PARASITES ARE EFFECTIVE

Timaru Herald, Volume CXLIV, Issue 20966, 19 February 1938, Page 10

PARASITES ARE EFFECTIVE

WHITE BUTTERFLY MENACE EXPERT INTERVIEWED “Given dry weather conditions, and once the parasite has been established in the district. South Canterbury will have nothing to fear from the white butterfly. If the conditions are right the parasite will go through them like a plague.” This statement was made to a reporter of “The Timaru Herald” yesterday by Mr J. Muggeridge. Associate Director of the Entomology Division of the Plant Research Bureau at the Cawthron Institute, who, accompanied by Mr C. C. Leitch, of the Department of Agriculture, Timaru. has just concluded a tour of the district in which he has closely studied the depredations of the butterfly and the work of the parasite (Pteromalus puparum). The butterfly, said Mr Muggeridge, confined its attentions to cruciferous crops (turnips, swedes, rape, chou moullier and the like). From investigations he has made recently in the Ashburton district he found that the butterfly was prevalent, but it was not felt that the district had much to fear as the parasite was very much in evidence. It was possible, in fact, that the butterfly had this year reached its peak in that area as the parasite would overtake it and reduce it to uneconomic proportions, providing always that weather of the normal dry autumn type was experienced. Mr Muggeridge explained that in the Clandeboye-Temuka district the butterfly was thick, and while he had observed that the parasite was present it did not seem that It was as well established as was the case in the Ashburton district. Another point was that the weather conditions in the area under review were such that there was almost a luscious growth of grass, particularly bordering the crops, and the parasite passed its pupal stage amongst that growth. The importance of that was demonstrated by the fact that parasitism was found to drop very considerably under those wet conditions (down to 30 and 40 per cent.) whereas under dry’ conditions the parasitism rose to about 98 per cent. A compensating feature, however, was that under wet conditions the crops appeared to be able to withstand the attacks of the butterfly while under dry conditions the attacks would be ruinous. From Timaru. as far as Palmerston south, the butterfly was well established and there was very little appearance of parasitism. On the other hand the butterfly was not sufficiently prevalent to be considered a menace to the crops. It was possible that the pest might increase to a certain extent for a while. Further south from Palmerston, the butterfly was comparatively absent and at the present stage there was nothing to fear in that locality. Population Density The cycle of the butterfly and the parasite in a normal season, continued Mr Muggeridge, was such that the population density of the butterfly rose for a period and to the casual observer had all the appearance of a serious outbreak. The corresponding rise of the population density of the parasite, while not readily observable, was also taking place and the apparent outbreak was thus checked. “I have every confidence in the ability of the parasite,” said Mr Muggeridge. “In my opinion were it not for the work of the parasite the growing of cruciferous crops would be impracticable.’’ He recalled that in Hawke's Bay the campaign against the butterfly proved very effective, but fluctations in the population density did occur in the earlier parts of the season. The parasite, however, was able to overtake these increases and an apparent outbreak faded out.

In Hawke’s Bay in the first year, Mr Muggeridge explained. 500 of the pupae collected showed parasitism to the extent of 48 per cent., but in the following season the percentage from 5000 rose to 90 per cent. In Manawatu where no parasites had been liberated, not one of the 20.000 pupae collected was parasitised. In the following year from 20.000 pupae collected 90 to 95 per cent, were parasitised. At the Cawthron Institute, said Mr Muggeridge. the butterfly chrysalids were collected in the previous autumn and held in cold storage from which they were taken during the winter months and exposed to parasite attack under controlled conditions. The chrystalids, after parasitism, were again transferred to cold storage and at a suitable time in the summer months they were removed from storage and placed in emergence cans. From these the adult parasites subsequently emerged, and were distributed to the various areas packed in glass tubes. Supplies for South Canterbury Mr Leitch explained that as supplies were available it was the intention of the Department to distribute the parasites throughout South Canterbury. They would first be liberated in crops where the butterfly was most prevalent and it w’as hoped to cover the whole of the district. All the requests for parasites, he said, should be made through the local office of the Department.