THE CITKUS THEIPS. 



11 



INCUBATION PERIOD. 



The time required for the development of the egg after deposition 

 varies considerably at different times of the year and is influenced 

 largely by the prevailing temperature. There is also some variation 

 among eggs deposited the same day, in the same plant, and kept 

 under precisely the same conditions of temperature. The incubation 

 period ranged from a minimum of 6 days to a maximum of 21 days, 

 the average for the entire season being 16 days. The records on 

 incubation in 1911 are given in Table II. The principal average 

 seasonal variations, together with the prevailing mean temperatures 

 for the periods covered, are shown in Table III. It will be seen that 

 in the cool weather of spring and early summer and again in the fall 

 the incubation period is about two and one-half times as long as in 

 the hottest period of summer. In like manner the egg stage and 

 other immature stages as well are short in exceptionally hot, dry 

 summers, such as those of 1908 and 1909, and considerably prolonged 

 in cooler years, such as 1911. 



Table II. — Incubation period of the citrus thrips, Lindsay, Cal., 1911. 



Rec- 



Num- 



ord 



ber of 



No. 



eggs. 



1 



11 



2 



61 



3 



15 



4 



6 



5 



2 



6 



1 



7 



1 



8 



1 



9 



8 



10 



31 



11 



2 



12 



11 



13 



7 



14 



5 



15 



3 



16 



1 



17 



1 



Deposited. 



May 4. 



...:do. 



do. 



do. 



.....do. 



do. 



do. 



do. 



May 5. 



do. 



do. 



May 6. 



do. 



do. 



do. 



do. 



May 9. 



Hatched. 



May 21. 

 May 22. 

 May 23. 

 May 24. 

 May 25. 

 May 26. 

 May 27. 

 May 28. 

 May 21. 

 May 22. 

 May 24. 

 May 22. 

 May 23. 

 May 24. 

 May 25. 

 May 27. 

 May 25. 



Dura- 

 tion of 



stage. 



Days. 

 17 

 18 

 19 

 20 

 21 

 22 

 23 

 24 

 16 

 17 

 19 

 16 

 17 

 18 

 19 

 21 

 16 



Rec- 

 ord 



No. 



Num- 

 ber of 



Deposited. 



May 9 . . 



:...do... 



do... 



July 17 . . 



do... 



July 22.. 



do... 



do... 



Aug. 1... 



do... 



Sept. 6.. 

 Sept. 8.. 



do... 



do... 



Sept. 18. 

 do... 



Hatched. 



May 26. . 

 May 27. . 

 May 28. . 

 July 23. . 

 July 24.. 

 July 28. . 

 July 29. . 

 July 30. . 

 Aug. 9.. 

 Aug. 10. 

 Sept. 23. 

 Sept. 21 . 

 Sept. 22. 

 Sept. 24 . 

 Oct. 8... 

 Oct. 10.. 



Dura- 

 tion of 

 stage. 



Days. 



Maximum . 

 Minimum. 

 Average . . . 



Days. 



... 24 

 .. 6 

 .. 16 



Table III. 



-Principal seasonal variations in incubation of eggs of the citrus 

 thrips, Lindsay, Cal., 1911. 



s 



Period of season. 



Range. 



Average. 



Mean 

 tempera- 

 ture. 



May 4 to May 28 . 



Days. 

 16-24 

 6-9 

 13-22 



Days. 

 18.8 

 7.3 

 17.0 



° F. 

 64.06 



July 17 to Aug. 10 



81.52 



Sept. 6 to Oct. 10 



62.18 







