UNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 907 =) 
Contribution from the Bureau of Entomolegy 
L. OC. HOWARD, Chief 
Washington, D. C. PROFESSIONAL PAPER October 20, 1920 
FUMIGATION OF CITRUS PLANTS WITH HYDRO- 
CYANIC ACID: CONDITIONS INFLUENCING 
INJURY. 
By R. 8S. Woeium,! Entomologist, Tropical and Subtropical Fruit Insect Investigations. 
CONTENTS. 
Page. | Page 
AAR ChIOMere aan sear ee ae 1 | General discussion of factors which influence 
The effect of hydrocyanic acid on plants..... Zee UM PALOMA Uys ono aces eee See oe 27 
Details of experiments. See Syn Se ph ee eet 3 | The concentration of the gas._._............- 28 
The effect on plant injury of temperature, jevEhe length ohexposure: jo. s2522sasees-) =” =n 29 
light, and moisture before fumigation... .. 4 | The physiological condition of the plant..... 30 
The effect on plant injury of temperature, | Atmospheric and light conditions............ 33 
light, and moisture after fumigation... .__. TOM SUMMA yep. 2 sae, eee we we ae tie 41 
The effect on plant injury of temperature, laluiberature cited... mee see eee 42 
light, and moisture during fumigation..... 22 | 
INTRODUCTION. 
The important factors long known to modify damage to the fruit 
and foliage of citrus trees under orchard conditions of fumigation 
with hydrocyanic acid include temperature, moisture, light, and 
physiological condition of the plant. Of these, light appears to 
have more completely influenced the application of this gas than any 
other factor and early confined fumigation to a night practice. 
Coquillett (3), the originator of orchard fumigation with hydro- 
cyanic acid, found early in his studies that citrus trees were less 
hable to injury by this gas when treated at night than in daytime, 
and explained that this result was due to decomposition of the gas 
by light and heat into other gases more injurious to the plants. 
Gossard (7), working with citrus trees in Florida, stated that ‘‘mid- 
day fumigation can hardly be practiced.”’ More recently Fernald, 
Tower, and Hooker (5), experimenting with cucumbers and tomatoes 
under glass, concluded that for such tender plants day fumigation, 
even in cloudy weather, is unsafe. 
Literature treating of the causes of fumigation injury is confined 
almost exclusively to the consideration of conditions, physiological 
1 Resigned September 11,1920. 2 Reference is made by number (italic) to “Literature cited,” p. 42. 
4533°—Buli. 907—20——1 ] 
