9 



In considering the agency of control afforded by the natural enemies 

 the fact must not be lost sight of that these are dependent on the scale 

 insects for their existence, and that, therefore, a fairly complete exter- 

 mination of the host insects means a like extermination of its enemies. 

 There is, therefore, a natural alternation or periodicitj^ in the abundance 

 of the scale insect and its parasites. A more even balance may be main- 

 tained to a certain extent by artificial introduction of the parasitic 

 insect the moment the scale has begun to be abundant, in this manner 

 assisting the early multiplication of the natural enemy. This is now the 

 practice with the fluted scale in California, South Africa, and Portugal. 

 To succeed in such efforts, it is necessary to have an efiicient parasite 

 or predaceous insect, and also regular breeding places where these may 

 be secured when wanted. These conditions may be naturally supplied 

 when a whole district, such as California, is under constant observation 

 and the localities where the parasite and scale are occurring together 

 are known, so that from such points the ladybirds or other enemies 

 may be collected and shipped to the districts needing them. 



THE DIRECT MEANS OF CONTROLLING CITRUS SCALE INSECTS. 



Scale insect enemies of citrus trees are directly controlled in two 

 ways: (1) By spraying the infested plants with some liquid insecticide, 

 (2) by subjecting them to the fumes of h3^drocyanic-acid gas, com- 

 monly designated as ' ' gassing. " Each of these methods of control has 

 its place. 



THE GAS TREATMENT. 



The gassing method (figs. 1-6) is undoubtedly the most effective 

 means known of destroying scale insects. It has been in general use 



Fig. 1,— Method of hoisting tent over orange tree. 



in California for fifteen years, and to a less extent elsewhere on citrus 

 trees, and the methods are now thoroughly perfected and highly satis- 



172 



