312 CITRUS FRUITS 



and a systematic and a logical program of fumigation is 

 practised no great amount of damage is caused by the insect 

 pests. 



Scale Insects. — The scale insects which are troublesome, in 

 almost every case, are foreign insects which have been intro- 

 duced into this country through shipments of nursery stock 

 and by other ways. Some of the scale insects not only 

 damage the plants by sucking out the juices, but they secrete 

 a sweet substance which gives a good medium for the grapth 

 of certain moulds. 



Fig. 129. — Citrus trees covered with tents preparatory to fumigating 

 them, taken at night when the operation is carried on. (After Quayle, 

 California Agriculture Experiment Station.) 



The most practical means of control of the scale insects 

 is by fumigating with hydrocyanic acid gas. Each insect 

 varies in its power to withstand the gas and separate dosage 

 tables have been worked out for each important scale insect. 

 The success of this gas in controlling the scale insect is in 

 its ease of generation and its exceedingly poisonous nature. 



The fumigation is done by the use of a tent placed over the 

 tree. The tents are made of the best duck and vary in size 

 from 20 to 36 feet for different sized trees. 



The cost of fumigating is about thirty cents for the aver- 

 age-sized tree. 



