A HUMID SUMMER

Evening Star, Issue 22890, 23 February 1938, Page 18

A HUMID SUMMER

FAVOURABLE TO CROP PESTS WHITE BUTTERFLY INCREASES [Special to the ‘ Stab/] WELLINGTON, February 23. 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 vary according to tho character of the season. This year, reports have been received by the plant research division of the Department of Agriculture 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 rise 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 Pteromatufl. ‘

Another interesting generalisation is that the Pteyomalus, 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 following season’s breeding of tho white butterflyi 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 Entomologist, 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-growing 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, whicn the entomologist has found already established 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 Pteromalus is fairly common in crops already examined, and that in all probability it will overtake the white butterfly before very long, for the pup® of the butterfly contains 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, however, is to forecast 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 little black insects, which have simply to be liberated l in the field where white butterfly flourishes, when they at once fly around to attack the white butterfly pup®. Another control measure suitable for small gardens is a dusting of the plants with derris powder, which, of course, is not economic nor practicable on the large scale. _ln addition to supplying farmers with the white butterfly parasite, the plant research division has been utilising field officers of the Department of Agriculture in their distributon.