INSECTICIDES. 203 



frequent intervals as the eggs hatch are necessary to control certain pests, ;'. c, as often as 

 twice a week for a month, if the houses are badly infested. 



The gas is best generated in stone jars which should be distributed at equal distances 

 through the house, and not set too close to the plants lest the near ones should receive an 

 overdose of the gas and be scorched. 



The jars are dosed with a measured amount of crude sulphuric acid and water (1 to 2), 

 and into these are dropped weighed amounts of cyanide of potash wrapped in thin brown 

 paper so as to delay the evolution of the gas for a minute, and thus allow the operator to 

 escape. To avoid the boiling over of the acid during the violent evolution of the gas, the 

 jars should be deep rather than shallow. The house should be shut tight and arrangements 

 made in advance to open it from the outside when the exposure is completed. The cyanide 

 of potash may also be lowered into the jars from the outside by means of strings; this is a 

 rather safer way since the generated gas diffuses through the air with great rapidity, i.e., 

 nearly as fast as a man can run. Sunset of a still day is the best time for commencing the 

 exposure. The house should be opened up after 1, 2, or 3 hours. 



The air space of each house must be calculated very carefully and for growing plants 

 not more than 0.15 gram of the cyanide of potash should be used for each cubic foot and 

 half this quantity for sensitive varieties. Any serious error in the calculation means, of 

 course, either failure of the treatment or destruction of the crop. The gas is deadly to man 

 and the higher animals, and exposures must not be made in hothouses connected with 

 stables or living rooms; and if dwelling houses are near, the doors and windows on that 

 side must be closed, or the rooms vacated. The potash salt is also very poisonous and must 

 be kept out of the reach of children and animals and handled with rubber gloves. 



A good remedy for red spider is a desideratum. Repeated syringing with water is 

 recommended. They are usually worse in dry seasons. 



For the destruction of larva, beetles, and bugs out of doors, a spray containing arsenate 

 of lead is effective, and foliage usually bears this poison very well, i.e., much better than 

 Paris green. Popenoe used 6 pounds to 50 gallons of water on potato foliage to destroy the 

 Coloradopotatobeetle. All the larvaewere killed in 48 hours and the plantswere notinjured. 

 Paris green is also an effective insecticide. It may be sprayed on the foliage, which is 

 sometimes burned ; or may be dusted on mixed with flour, air slaked lime, or land plaster 

 (1 part to 30 or 50). 



Both lead arsenate and Paris green may be combined with Bordeaux mixture, so as to 

 avoid two separate sprayings. 



For suggestions respecting trap crops see pp. 282, 295, 296. 



Many of our experiment stations now publish annual spraying calendars and other 

 literature, giving the principal formulae, and usually these publications may be had upon 

 application. 



