6 farmers' bulletin 880. 



tilating the greenhouse upon the completion of an exposure. It is 

 inadvisable to fumigate on hot, humid nights, when the temperature 

 in the greenhoiLse can not be lowered readily to the desired limit. 

 The best temperature for fumigation is between 55° and 68° F. 



The interval between fumigations naturally should bo governed 

 by the reappearance of the insect under control. With small dos- 

 ages, which are imperative when fumigating a greenhouse containing 

 an assortment of plants, it is possible to kill only the larvae of scale in- 

 sects, the adults and first larva stages of the greenhouse white fly, 

 the adults of the Florida fern caterpillar, greenhouse leaf-tyer, and 

 loopers, and a certain percentage of aphids. The eggs and pupae 

 of most greenhouse insects offer considerable resistance to hydro- 

 cyanic-acid gas, and, furthermore, the overlapping of broods neces- 

 sitates several fumigations at short intervals. It has been proved 

 repeatedly that three or four fumigations at short intervals will give 

 practical control. 



CHEMICALS REQUIRED FOR FUMIGATION. 



The chemicals required in fumigating with hydrocyanic-acid gas 

 are sodium cyanid (NaCN) or potassium cyanid (KCN), sulphuric 

 acid (HjSO^), and water (HjO). Potassium cyanid has been super- 

 seded recently by sodium cyanid in the generation of tliis gas, and 

 the former is rarely used nowadays in fumigation. Sodium cyanid 

 for fumigation purposes should be practically free from clilorin and 

 should contain not less than 51 per cent of cyanogen. It may be 

 purchased either in lumps or in the shape of an egg, each "egg" 

 weigliing approximately 1 ounce. The latter is easily handled and 

 the necessity of weigliing each charge is obviated, provided, of course^ 

 the dosage is in ounces. For example, if the greenhouse requires 10 

 ounces of cyanid, 10 "eggs" are used. In small dosages, however, 

 where the cyanid is measured in grams, it is necessary to use small 

 lumps or break up the "eggs." 



Cyanid is one of the most 'poisonous substances Jcnovm and should 

 he stored in air-tight cans, plainly labeled, and Jcept out of reach of 

 those unacquainted with its poisonous nature. 



Commercial sulphuric acid (about 1.84 sp. gr. or 66° Baume) 

 that is approximately 93 per cent pure is commonly used and gives 

 very satisfactory results. Tlie acid should be kept in a glass re- 

 ceptacle, properly labeled, and tightly corked with a glass stopper. 



DETERMINING THE AMOUNT OF CYANID TO BE USED. 



Satisfactory results are obtamed only where it is possible to over- 

 come the resisting power of the insects without overcoming the resist- 

 ing power of the plant. Under favorable conditions greenhouses 



