44 FUMIGATIOISr INVESTIGATIONS IN CALIFORNIA. 



less satisfactory results in eradicating the scale, in this experiment. 

 We know that a leakage of gas takes place through the tent and that 

 more gas will escape through 2 square feet of cloth than through 1 

 square foot in a given time. It will be shown m one of the following 

 discussions that the leakage surface of tented trees is proportionately 

 much greater for smaller trees than for larger ones. This would lead 

 us to expect a greater escape of gas and consequently the requirement 

 of a heavier dosage rate with the smaller than with the larger trees. 

 The last experiment demonstrated the correctness of this deduction. 



THE LENGTH OF EXPOSURE. 



All considerations were the same in experiments Nos. 1 and 2 ex- 

 cept the length of exposure. In using a 2-ounce dosage rate, wo were 

 able to destroy the purple scale in all of its stages on the leaves and 

 branches with a thirty-minute exposure, whereas with a one-hour 

 exposure we were able to accomplish the same results by using a H- 

 ounce dosage rate. This demonstrates that decidedly better results 

 can be secured by leaving the tents on the trees one hour than is 

 possible with thirty minutes gassing. Whether more favorable 

 results can be accomplished in one and a half hours than in one hour 

 can not be determined from these experiments, since the trees in 

 •experiment No. 3 were of a smaller size than those in experiments 

 Nos. 1 and 2. This matter of the large or small size of the trees is a 

 vital factor in affecting the results obtainable. 



Judging solely from the data at hand, we are forced to the conclu- 

 sion that one hour is the more satisfactory length of exposure. Fur- 

 ther experiments may show that a longer exposure will produce better 

 results, or even that a forty-five or fifty -minute exposure will produce 

 results as satisfactory as are obtainable in one hour. We hope in the 

 near future to be able to fully settle this question. Until this is done, 

 however, it would appear advisable to adhere to the one-hour length 

 of exposure which is now generally employed in southern California. 

 The considerations upon which this conclusion is based are as follows: 



Experiments have demonstrated conclusively that with an expo- 

 sure of one hour we can obtain decidedly better results than with an 

 exposure of thirty minutes. If we give the tree an exposure of 

 thirty minutes, it will require a considerably larger amount of cyanid 

 to accomplish the same result. It requires approximately one hour 

 for an outfit to go through the complete operation of preparing the 

 chemicals and shifting 30 to 33 tents — the number usually employed. 

 The tent pullers, by the time the end of the row is reached, are usually 

 as much as five minutes, sometimes more, ahead of the one who 

 handles the chemicals. As a result, the last trees of a row are ex- 

 posed to the gas about fifty-five minutes, or a little less, under the 



