544 CITRUS FRUITS AND THEIR CULTURE. 



who mix the two practices. The last-mentioned class get 

 all the bad effects of the other two practices and none of 

 the good. They do enough spraying to destroy or drive 

 away their friends, too little to destroy their foes, and 

 consequently they leave themselves to the tender mercies 

 of the latter. It should be distinctly understood that there 

 is no intermediate ground. The citrus-fruit grower must 

 either spray and do it thoroughly and systematically, just 

 as he would cultivate, fertilize or irrigate, or if he decides 

 to depend upon certain fungi and insects to control the 

 insect foes which attack his trees and fruit, he must leave 

 the field entirely to them and do everything in his power 

 to foster and assist them. There must be no mixing of 

 practices. 



Which plan is the best to adopt no one but the 

 grower can determine, and then only after having obtained 

 a thorough knowledge of his own conditions. 



To successfully control insects, by natural agents, cer- 

 tain conditions are necessary. The insects to be controll- 

 ed, must be gregarious and sufficiently numerous in the 

 beginning at least, to enable the controlling agent to be- 

 come thoroughly established. Thereafter the host insect 

 must always remain in sufficient quantities to supply suffi- 

 cient food for the existence of the friendly insect or fun- 

 gus as the case may be. Lacking this, the grower must 

 see to it that a, sufficient number of the injurious insects 

 are kept to insure the perpetuation and multiplication, to 

 a considerable extent, of their fungous and insect enemies. 

 In short the latter must be bred and kept for use in case 

 of outbreaks. Then when injurious insects are present 

 in alarming numbers in a section, the controlling agents, 

 fungous or insect, may be introduced and established. 



