Seeking Reason for White Butterfly Increase

Manawatu Times, Volume 63, Issue 45, 23 February 1938, Page 9

Seeking Reason for White Butterfly Increase

THEORY OP EBB AND PLOW WITH PARASITE WELLINGTON, Feb. 22. Farming and gardening have their problems, even when conditions are apparently quite favourable to growth, for they are equally good for some kinds of pests, the extent of which varies according to the character of th* season. This year, reports have been received by the Plant Research Department of an unusual prevalence in many parts of the country of the white butterfly. It has caused much alarm to farmers in Canterbury and the Manawatu, and there are other districts which have called for special attention. The experts who assist producers in coping with a great variety of destructive pests state that the explanation of a sudden Tise in numbers of white butterfly is somewhat uncertain, but is most probably due to climatic conditions involving fairly frequent rain interspersed with hot, humid weather, which is more favourable to the pest than to its imported parasite, the pteromalus.

Another interesting generalisation is that the pteromalus, which has been well distributed throughout the Dominion during the last few seasons, does its work so well that it reduces its food supply, and cannot reproduce in sufficient numbers to cope with the follow-, ing season’s breeding of the white butterfly. This means an ebb and flow between the rival insects, and there is certainly no balance in the position at present, possibly because great variations in the seasons upset the position. The Government entymologist, Mr. J. Muggeridge, who was responsible for the introduction of the parasite of the white butterfly, has been travelling through the Dominion investigating the number of complaints of the undue prevalence of the pest, particularly in the turnip-grownig areas of the South Island and portions of the North Island. The division has received a large number of requests for supplies of the parasite, which the entymologist has found already etablished in a number of districts. The demand has been so extraordinary that in a number of cases delay has occurred in supplying the parasite, but it is reassuring to farmers to learn from the experts that the ptero-

malus is fairly common in crops already examined, and that, in all probability, it will overtake the white butterfly before very long, for the pupae of the butterfly contain large numbers of parasite eggs, and multiplication of the parasite should take place on a large scale.

It is pointed out that the numbers of white butterfly, which are so prominent as they fly over crops to feed, often create a false impression of the damage they are doing, for it has been found that a great deal of trouble with turnip crops has not been due to the butterfly, but to the diamond-back moth, which is working fairly actively in a number of farming areas, and is, unfortunately, well distributed through the Dominion. As a measure of control of the white butterfly the imported parasite, which is bred at Nelson, has been proved to be highly effective. The problem, howover, is to forecast the probable demand from farmers, for during the last few seasons there has been very little call for supplies. However, when the call comes, they are sent out in little test tubes containing a hundred or so of the little black insects, which have simply to be liberated in the field where the white butterfly flourishes, when they at once fly around to attack the white butterfly pupae. Another control measure suitable for small gardens is a dusting of the plants with derris powder, which, of course, is not economic or practicable on a large scale. In addition to supplying farmers with the white butterfly parasites, the Plant Research Division has been utilising field officers of the Department of Agriculture in their distribution.