FUMIGATION OF CITRUS TREES. 



59 



THE BLACK SCALE. 



To specify a certain dosage for use at all times against the black 

 scale is impractical. The reason is that this insect is more difficult 

 to destroy in some stages of its development than in others. While 

 young and in a soft condition it can be destroyed by a light dosage. 

 As the insect approaches maturity its body becomes leathery and 

 tough, which renders it difficult to destroy. The eggs require even 

 a heavier dosage. The great variation in development among the 

 black scale results that at most times of the year trees will contain 

 insects in all stages of development, from those recently hatched to 





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49 



Fig. 11.— Dosage schedule No. J, for potassium cyanid. (Original.) 



tough or even leathery ones. In such cases a dosage sufficient to 

 destroy the more resistant individuals is necessary. 



Experiments during this investigation, as well as observation of the 

 work of practical outfits, has demonstrated that in the younger and 

 tenderer stages this insect can be destroyed by one-half of schedule 

 No. 1. Very few of those in the later stages of development are 

 affected by such a dosage. A three-fourths schedule not only 

 destroyed those in the earlier stages of development, but also those 

 of a leathery nature and many that are tough and full grown, as 

 evidenced by their size, together with the absence of the character- 

 istic letter "H" on the upper side. A considerable percentage of 

 eggs is also destroyed at this strength. A full schedule No. 1 dosage 



