150 The Spraying of Plants. 



" One would suppose that an [orange] tree having a dense 

 foliage would fill up the space within the tent and require less 

 gas to be effective. But the cold surface of the leaves con- 

 denses the gas, and fumigators find that a slightly heavier 

 charge of chemicals is necessary for such a tree, and where the 

 foliage is scant a less amount than is given in the table will 

 answer. Some orchardists and fumigators consider that the 

 work has not been effective unless some of the leaves or tender 

 twigs have been injured. This is not necessary, for in our 

 early experiments we have treated trees and killed the scale 

 without even injuring the most tender twig or blossom. As 

 the trees recover very quickly, even when seriously scorched, a 

 slight burning is no detriment and is evidence that the work 

 has been effective, except in the case of ' black scale ' (Lecanium 

 Olece), during the early summer when the eggs are under the 

 females. The proper time to fumigate for this scale is during 

 the fall or early winter, when they are in the larva state." 1 



" In order to make the canvas used for fumigation perfectly 

 air-tight, to prevent the gas escaping, the tents have been 

 treated with a light coat of boiled linseed oil. The great 

 objection to the oil has been that it had a tendency to stiffen 

 the canvas and add considerably to its weight, so a cheaper 

 and more flexible preparation was sought. The following 

 mixture, used by Commissioner Scott, of Los Angeles County, 

 [Cal.] during the past season, made the tents gas-tight and 

 left the canvas soft and pliable. The chief essential ingredient 

 is a supply of common prickly-pear cactus (Opuntia Engelmanni) 

 that grows in abundance in the southern counties of the state. 

 It is the flat-leaf species, and parties living in sections to which 

 it is not indigenous could have it sent in boxes. To make the 

 cactus extract, chop up enough cactus to fill a barrel two-thirds 

 full, then fill up the barrel with cold water. It should stand 

 for twenty-four hours, when it will be ready for use. Do not 

 prepare more than is required for immediate use, otherwise it 

 will sour and become worthless. Stir well, then strain ten gallons 

 of the liquid into another tub or barrel ; dissolve tw r o pounds ot 

 common glue and add to the cactus extract, with sufficient 

 yellow ochre or Venetian red to give it a good body. After 

 thoroughly mixing the ingredients, it is ready for use. Both 



i Craw, California State Ed. of Hort. 1894, Bull. 68, 18. 



