1T.—34

Field-survey work conducted during the present season has not been completed, consequently a proper picture of the results cannot be presented at present. We have found, however, that A. cerophaga, which was liberated in the North Island in 1936-37 and which apparently failed to become established there, is now common and appears to be fairly widespread. Again in the South Island this parasite has increased and spread at a considerable rate. We cannot say what the full effects of the presence of Angitia cerophaga will be, but the position now is far more hopeful than when we made the first liberations, and the hyperparasite referred to above, which we felt might reduce the good effects of cerophaga, appears now to have little influence on the position. A. cerophaga is now playing a part in reducing the diamond-back-moth population, but whether the reduction will be sufficiently low to be economically important we do not yet know. Our field-work to date, however, gives everincreasing evidence of the effect of the parasite. In Hawke's Bay during the season, in Nelson, and as far south as Amberlcy the diamond-back moth at the time of making the field surveys was very little in evidence. From Ashburton, however, to south of Timaru the position was different. In the vicinity of Ashburton itself a good deal of variation occurred. Some of the crops were relatively clean, though a few were severely attacked by the moth. Considering the very dry season which occurred there, however, it was surprising that the crops had not suffered far more than was observed. In Hawke's Bay a comparison of the population density from 19th to 26th January, 1941, for a similar period in 1940 and 1939 gave a figure of 4-9, as compared with 51-5 and 15-4 respectively for the previous two years. We cannot say, of course, how much the drop in population was due to climate or other circumstances. Diadromus oollaris. —This parasite attacks its host in the pupal or chrysalid stage. During the past season an effort was made to breed sufficient for liberation in the main crucifer-growing areas in the North Island where no previous liberations have been made. Although the numbers of parasites obtained was below the figure estimated at the beginning of the season, liberations were made in the selected areas, and, in addition, in one area of the South Island. The following are the districts in which liberations have been made, and the numbers liberated : — District. Pupae. Adulta. Dates of Liberation. Auckland .. .. .. 5,500 3,900 25/10/40 to 18/12/40 Levin .. .. .. .. 4,100 6/1/41 to 10/2/41 Ashburton.. .. .. .. 2,400 12/3/41 to 26/3/41 Alexandra .. .. .. .. 650 26/9/40 Totals .. .. 5,500 11,050 26/9/40 to 26/3/41 From both Auckland and Levin, satisfactory recoveries of parasites have been made in the areas of liberation. Again, in the field surveys, Diadromus was found to be common in the Hawke's Bay and parts of the Masterton area. It is well established in Nelson, Marlborough, and has reached as far south as Amberley. Efforts are being made to get it spread as far and wide as possible through artificial rearing and distribution. Comment.—lt is quite likely the results from this work will be far-reaching. It is too early yet to say what the final position will be, but as time goes on it appears more hopeful. It is advisable, and perhaps necessary to introduce other species in order to improve the position. When we are successful in getting both of these parasites thoroughly established throughout New Zealand, and possibly some other closely-allied species, the effect on the population density of diamond-back moth should be conspicuous. (3) Pieris BAPAB (White Butterfly). Reports have come in during the season of the presence of white butterfly in various localities, and in many cases severe damage was alleged. It has not been possible to investigate the position in every case, but when possible an examination was made, and in practically every case damage thought to be due to the white butterfly was due to the presence of the diamond-back moth. There is little doubt that Pteromalus puparum is maintaining good control of the butterfly. There are places, of course, where the butterfly is able to increase to serious proportions, but, fortunately, these are very limited, and as a rule the parasite is able subsequently to overtake its host. It is felt, however, that the introduction of further species of parasites should be undertaken, and in this connection we hope to receive supplies of Apanteles rubecula and Compsilura concinnata for the coming season. In the past small supplies of A. rubecula have been received, but, unfortunately, success in establishing a colony was not realized. Both of these parasites should prove helpful in reducing the butterfly in those localities where the present parasite is less effective. (4) Oxycanus si\* (Subterranean Grass Caterpillar). Field Surveys. —Field surveys in the autumn of 1940 disclosed that there was no general infestation in Canterbury, but that the insect was present in pastures throughout Otago and Southland and was particularly serious in North Otago. Chemical Control.—A comprehensive series of chemical control experiments was laid down in June on two areas near Oamaru. The poisons tested included several arsenical poisons, sodium fluoride, and derris. Three methods of application were tested —viz., poison baits, sprays, and dusts. The final result of the experiments showed that a poison bait containing paris green gave a reduction in insect population of 80 per cent, at a cost (for materials alone) which varied between 6s. and 12s. per acre, depending on the rate of application. This treatment, on the score of low cost and high

* Previously known as Porina.

3—H. 34.

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