Conference in London, there were more speakers opposed to this method than in favour of it, and the great majority were in a position of doubt, not knowing what to think about it. This was inevitable, of course, in so conservative a centre as London, and in view of the backward state of entomology in the Empire as compared with America. The best speech in favour of the biological method was made by a visitor, Dr L. 0. Howard. \New Zealand already owed more to Dr Howard than to any other living entomologist, for it was through him that we had been enabled to introduce a number of most valuable enemies of some of our worst insect pests, and to make marked progress in the work of establishing them permanently within the Dominion. One of the curses of entomology, as, indeed, of all branches of science to a greater or less degree, was facile generalisation from insufficient facts. “My experience with biological control,” said Dr Tillyard, “ leads me to state hare that no generalisations are worth anything in this case, for the simple reason that the amount we know about the habits and psychology of insects is so small compared with the vast amount still unknown, that there is only one safe method of procedure—namely, to test every case on its own merits and as thoroughly as possible.” To illustrate this, the speaker outlined the recent history of the control of woolly aphis in New Zealand and Australia. There were, in America, the original home of the pest, three types of insects concerned in its natural control—viz. (1) certain Syrphid flies, (2) the ladybird Hippodamia convergens, and (3) the chalcid wasp Aphelinus mali. As closely similar Syrphid flies already existed in New Zealand, he did not make any attempt to introduce the American species. In considering the other two, all the evidence was in favour of Hippodamia. This ladybird had a wonderful record in California, and was rightly looked upon as one of the most valuable of known beneficial insects. Large sums of money were spent in rearing, collecting, and distributing it, and every fruitgrower was fully convinced of the benefits it conferred upon him. Aphelinus mali, on the other hand, was not thought much of, but, when it was introduced from two or three localities with widely different climates, and the biological races received from these were crossed it was very .successful in New Zealand. At the present time woolly aphis was well under control in New Zealand owing to the work of this parasite, which had proved of the greatest help to orchardists throughout the Dominion. Hippodamia convergens, on the other hand, though liberated in thousands throughout the Nelson province, had noc since been seen or heard of, and it was probable that this insect’s known habit of seeking the tops of high mountains, on which to hibernate, had nroved its undoing in New Zealand and in every othex country in which it had been tried After dealing extensively with the earwig pest and the means of combating it, Dr Tillyard referred to another serious orchard pest, the codlin moth. Only the most careful and rigid attention to the spraying schedule would ensure to an orchardist freedom of his crop from the injury done by it. In New Zealand there wore known to .bo two distinct broods of this moth, ami the main effort of the orchardist was in the direction of getting his arsenical sprays on to the trees at the right times <to check the newly-hatched larvae of these two broods before they could enter the apples. Recent observations, however, indicated that there was a more continuous succession of moths throughout the summer than was generally supposed. The two broods were, in fact, only the crests or maxima of the wave-like curve on the graph indicating the number of emergences week by week from October to May. Two entomologists in the University of Colorado discovered a few years ago that codlin-moth could be easily trapped by moans of fermented apple-juice placed in jars hung in the upper third of the trees. This experiment had been repeated recently in a Nelson orchard and also by a South Australian grower. Both got closely similar results, which were rather remarkable when one considers that they were carried out on clean orchards where codlin-moth had been controlled by careful spraying for many years. In the Nelson orchard,- 55 small tins were used, hung between the upper limbs of consecutive trees in one section of the orchard. During November and December, when the experiment was being carried on, high winds prevailed' nearly all the time, a most unusual state of affairs for Nelson. A number of tins were blown about violently and their contents spilled, so that the total recorded fell short of the actual catch. The supply of apple-juice gave out on December 23. For the period of seven weeks, the following catch was secured: —Number of codlinmoth 505, of which 218, or 72 per cent., were females; number of leaf roller moths 198; number of other insects 5463, of which 7 per cent, could be classed as beneficial, the rest being either injurious species such as ant worms, etc. of no account. THE CONTROL OF NOXIOUS WEEDS BY INSECTS. Dr Tillyard proceeded to deal with the control of noxious weeds by their insect enemies. Until quite recently this was almost a virgin field, as the only work that had been done along these lines was the control of lantana in the Hawaiian Islands by means of a Lycaenid butterfly and a small fly. These insects attacked the buds and the fruit of the weed, with the result that it was quickly prevented from further spreading; what was lalready there was grubbed out in very thorough fashion, except in inaccessible places, and the menace was now entirely removed. Those who knew the hold that lantana had obtained along the east coast of Australia would agree that this achievement was in itself a notable one. The speaker summarised the course of events which led up to the present system of prickly pear control by moans of its insect and fungus enemies, and explained tent the method adopted by the Australian scientists in testing their insects was that known as the starvation test method. The larvae is segregated into two cages, one with supplies of prickly pear and the other with the plant to be tested, say, pumpkin. As hunger increases they must do one of three things:—(l) They may feed on the new plant, find it digestible, and continue to feed and grow on it; (2) they may eat the new plant, find it indigestible and turn sickly, finally dying of indigestion or starvation ; (3) they may absolutely refuse to touch the new plant, dying straight out from starvation. The general opinion at the present time in Australia was that the problem was in course of solution by the utilisation of the insect enemies alone. A few years back, the then Minister of Lands in Now South Wales, visiting the Prickly Pear Laboratory in the north-west of that State, and seeing the work that was beginning there with the insects, remarked that it was the one bright spot in an otherwise hopeless situation. In 1925 the officer in charge of the principal laboratory in Queensland was able to write as follows: “There is now no doubt at all, in our opinion, that in time, by biological means, we shall surely control the prickly pear pest in Australia.”