December, ’20] WOGLUM AND ROUNDS: DAYLIGHT ORCHARD FUMIGATION 483 
uniformity of results throughout the bottom of a tree fumigated at 
night, whereas in sunshine work the poorest kill is in the more shaded 
part which is also usually most severely infested, the figures just pre¬ 
sented would appear to show that a 50-minute night exposure at a 
cool temperature is, under some conditions, superior to a 30-minute 
sunshine exposure at a much higher temperature. 
The greater effectiveness of daylight fumigation to that at night 
with the same dosage and exposure is clearly evident. The temperature 
averages many degrees higher during the day which correspondingly 
increases insect activity as well as susceptibility to the gas. This 
superiority is most noticeable on the sunward side of the tree. In this 
connection it must be noted that the severest infestations of such 
scales as the black and purple are on the more shaded parts of the 
tree where the superiority of sunshine work is least apparent. This 
condition is of primary importance in regulating any reduction of 
dosage for sunshine work, for to accomplish results with a reduced 
schedule equivalent to night work such reduction of dosage must be 
made with regard to the scale-kill on the shaded part of the tree where 
it is most difficult to destroy. The irregularity of scale-kill is one 
of the greatest drawbacks to the daylight practice. 
Factors Limiting Daylight Work 
Sunshine coming in contact with plants immediately after fumiga¬ 
tion and before they have fully recovered their normal physiological 
activity is a factor of the greatest concern from the standpoint of 
injury. In fact there appears to be no other meteorological condition 
which so intensifies plant injury. Therefore, in conducting daylight 
fumigation one is constantly menaced with a factor of great danger. 
The effect of the sunshine is modified by its intensity (this depends 
mainly upon the height of the sun above the horizon and the clearness 
of the atmosphere), by the physiological condition of the plant, by 
the concentration of gas and by the length of exposure. Thus in 
entering the field of daylight fumigation we are entering a field of 
complex nature. Of these factors the strength of gas, the length of 
exposure and the temperature or intensity of the sunshine are entirely 
tangible and necessarily form the basis of daylight fumigation proce¬ 
dure. Any plant will withstand a certain amount of gas under the 
most severe conditions without injury, and within certain limitations 
this concentration is safely increased as the length of exposure is de¬ 
creased. This very point appears to be the keynote to successful 
daylight fumigation. Night fumigation with dosage schedule J and 
No. I 1 has been followed with safety for many years with exposures 
1 Bui. 90, Bur. Ent., U. S. Dept. Agr. 
