22 BULLETIN 616, U. S. DEPARTMENT OF AGRICULTURE. 
Table VI. — Duration of the life cycle of the citrus thrips, Lindsay, Cal., 1911. 
Period covered. 
Average 
length of 
egg stage. 
Average 
length of 
larval 
stage. 
Average 
length of 
pupal 
stage. 
Average 
duration of 
life-cvcle. 
Mean 
daily tem- 
perature. 
May 3 to June 9... 
June 2 to Sept. 5... 
Aug. 31 to Sept. 26 
Days. 
18.8 
7.3 
17.0 
Days. 
8.2 
4.4 
Days. 
5.3 
33.9 
15.2 
29.1 
62.67 
76.03 
68.12 
SEASONAL HISTORY. 
About the middle of October the thrips begin to diminish notice- 
ably and as the temperature goes lower, through Xovember and 
December, they gradually disappear. Occasionally adult thrips will 
be found on the trees as late as December, from which it has been 
inferred that the winter is passed in this stage. As a matter of fact 
none of them lives through the month of January, and even the most 
painstaking search has never revealed a single specimen of larva, 
pupa, or adult later than January 5. Dead adults occur in increasing 
numbers on the leaves in October and Xovember. corresponding with 
the period in which the living insects disappear most rapidly. It 
was determined by experiment, verifying field observations, that the 
winter is passed successfully in the egg stage only. Large numbers 
of adults and larvae confined in the fall on orange plants, practically 
exposed to the prevailing weather except for being sheltered from 
rain, began to die early in Xovember, very few larvae pupating and 
these few dying as pupae. All specimens were dead by December 26, 
but the eggs deposited produced larvae the following spring. 
The earliest time at which the citrus thrips have been found in 
the spring was March 25. There is, therefore, a period of from 8 to 
11 weeks in January, February, and March in which feeding, ovi- 
position, and all other activities of the insect cease, but which can 
not be called a hibernating period in the strict sense. The late issuing 
larvae, as well as the pupae and adults found in Xovember and De- 
cember, feed during the warm part of the day until the first severe 
frost kills them. The date of issuance of the first spring larvae will 
depend upon the character of the season, coming early when the 
mean temperature for February and March is high and being delayed 
by a late cold spring. 
MOVEMENTS OF THE THRIPS IN RELATION TO THE BLOSSOM AND GROWTH 
PERIODS OF THE WASHINGTON NAVEL ORANGE. 
As the stems from which the first spring larvae issue harden, the 
insects wander in search of better food and are soon found working 
up onto the new spring growth, which is usually 8 or 10 inches long 
by the time the larvae have attained considerable numbers. Orange 
