10 



factory. Gassing should undoubtedly be employed wherever the 

 expense of the treatment, which is the one objection to it, is not an 

 object as measured hy the value of the crop protected. For most 



Fig. 2.— Tent carried over tree bj- the falling of pulleys. 



species of scale insects, one good gassing is worth as much as two 

 or three sprayings, and, when done at the right season and properl}", 

 it very frequently will almost, if not quite, exterminate the scale 

 insects from the treated trees, giving them comparative immunity 



Fig. 3. — Tent in position for fumigation. 



often for two or more 3"ears. The use of hydrocyanic-acid gas is, 

 therefore, strongh' urged wherever the conditions warrant it. Gassing 

 is especially desirable for trees that have a dense habit of growth, 

 such as the orange, which develops a large, thick head, the spraying 



172 



