White Butterfly Pest

Dominion, Volume 28, Issue 58, 1 December 1934, Page 9

White Butterfly Pest

Sir, —Until recently 1 the white butterfly was»not known to attack plants except those of the cabbage family. Experiments carried out many years ago showed that the butterfly larvae could be induced to consume other plants if they were sprayed with mustard oil, the substance responsible for the positive chemotropic influence in their normal food plants. So essential was it that where it was absent the larvae starved rather than eat plants which were not treated or which did not normally contain it.

It is interesting as well as being extremely important to note that the butterfly is changing its habits. It has recently been observed depositing eggs on silver beet, and the full-grown' caterpillars have been found eating it. Silver beet, red beet, and that important farm crop, mangolds, have all evolved from a common wild plant resembling silver beet. The relationship is therefore very close, and any fears one may have for the future of the mangold crop would be amply justified. This crop supplanted turnips and rape on many farms owing to its supposed immunity to attack by the butterfly, but now this supposed immunity cannot be relied upon. - This change of habit is quite in accordance with changes of habit which take place when a pest gains a footing in a new environment. Such a change may be brought about by various conditions, tlie most important of which is a sudden check in food supply. While on this subject of change of habit, it may be pointed out that those who advocate the importation of parasites to. deal with noxious weeds always claim that experiments are conducted to ensure that the parasite will not attack plants of economic importance. This is absolute rubbish. No one can be sure, by the results obtained under artificial conditions, what any parasite is going to do under natural conditions. The white butterfly is a ease in. point, and any attempt to import parasites to deal with noxious weeds, such as blackberry, should be strenuously opposed. Parasites to deal with insect pests such as white butterfly come under an entirely different heading. To return to the question of white butterfly, there appeared recently a note in the Press concerning the use of salt to combat it. This is not a new remedy. I have frequently used salt, and it is included in some of the older detergent mixtures used by orehardists during the dormant period. The salt .is particularly useful in destroying insect eggs. The effect of salt is Twofold. It is partly corrosive in action, and. as is the case with all deliquescent chlorides, has a great affinity for water. It is this latter action that is so valuable in destroying insect eggs. It depletes the eggs of their liquid contents. It would aet this way with caterpillars and pupae too, especially, in conjunction with its corrosive action. Common salt is frequently. used. as a spray for carnations, especially in. dealing with thrips, which produce malformation of. flowers. There is no reason why common, salt should not be used more frequently for spraying purposes, except, of course, on plants with very tender foliage.—l am, etc., LESLIE A. HAY. November 30.