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 



Average 



length of 



larval 



stage. 



Average 



length of 



pupal 



stage. 



Average 

 duration of 

 life-cycle. 



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 

 6.8 



Days 



6.9 

 3.5 

 5.3 



Days. 

 33.9 

 15.2 

 29.1 



' F. 

 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 November 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 November, 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 November, 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 November 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 



