146 SCIENCE AND FRUIT GROWING 



Hellebore, consisting of the root -stock and roots of Veratrum 

 album (Linn.) and V. viride (Ait.), is effective in destroying cater- 

 pillars on gooseberries, etc. : it acts chiefly by poisoning their 

 food, but is. probably also injurious to them by contact with 

 their bodies. As it is less poisonous than arsenical compounds, 

 and loses its properties rapidly on exposure to air, it may be used 

 with safety within a short time of the ripening of the fruit. 



Pyrethrum, obtained from the ground heads and stems of 

 Chrysanthemum (Pyrethrum) roseum (Web. et Mohr), and C. 

 Marschallii (Aschers.), and C. (Pyrethrum) cinerariafolium 

 ((Trav.) Bocc.), contains an oil which appears to act on insects 

 only by contact. Besides being used in the dry form, it is used 

 as a decoction, and also as a fumigant. 



Liquids 



A. Insecticides. For a determination of the nature of the 

 insecticide which is likely to be effective in any particular case, 

 a knowledge of the life-history of. the insect is necessary, to 

 indicate at what period it is most vulnerable, and by what means 

 it may best be combated. Insects which chew the leaves are 

 best dealt with by poisoning their food, whilst others which pierce 

 the leaves and bark, extracting the juices from below the surface, 

 are best attacked by the application of some corrosive substance 

 to their bodies (p. 14). The most effective procedure, where 

 practicable, is to destroy the eggs of the insect : for this a cor- 

 rosive substance is required, and such can only be applied to 

 the plants while these are in the dormant condition. For 

 facilitating insect eggs being reached by a fluid, as well as for 

 reducing the number of crevices in which insects can deposit 

 their eggs, the application of a detergent wash in the winter, 

 to remove dead bark, moss, lichen, etc., from the trees, is an 

 important adjunct to an insecticide (p. 14). 



Many of the most popular insecticides and fungicides have 

 come into use by accident, and, although practical experiments 

 have introduced considerable improvements in them, much 

 uncertainty still prevails as to the method of their action. 

 It is remarkable that in very few cases has science provided 

 satisfactory substitutes for these chance selections. 



The chief classes of insecticides are (i) vegetable poisons 

 consisting of alkaloids, (2) soft soap, with or without excess of 

 alkali, (3) sulphur, and sulphides, (4) phenol, (5) paraffin oil, 

 (6) arsenical compounds, (7) lime. 



(i) Of the vegetable poisons, hellebore and pyrethrum have 



