124 PROCEEDINGS OF THIRTY-SIXTH FRUIT-GROWERS ' CONVENTION. 



form, the task of the apple grower would be a hopeless one. So. in 

 the hypothetical orchard we will assume, as is always the case, that 

 only a limited number need be considered. The sap-sucking insects 

 most likely to be present are the San Jose scale, greedy scale, and 

 woolly aphis. The leaf feeders and other chewing insects will include 

 the codling moth, tussock (horned) caterpillar, cankerworms, and the 

 tent caterpillars. Of the fungus diseases, the apple powdery mildew, 

 apple scab, and w T ood rot or sappy bark disease are most important. 

 A physiological disorder, known as "leaf spot." may also be present. 



Even this limited list may have only a few representatives in a given 

 year. In the coast districts those recurring most persistently are the 

 codling moth and apple powdery mildew, with the scales and woolly 

 aphis a close second. The apple scab is more often absent than present, 

 and the caterpillars are subject to very marked fluctuations extending 

 over a period of years. 



The insects here considered are divided into two classes, according 

 to the manner in which they take their food, that is sucking and 

 chewing. The scale insects and aphids suck the juices from the inner 

 tissues without eating the surface, while the caterpillars eat the sub- 

 stance of leaves and fruit. 



The San Jose scale is an insect which passes the greater part of its 

 life under an armor or scale formed by the secretions of special glands. 

 The youngest stage of the insect resembles a minute louse and crawls 

 about over the tree in search of a suitable spot to locate for the 

 remainder of its life. These young are also carried by the wind, birds 

 and other agencies to distant trees, thus spreading the infestation. 

 Potentially, these young are not very potent under balanced conditions. 

 Not more than one hundred succeed in locating and the number may 

 be much less. 



After locating the insects are still subject to death from various 

 causes so the number reaching maturity is still further reduced. When 

 the San Jose scale it probably does not change location, and certainly 

 is unable to do so after a time, for the legs are lost. This scale matures 

 quickly, for as high as ten generations have been recorded in a year. 

 The young are born alive and continuously for a considerable period. 

 In California the winter is passed by both adult and immature insects, 

 the first young appearing in February and March. 



The injury caused by the San Jose scale is caused by the injection 

 of a toxic substance into the tissues of the bark, and is proportional to 

 the number of insects present. The scale may be so numerous as to form 

 a continuous crust over twigs, limbs and even the trunk, or there may 

 be only a scattered infestation on the small twigs. In the first case 

 death of limbs and perhaps the whole tree may ensue, but with the 

 lesser infestations the injury may be confined to a few red spots on the 

 fruit, caused by individuals which have located on the apples in May 

 and June. The red stain is quite a constant character, and the inner 

 bark of infested branches is deeply colored. The San Jose scale is not 

 confined to the apple, but attacks several other species of trees and 

 shrubs. It is supposed to be a native of northern China. 



A number of other scale insects are closely related to the San Jose 

 scale, and have similar life histories. We have spoken of the greedy 

 scale, this is a larger species than that just described, and has a more 



