THE CITRUS THRIPS. 7 



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 



