FUMIGATION OF CITRUS TREES. 71 



With these records as a basis, the writer advises that fumigation be 

 stopped at 38° F. or below. It is known that there are instances in 

 which fumigation can be carried on with impunity at a temperature 

 lower than 38° F. ; in fact, there are records of fumigation carried on 

 as low as 33° F. without any considerable injury. The fact remains, 

 however, that there are authentic cases of severe injury at tempera- 

 tures below 38°, and any person carrying on work at these lower 

 temperatures runs the risk of inflicting injury, and with products of 

 the commercial value of citrus fruits one can not well afford to assume 

 such risks. 



LIGHT. 



Coquillett, in the course of Iris early investigations in the use of 

 hydrocyanic-acid gas, determined that daylight fumigation was more 

 injurious than fumigation at night. This he attributed principally 

 to the fact that the actinic rays of light decomposed the gas into other 

 gases of a more injurious nature. Commercial fumigation work 

 since then has been carried on exclusively at night. Occasionally a 

 person of questioning mind has attempted some experiments hoping 

 to dispel the old idea of daylight injury, but current information has 

 it that these attempts have never been successful. 



Some records of daylight work have been taken during this investi- 

 gation. In one instance about 50 large orange and lemon trees were 

 treated , partly while cloudy and partly in the sunshine. These trees 

 had the whole upper half burned back — branches as well as leaves 

 and fruit. It was the severest injury to citrus trees the writer has 

 ever seen. 



One very cloudy afternoon about 25 trees (orange and lemon) were 

 fumigated for red scale with a three-fourths schedule. The tempera- 

 ture ranged between 70° F. and 80° F. The lemon trees were only 

 slightly affected, but the orange trees were very severely damaged, a 

 large part of their tops being burned back. 



The first row of trees fumigated at night and the last in the morn- 

 ing are frequently more or less injured. The cause of this injury in 

 the former case is that the work is sometimes commenced before the 

 sun has set and while it is still warm. In the latter case the sun has 

 come up before the tents were removed from the last row of trees. 

 The injury under such conditions is most apparent from fumigation 

 carried on in the late summer and early autumn when the weather is 

 warm. On the cool days late in autumn the treatment can be and 

 is carried on with impunity at a degree of light at which injury would 

 result on the warm days earlier in the season. Judging from the 

 experiments and many observations which the author has made 

 respecting the eifect of fumigation during the daytime, the contention 

 that daylight work is generally unsafe is entirely correct. Although 

 67330°— Bull. 90—12 6 



