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, , . . •■,,-, -, i , 
Erigone sp., which preys ward season, in which the trees made but 
largely upon the citrus thrips. gl ow an( | 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. 
