28 BULLETIN 616, U. S. DEPARTMENT OF AGRICULTURE. 



death or the attack resulted from the excessive moisture formed in 

 the rearing jar by the orange leaves. It results only from abnormal 

 conditions and is plainly unimportant in the field. 



IMPORTANCE OF NATURAL AGENCIES IN CONTROLLING THE THRIPS. 



The citrus thrips appears to be subject to controlling influences 

 the nature of which is difficult to determine completely. The most 

 severe infestation recorded against the insect occurred in Tulare 

 County in 1909, when 90 per cent of the entire crop of navel oranges 

 of that county was thrips marked, and close to 20 per cent of it was 

 so badly scarred and distorted that it was unsalable at a profitable 

 price. In 1910 the insects failed to appear in anything like their 

 numbers of 1909 and the injury was less than 50 per cent of what it 

 had been during that season. This naturally affected the results 



due to spraying by the growers in 1910, 

 even where the work was done thoroughly, 

 as in many cases unsprayed fruit was in- 

 jured so slightly that the difference in 

 returns between sprayed and unsprayed 

 fruit was so small that very little profit 

 was derived from the operation. In 1911, 

 thrips were still less numerous and more 

 scattered early in the season and it looked 

 as though the injury would be very slight. 

 Owing, however, to an unusually back- 



Fig. 10. — Immature spider, ° . ■,.■,.-, . T 1 , 



Erigone sp., which preys ward season, in which the trees made but 



largely upon the citrus thrips. g l ow an( l scan ty growth, the insects Were 



forced to depend to a greater extent than 

 usual on the fruit for food, on which they concentrated in the lat- 

 ter part of the season. The marking was late in appearing, but at 

 the end of the season proved to be somewhat more extensive than 

 in 1910. This illustrates the fact that the abundance of thrips is 

 not always a reliable index of the extent of expected damage to the 

 fruit. 



The chief factors influencing the decrease of thrips subsequent to 

 1909 are undoubtedly the climatic changes and relative inadequacy 

 of the food supply. In 1910 and 1911 the late, cold spring was the 

 cause of the slowness with which the insects increased during April, 

 May, and June; while the same climatic condition, in retarding the 

 growth of the trees, further checked the increase of the insects by 

 reducing their food. Furthermore, it forced the insects to scatter 

 widely over many food plants, thus hindering reproduction and 

 greatly retarding oviposition by causing dearth of suitable plant 

 tissue. 



