CROP PESTS

Wairarapa Times-Age, 7 March 1940, Page 9

CROP PESTS

DIAMOND BACK MOTH & WHITE | BUTTERFLY J POSITION IN THE WAIRARAPA. INDICATIONS OF RELATIVE. DAMAGE. Observations over various parts of the Wairarapa valley have revealed that 80 per cent or 90 per cent of the damage suffered by crops of rape, turnips. swedes, etc., this year is due to the diamond back moth, states Mr N. Lamont, Instructor in Agriculture. Masterion. Being small, inconspicuous in colour, and because of their habit of flying by night, and resting by day under leaves, the number of moths present in crops is frequently not appreciated. On walking through crops, however, one will find the diamond back moths fluttering up in thousands within a radius of a yard of so of the disturbance. Many white butterflies can, of course, be seen —particularly where they have been attracted to an area for some reason such as shelter or the presence of flowering weeds. It can be said without fear of real contradiction however, that in all crops seen, one would count the diamond back moths in thousands, while counting white butterflies in scores. A much more reliable indication of the relative damage actually caused by the two pests can be obtained in other ways. First, there is the identification of the caterpillars. A small white butterfly caterpillar can be distinguished from a diamond back caterpillar, of similar size by the latter’s habit of suspending itself by a thread from a leaf when .it is touched. The white butterfly caterpillar, of course., rapidly becomes very much larger than the diamond back caterpillar, and can be identified by its soft green colour with three fine light brown lines, one running down the back, and one down each side. Another measure of their relative importance is given by the typical kind of damage done by each. Where the leaf of. a plant is liberally doted with small holes, many still retaining the leaf cuticle and appearing like little windows, one can be assured that the diamond back is responsible. The white butterfly caterpillar on the other hand will cut out quite large pieces from the leaves —even cutting through the leaf ribs. The small holes made by the diamond back may join up and leave a large cavity but. if the damage is reasonably fresh, a fairly reliable estimate of the relative damage caused by these two pests can be obtained in this way. It is the usual habit of the white butterfly caterpillar to climb up a fence post to pupate. Two or three years ago it was possible to find perhaps half a dozen pupae (or chrysalids) on every post, but this year considerable time has been spent along fence lines before even one or tw6 have been found. The object of seeking out these chrysalids is, of course, that it is in this stage that the white butterfly is attacked by the parasite, and although only a few have been found, they have been sufficient to establish that the parasite is still being thoroughly effective in further reducing the white butterfly population. It is perhaps poor consolation for a farmer, whose crop is practically ruined, to know that it is not the white butterfly that is most responsible. There must be some satisfaction to be derived. however, from the fact that the depredations of this pest have been so successfully checked by the parasite liberated for the purpose and we can but hope before long, similar success will attend the efforts to find a control for the diamond back moth.