72 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA. 



the rays of light appear to hive a direct effect in producing injury, 

 these investigations have shown that this injury is intensified by the 

 presence of heat, and the writer contends that burning during the 

 daytime is directly attributable to heat as well as to the light itself. 

 Fumigation should never be attempted in the daytime, even on 

 cloudy days. Carry on the work at night and do not commence the 

 operations until the sun has disappeared and the shades of darkness 

 are approaching. Remove the last row of tents in the morning before 

 the sun rises. 



WINDS. 



Fumigation should never be attempted during a heavy wind, for 

 two reasons: First, the gas escapes out of the tent so that poor work 

 results; second, injury to the trees might result. Dr. Morrill has 

 called attention 1 to the variability of results on trees fumigated during 

 a stiff breeze, stating that on different parts of the same tree he found 

 the killing to vary from 30 to 100 per cent. 1 Observations made dur- 

 ing this investigation cover instances in winch the gas was driven 

 from tents b}^ winds in a very few minutes. The trees, of course, 

 required a second treatment. The burning of trees fumigated during 

 winds has frequently been observed. In light winds the injury 

 appears to be more prevalent on that side of the tree from which the 

 wind comes. Heavy winds appear to produce the burning fully as 

 much on the opposite side, or may affect the entire tree. A sudden 

 change in temperature accompanying a wind appears to be especially 

 severe in its results. Undoubtedly some of the cases of severe burn- 

 ing during " electric" weather are due partly to the wind as well as 

 to the heat. 



The author's experience has led to the conclusion that fumigation 

 should never be carried on during a windstorm. As soon as a breeze 

 arises sufficiently strong to "flap" the tents, it is well to discontinue 

 work until calmer weather. 



INJURY TO SPRAYED TREES. 



Distillate oils are still used by a few orchardists in combating scale 

 pests. As these men become discouraged with the oil treatment they 

 adopt fumigation. In this connection it appears desirable to state 

 a recent experience in the fumigation of trees previously sprayed with 

 an oil combination spray. 



A lemon orchard of 40 acres was sprayed early in the autumn with 

 a combination of Bordeaux mixture and distillate oil. Two months 

 later about 25 acres of this orchard were fumigated partly with a No. 

 1 and partly with a three-fourths schedule. This fumigation con- 



1 Bui. 76, Bur. Ent., U. S. Dept. Agr., p. 12, 1908. 



