44 APPLIED ENTOMOLOGY 



With stomach poisons, however, the poison is not necessarily eaten 

 by the insect as soon as it falls on the plant, but must or should remain 

 there for some time, as the insects may appear during a period of several 

 days or even weeks. During this time much, probably most, of the 

 poisonous dust would be blown off and the treatment be of little value. 

 In spite of this difficulty, much successful work has been done with dry 

 stomach poisons, and they have many advantages over sprays under 

 certain conditions. 



It has been found that when stomach poisons are mixed with water 

 and sprayed onto plants in the form of very fine droplets, the spray 

 appearing like a fine mist, each droplet soon dries, leaving behind it 

 the poison it contains, adhering to the leaf, where, unless washed off by 

 rain, it will remain a long time. This has led to ths general adoption of 

 spraying, both with stomach poisons and contact insecticides, despite 

 certain difficulties which have developed. 



STOMACH POISONS 



Arsenic is the basis of nearly all the commonly used stomach poisons, 

 for though probably more than 50 materials have been tested, only a 

 few have proved at all satisfactory, and with two or three exceptions, 

 useful only under special conditions, they have all been arsenical com- 

 pounds. It would seem natural under these circumstances to use 

 common white arsenic (As2Os) as the stomach poison, it being, when 

 pure, 100 per cent arsenic (arsenious oxid). But it is found that arsenic 

 dissolves to some extent in water, and that thus dissolved it destroys 

 (" burns") the places on the leaves on which it falls. This result is as 

 bad for the plant as it would be to have the leaves eaten, for the object 

 of spraying is to prevent injury or loss of leaf surface. Because of its 

 solubility in water, therefore, arsenic, as such, is not employed as a spray, 

 but combinations of it with other materials, not, or only very slightly 

 soluble, have been selected for use instead. This produces another diffi- 

 culty. A combination with lead can be obtained for example, which is 

 almost absolutely insoluble in water and therefore entirely safe for use 

 as a spray. But in this material only about one-quarter of it is arsenic, 

 so that an insect, speaking in a general way, would be obliged to eat 

 about four times as much before being poisoned, as would be the case had 

 the material been arsenic instead. By the use of more or less insoluble 

 combinations of arsenic with other substances, then, reduced injury to 

 the foliage can largely be secured, but a larger leaf surface is consumed by 

 the insect before the poisonous dose is obtained. This is a small matter, 

 however, as compared with the protection of all the foliage on a tree from 

 injury by the spray. 



Another difficulty in the use of sprays is the weight of the poison 

 mixed in and carried by the water. It has just been pointed out that 



