INSECTICIDES AND THEIR USES 37 



an under-spraying of the leaves in order to reach the pests. This 

 is frequently of difficult accomplishment owing to the thick 

 growth of the plants after they have got well started, and the 

 fact that many leaves overlap others. An example of the dif- 

 ficulty experienced in spraying for aphides is afforded in the 

 pea aphis, particularly where peas are grown broadcast, which 

 does not permit the driving of a machine through the fields. 

 The leaves interlace and intertwine in such a manner that the 

 insects cannot be reached. The same is the case with melons 

 after they have attained any growth. There is also danger of 

 injury to the vines by the wagon wheels and the horse in going 

 through the fields. 



For success with this remedy it is in many cases absolutely 

 necessary that the emulsion should be applied so that it will 

 actually come in contact with or strike the insects against which 

 it is directed. 



Kcroscnc-milk emulsion is sometimes used, but it is hardly 

 as satisfactory as kerosene-soap emulsion, since the soap in the 

 latter has also considerable killing and repelling power. 



Carbolic-acid emulsion. — For some purposes it is desirable to 

 add to kerosene emulsion a quantity of carbolic acid, e. g., for 

 the treatment of various root-infesting insects, such as root- 

 maggots affecting seed-corn, onion, and cabbage. This emulsion 

 is prepared at the rate of i pound of soap, I gallon of water, 

 and Yz gallon of crude carbolic acid, and is diluted with from 

 35 to 50 parts of water. It has been found quite effective 

 against root-maggots, the plants showing no injury due to the 

 insecticide. It should be applied a day or two after the plants 

 are up, or, in case of crops that are transplanted, the day; after 

 they are set in the field, and should be repeated every ;week or 

 ten days until about the latter half of May. Carbolic acid is 

 at once a repellent and a contact and stomach poison. It should 

 be handled with care as- it is corrosive! 



Corrosive sublimate (Bichlorid of mercury) is prepared by 



