96 SOME SOUTH INDIAN INSECTS, ETC. [CHAP. XI. 



only pays to use insecticides when damage is imminent or actually 



being done. The conditions of agricultural holdings in the Plains 

 of India are so special thai insecticidal methods for the control of 

 crop-pests are hardly within the means or reach of the ordinary 

 cultivator, but even at present there is large scope for their employ- 

 ment In Zamindars, Agricultural Stations and Associations, and 

 by all owners of garden and orchard property, whilst in the Hill 

 Districts the more permanent plantations of tea, coffee, etc.. have 

 mostly found by experience the economic utility of insecticides. 



Insecticides are usually applied in the form of powders, liquids 

 or gases, but only the first two of these will be considered here, as 

 gaseous insecticides are not applicable to crops under ordinary 

 conditions in India and will therefore be dealt with in the next 

 under the heading of "Special Methods." Broadly speak- 

 ing, insecticidal substances may be divided into Stomach Poisons 

 mtacl Poisons, and it is with respect to their correct usage 

 that some knowledge of the insects concerned is requisite. Stomach 

 are applied to the plant which is attacked in order thai 

 they may be eaten by the insect and kill it when taken into the 

 system through the alimentary tract ; they can therefore only be 

 used for insects which eat solid food, such as leaves or stalks of 

 plants, or in other words insects which have biting mouthparts. 

 Insects with sucking mouthparts obtain their food by thrusting their 

 mouthparts into the plant and sucking its juices; therefore they 

 cannot be poisoned by substances applied only to the outside 

 surfaces of the plant and it is impracticable to inject poisons into 

 the plant which will kill the insects without harming the plant. 

 For sucking insects we therefore use Contact Poisons which kill 

 them either by blocking their spiracles and so asphyxiating them 

 or by direct poisonous action or by evolving poisonous gases; so 

 far, however, as the cultivator is concerned, the insect is killed by 

 Hon. Contact insecticides can of course be Used to kill 

 insects with biting mouthparts, but it is more economical to use 

 Stomach Poisons for these. To siun up, Stomach Poisons are 

 applied to the plant in order that they maybe eaten by those 

 insects which have biting mouthparts ; Contact Poisons are applied 

 to the insects themselves, and only incidentally to the plant, in 

 order to suffocate insects with sucking mouthparts. 



Before considering the insecticides themselves, a few general 

 remarks on their application will not be out of place here. In 

 the first place it must be noted that no exact scale of dosage 

 can be laid down as universally applicable, although for ordinary 

 practical work an approximately correct dosage may be used. 

 Too large a quantitj ol an insecticide may kill the plant as well 



