No. 4.] GREENHOUSE PESTS. 393 



has not been discovered. Then different plants differ in the strength 

 of the fumes which they can stand without injury, and this must be 

 taken into consideration. In some cases if the gas be used strong 

 enough to kill all the insects present the plants will be seriously in- 

 jured, and it is necessary in such cases to use it weaker than is desir- 

 able, and repeat the fumigation oftener. 



Where a house is filled with but one kind of plant, the problem 

 how strong the fumigation can safely be made is much easier than 

 where many kinds are present. But just how much gas any plant can 

 stand under all conditions has not been ascertained, nor even whether 

 a strong treatment for a short time or a weak treatment for a long 

 time is the safer. For tomatoes it has been found at the Massachu- 

 setts Experiment Station that one-third of an ounce of cyanide to 

 every thousand cubic feet of house for forty minutes at night is safe 

 for the plants and quite effective against the white fly. In Maryland 

 three-fourths of an ounce per thousand cubic feet, the exposure to 

 the gas continuing over night, caused no injury; but it is probable 

 that the house used for these tests was not as tight as the Massachu- 

 setts one, as this strength injured the plants in this State. 



It is necessary to be sure of the quality of the cyanide used in fumi- 

 gation. Most drug stores carry a grade of about 50 per cent strength; 

 but it is necessary to obtain the 98 to 99 per cent strength, to be at 

 all certain of results. This should be obtained in one pound sealed 

 cans, which state the strength on the outside, and which should not 

 cost over 50 cents per pound. The commercial grade of sulfuric acid 

 is as good as is needed, and its cost should not be more than 10 cents 

 per pound. From this it can be seen that fumigation with cyanide 

 is not an expensive process. 



At the present time our knowledge is not sufficient to enable us 

 to use the gas strong enough to kill all the insects. A single fumiga- 

 tion well below the point of danger to the plants will kill the plant lice, 

 most of the white fly, thrips, and some of the mealy bugs. Against their 

 eggs, however, and against most scale insects, it is of little value. But 

 the eggs will soon hatch, and a repetition of the treatment will then 

 reach the young which hatched after the first fumigation. In practice 

 it has been found that fumigation with one-third of an ounce of cyanide 

 to each thousand cubic feet in the house, repeated three times at 

 intervals of twelve to fourteen days, will practically free a house from 

 insect pests except scales and red spiders. 



This holds good for tight houses only. If the house is loose and 

 much of the gas becomes quickly lost, larger amounts must be used, 

 but how much larger these may be will depend entirely on how loose 

 the house is. 



Where the house is a long one, it is well to divide the charge into two 

 or more equal parts, and start the fumigation at the jar farthest from 

 the door; then drop the other charges of cyanide into their jars as 

 these are passed on the way out. 



