492 INJURIOUS AND BENEFICIAL INSECTS OP CALIFORNIA. 



leakage of tent materials and these will make it possible to grad- 

 ually correct the present dosage schedules, which at best are only 

 tentative schemes which have given good results in practice. 



LENGTH OF EXPOSURE 



The time actually required to generate the gas is very short, but 

 the tents are left over the tree for a considerable time in order to 

 adequately kill the scale insects. The time usually allowed is forty- 

 five minutes. Some shorten the time to thirty minutes, while others 

 insist upon a full hour. 



TIME OF OPERATION 



Fumigation for red scale, yellow scale and purple scale may be done 

 with fair results at most any time of the year, but if black scale is also 

 present with any one of these the work is usually done during the 

 late fall, winter and early spring months, when the black scale is in 

 the immature stage, in order to avoid the presence of the mature and 

 very resistant gravid females, which do not easily succumb to ordi- 

 nary treatment. Treatment for the gray citrus scale should be mad'.' 

 from August 15th to September 15th. 



TEMPERATURE 



Fumigation is ordinarily done during the night, when there is less 

 danger to the plant. Cool, cloudy days may admit of some work, 

 but all day operations are liable to result in severe burnings of the 

 fruit and foliage. Excessive coldness is also likely to cause disastrous 

 results to the crop, especially when accompanied with much humidity. 

 It is always advisable to keep a thermometer with the outfit and to 

 suspend operations when the temperature is 70 degrees Fahrenheit or 

 more above zero or when it is 36 degrees Fahrenheit or less. Low 

 temperature should be avoided, especially during damp or wet nights. 



Hot, electric winds are also claimed to produce severe burnings 

 and work is generally suspended on such occasions. 



BORDEAUX MIXTURE AND BORDEAUX PASTE 



Orchards previously sprayed with Bordeaux mixture should not be 

 fumigated, for there have been severe burnings to fruit and foliage 

 as the results of fumigation. It is still a matter of some doubt if 

 injury follows when only the tree trunks and larger limbs are painted 

 with Bordeaux paste. . Recent experiments conducted in San Diego 

 County by J. A. Prizer seem to point conclusively that no damage 

 is done in such instances. 



