16 FUMIGATION INVESTIGATIONS IN CALIFORNIA. 
The black scale confines its attack mainly to the branches, vet it is 
commonly found on the leaves during its earlier stages of develop- 
ment and sometimes matures in this situation. Seldom does it 
mature on the fruit. The red and purple scales infest the branches, 
leaves, and fruit. The yellow scale occurs on the leaves and fruit. 
Occasionally it is found to a very slight extent on the branches. 
The more directly injurious effect to the tree resulting from the 
attacks of the red, purple, and yellow scales appears to the writer to 
be due to their ability to produce some toxic effect in the host plant 
in addition to the injury caused by the removal of sap. These scales 
cause a discoloration of the plant cells at the place where the sap is 
extracted, whereas the larger black scale causes no discoloration 
whatever. 
METHOD OF PROPAGATION OF THE MORE INJURIOUS SCALE PESTS. 
The young purple and black scale-insects hatch from eggs deposited 
by the adult,- while the red and yellow scales produce their young 
alive. The red and yellow scales are thus susceptible to the applica- 
tion of remedial measures at any time throughout the year. The 
eggs of the purple scale are much more difficult to destroy than the 
insects, for the latter can be killed readily in any stage of develop- 
ment although more easily in the early stages. The black scale is 
capable, after it has reached its mature and leathery condition, of 
resisting extreme insecticidal applications. Its eggs, also, are quite 
as resistant as the mature insect, if not more so. In its early stages, 
however, it can be readily destroyed by the proper insecticides. 
In all species the different broods on citrus trees are seldom, if ever, 
distinct, but overlap one another to varying degrees. At certain 
periods the breeding is more marked than at others for each of these 
insects; yet it is possible to find adult red, yellow, purple, or black 
scales in the egg-laying stage at any time throughout the year in 
any extensive citrus locality in southern California containing thrifty 
trees and in which these scales are known to thrive. This over- 
lapping of broods is due largely to the forcing and artificial conditions 
of citrus culture. 
METHODS USED IN THE CONTROL OF SCALE PESTS OF CITRUS TREES. 
The methods generally resorted to in the control of citrus insect 
pests are (1) fumigation, (2) spraying, and (3) the use of beneficial 
insects. 
The question of beneficial insects is too large for discussion in this 
limited report; suffice it to say that their work is of the highest 
importance in many respects. 
Sulphur sprays are employed against the red spider and the silver 
mite of the lemon. 
