INSECTICIDES AND THEIR USES 39 



as a means of riddance of these pests. The latter is most ap- 

 plicable to such plants as cabbage, which withstands a high 

 degree of temperature, which would be hurtful to less hardy- 

 plants. A water thermometer is employed, and care is observed 

 that the temperature does not reach far above 150°, the 

 scalding point for most plants. From 125 to 135° is sufficient 

 for the destruction of most insects, but applied still hotter where 

 possible is still more effective. Cabbage will withstand a tem- 

 perature of 180° without special harm. When setting out cab- 

 bage and some other plants they can be freed from "lice" by 

 dipping them into water heated to between 125 and 150°. Hot 

 water cannot be thrown in a spray owing to its cooling too 

 rapidly, and it is necessary if it is to be thoroughly effectual 

 for it to actually strike the insects which it is desired to destroy. 

 Where only a few plants are to be protected and it is possible 

 to reach the "lice" with water applied with a garden hose, 

 syringe or spraying machine their work can be checked. Such 

 as come in direct contact with a stiif spray at an ordinary 

 temperature are unable to survive, while many that are dis- 

 lodged cannot return to the plants as most of them are wingless 

 and unable to crawl any distance, particularly if the ground be 

 dry and hot. 



INSECTICIDES WHICH KILL BY SUFFOCATION 



Under this caption will be included insecticides which do not 

 properly fall under the heading of either stomach or contact 

 poisons. Of these are pyrethrum ; tobacco, which acts both as 

 a repellent and, when vaporized, as a destroyer of aphides and 

 thrips ; bisulphid of carbon and hydrocyanic-acid gas, two deadly 

 gases, and some others. 



Pyrethrum.— St\^r?i\ forms of insect powder, the pulverized 

 dry flowers of different forms of daisies, are on the market 

 under such names as Persian and Dalmatian insect powder. 

 One of these is buhach, made exclusively from Pyrethrum 



