THE CITRUS THKIPS. 
thrips marking; 9.5 per cent had received a few scratches which 
would in no way affect their value, and only a few oranges were 
characteristically and severely marked. 
INJURY IN ARIZONA. 
The amount of thrips injury to citrus in Arizona varies greatly 
in different groves, depending upon the health and vigor of the 
trees. It has never been as serious in Arizona as in the San Joaquin 
Valley, Cal. In the poorer conditioned groves the fruit injury 
varies from none to 25 per cent scarred sufficiently to reduce the 
grade. In some seasons the injury has run much higher than this 
in certain groves. Thus Morrill (9) in 1912 stated that in the various 
groves injury to the 1911 orange crop ranged from none to about 
60 per cent scarred sufficiently to affect the market value, and cal- 
culated that there was a 28 per cent reduction of oranges to second 
grade and a 25 per cent to third grade, and that about 1 per cent 
were culled partly or entirely because of thrips marking. 
INJURY TO TREES. 
In many orchards in the San Joaquin Valley the foliage was 
subjected to prolonged attack from thrips until the functions of 
the leaves became so disturbed that the trees were prevented from 
reaching their normal size and fruiting capacity. Stunting due to 
thrips feeding begins in the nursery. In seasons of severe infesta- 
tion the leaves and stems of nursery trees are so badly scarred and 
twisted as to appear blighted, preventing sale at a fair price, and 
often the growth of the trunk is so retarded that the trees must be 
held a year or more beyond the proper time for sale in order to 
meet the size requirements. It sometimes happens that this class of 
stock is sold along with better trees and, thrips attacks continuing 
for several years in the orchard, the trees remain undersized and 
relatively unproductive. 
DISSEMINATION. 
The spread of the citrus thrips from one citrus-growing section 
to another is accomplished mainly in the egg stage. The shipping 
and planting of nursery trees occurs principally from February 
to May, the transfer of trees being well under way before first- 
generation larvse begin to issue from the eggs deposited the pre- 
ceding fall. Quarantine inspection is entirely inadequate to pre- 
vent the introduction of the insect into new districts, since it is 
practically impossible to detect the eggs of the species. Complete 
defoliation of the trees prior to shipment accomplishes the destruc- 
tion of many of the overwintering eggs, but a proportion of them 
escape because of insertion into the bark of the smaller branches. 
13138°— 18— Bull. 616 2 
