Amount of potassium cyanide to use and length of exposure.— 
Experiments and practice have shown that the white fly is destroyed 
in all except two stages (egg and late pupal) by an amount of potas- 
sium cyanide which is extremely small as compared with the amounts 
generally recommended for other insect pests. As small an amount 
as 0.005 gram” per cubic foot of space, or between one-fifth and one- 
sixth ounce per 1,000 cubic feet, for three hours’ exposure, has been 
used with success, ? while as large an amount as 1 ounce per 1,000 cubie 
feet for an ‘‘all-night exposure,” in a house containing infested toma- 
toes, has been reported to have given, in one instance, a like result.¢ 
On the other hand, Mr. E. C. Rittue, of the Bureau of Plant Industry 
of the Department of Agriculture, in attempting to control the 
white fly infesting tomatoes in a greenhouse on the grounds of the 
Department, found that 0.01 gram per cubic foot, or one-third ounce 
per 1,000 cubic feet, slightly injured the plants when the exposure 
exceeded thirty minutes. ‘This treatment for thirty minutes destroys 
only the adults. The greenhouse is a new one and, judging from the 
great difference in the results obtained there and in other houses 
whose fumigation has been recorded in various publications, it is 
tighter and does not allow the gas to escape as readily as does the 
average forcing house. 
This shows that the greatest difficulty attending the use of hydro- 
cyanic-acid gas, in greenhouses containing plants as susceptible to 
injury by it as are the cucumber and tomato, is the difference in the 
tightness of different greenhouses. It is consequently impossible 
to give specific directions which will be suitable under all cireum- 
stances. A fumigation with hydrocyanic-acid gas which will kill 
adults only is not effective for practical use in checking the multipli- 
cation of the white flies. Rather than this, the method of control 
described under the heading, ‘t Treatment when the use of hydrocyanic- 
acid gas is undesirable,” is greatly to be preferred. 
In most greenhouses, probably 0.007 gram of potassium cyanide for 
each cubic foot of space for an exposure not exceeding three hours 
represents the amount which will prove most effective for treatment 
of the insects without injury to tomato or cucumber plants. In many 
cases 0.01 gram per cubic foot has been found suitable for the same 
exposure, but this should not be used except in loose greenhouses 
where, after trial, a smaller amount is found ineffective. In all green- 
houses when an attempt is to be made to control the white fly with 
hydrocyanic-acid gas, it is advisable to first use not more than 0.005 
gram per cubic foot of space for a three hours’ exposure. If this 
amount is sufficient for the house, none of the adults will recover after 
the fumigation, though in the course of two or three days many more 
428.55 grams=1 ounce. > Mass. Station Tech. Bul., No. 1, p. 46. 
¢ American Gardening, XIX, p. 741. 
