INSECTICIDES AND FUNGICIDES: FUMIGATION 55 



arsenate of lead brings about a decomposition, as a result of which con- 

 siderable soluble arsenic is formed, and the efficiency of the lime-sulfur 

 is also reduced about one-third. This may be avoided, however, by 

 slaking 5 Ib. of quick lime and adding this to the lime-sulfur before putting 

 in the arsenate of lead. 



Lead arsenate can be combined with nicotine sulfate, and in some cases 

 at least, with kerosene emulsion. With soap, acid lead arsenate decom- 

 poses to some extent forming a soluble arsenate which is dangerous on 

 foliage. Small amounts of soap added as a " sticker," however, are often 

 advantageous even in spite of this decomposition and are frequently 

 recommended, the gain by the addition of the soap being greater than 

 a small injury by burning. 



FUMIGATION 



In theory, fumigation is the best method for the control of insects. 

 A gas will reach every portion of a room or a plant, penetrating where no 

 spray can reach, so that insects no matter how well concealed in crevices, 

 under bark or in other locations, will be reached. Still, practical diffi- 

 culties in the use of poisonous gases are numerous and result in a restricted 

 use of this method of treatment. 



The gases used for the destruction of insects act either as actual 

 poisons which enter the body through the tracheal system and directly 

 attack the tissues, or combine with the oxygen of the air and thus remove 

 it from availability by the insect, which suffocates in consequence. In 

 either case, successful fumigation depends upon the liberation of a suf- 

 ficient quantity of the gas or vapor to make it strong enough to kill the 

 insect. 



This at once eliminates trees, bushes and crops growing out of doors 

 from consideration, unless they or their products are so valuable as to 

 make the use of gas-tight tents to cover them during treatment worth 

 the expense, which is considerable. Accordingly, fumigation is generally 

 made use of only with citrus trees, and in houses, greenhouses or other 

 places capable of being tightly closed. Under the conditions mentioned, 

 however, it is an excellent method for insect control, though where plants 

 are to be fumigated, it is usually done at night as the gases or vapors are 

 less liable to cause injury then. 



The fumigants most often used are carbon disulfid, nicotine, sulfur 

 and hydrocyanic acid gas. 



Carbon Disulfid (CS 2 ). This is obtained in liquid form but becomes 

 a gas on exposure to the air. Impure grades are as good for fumigation 

 as the purified article. The number of cubic feet in the space to be fumi- 

 gated is calculated, and in general from 10 to 20 Ib. of the disulfid 

 are used for each 1,000 cu. ft., though if the place is very tight, less than 

 this will be needed. As the gas is considerably heavier than air, the usual 



