FUMIGATION OF CITRUS TREES. 53 



damp nights near the coast are exactly as satisfactory as on dry ones. 

 Even if the gas is absorbed by moisture the tents become so mucli 

 tighter from being moist that any negative effect from the dampness 

 is offset. 



The character of the tenting material used directly affects the dosage 

 required. Most of the ducks and drills now used in California (see 

 p. 11) are about equally gas-tight. The recommendations of dosage 

 given in this bulletin are for these tenting materials. With the 

 special new drill experimented with during these investigations (see 

 pp. 11-12) one-fourth less dosage is required. Any tenting tighter 

 than the cloth commonly used for this purpose in Calif ornia will also 

 require less dosage. 



THE PURPLE SCALE. 



Preliminary experiments to determine the dosage required for the 

 destruction of the purple scale were undertaken at Orange, Cal., 

 during the month of November, 1907. Orange trees severely infested 

 with the purple scale in all stages of development were treated with 

 dosage rates varying from three-fourths of an ounce of cyanid per 

 100 cubic feet up to 2\ ounces per 100 cubic feet. The cubic contents 

 of the trees varied but little, the trees ranging from 11 to 14 feet in 

 height. The 1-1-3 formula was followed. Exposure lasted one hour. 

 After a period of about two months an examination of the results of 

 this experiment was made. To show the care with which the exam- 

 ination was conducted in this as well as all other experiments against 

 the purple scale it might be mentioned that in each case the scales 

 were overturned and examined with a high-power hand lens. In 

 those instances in which the entire contents of the scale were not at 

 once revealed, the delicate ventral scale was ruptured and the con- 

 tents scraped out. Through this method not a single egg could 

 escape observation. 



As a result of this experiment it was found that all insects were 

 destroyed on the leaves and branches by a f-ounce dosage rate, 

 that all insects and over 99 per cent of the eggs were destroyed at a 

 1-ounce dosage rate, and that all eggs on the leaves and branches were 

 destroyed at a 1 J-ounce dosage rate. Very little fruit was on the trees, 

 yet, where present, normal eggs were found on the fruit after a dosage 

 as high as a 1 f-ounce rate. 



In another experiment in which the trees were considerably smaller, 

 some being not more than 7 feet tall, it required a 2-ounce rate to 

 eradicate the eggs on the leaves and branches, even though the length 

 of exposure was one and one-half hours. This condition shows that 

 smaller trees require a much heavier dosage proportionally than large 

 trees to offset the leakage of gas, as explained on pages 33-34. 



During July, 1908, an orange orchard near Whit tier, consisting of 

 about two acres of trees averaging about 7 to 9 feet tall which were 



