476 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 13 
Chairman A. L. Melander: Mr. R. S. Woglum and M. B. Rounds 
have sent in a paper which will be read by the Secretary. 
DAYLIGHT ORCHARD FUMIGATION 
By R. S. Wogltjm and M. B. Rounds, XJ. S. Bureau of Entomology 
Orchard fumigation of citrus trees in this country has, with few 
exceptions, been confined strictly to night operations from the earliest 
days of its commercial adoption. During hot weather trees have not 
been covered until sundown, but with the advent of cool weather in 
late autumn, fumigation was frequently started while the sun was 
still up; and in winter full exposure to gas was sometimes made on 
cool days. Occasionally a venturesome fumigator attempted day¬ 
light work in the winter without regard to temperature and, although 
temporary success sometimes followed, sooner or later severe injury 
was experienced and return to the night practice invariably followed. 
In 1918 a situation arose in California which necessitated prolonga¬ 
tion of the fumigation season well into the winter and an outgrowth 
of this in Orange County was a large amount of daylight work, in 
part performed in the bright sunshine at comparatively high winter 
temperatures. This was carried on without experiencing the severe 
injury which had always proved a check in former years. One opera¬ 
tor in particular was so impressed with the possibilities of daylight 
work that he practically abandoned night fumigation throughout the 
season of 1919. The past winter, 1919-20, saw an additional number 
of outfits fumigating very extensively during the daytime. 
A special investigation of daylight fumigation was started by the 
writers in 1919 with the object of ascertaining the reasons for the 
greater freedom from injury at the present time with liquid hydrocy¬ 
anic acid than in former years under the pot and machine methods of 
generation, and also of determining if a system of daylight fumigation 
both practical and safe could be developed. Partial results of this 
investigation are presented herewith. 
Prior to 1916 orchard fumigation was performed with a highly 
heated gas generated in the field. The introduction of liquid hydro¬ 
cyanic acid was accompanied by a new method of field application 
which produced a cool gas at the lower part of the tree and resulted in 
very different diffusion from that of the heated field-generated gas. 
This was pointed out by the senior writer 1 who showed that at warm 
temperatures the scale-kill in the case of pot-generated gas is best 
toward the top of the tree whereas with liquid hydrocyanic acid it is 
best toward the bottom. Gas concentration is proportional to scale- 
kill. 
1 Jour. Econ. Ent., V. 12, No. 1, 1919. 
