50 



BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE. 



Table XIII. — Comparative number of pupae, and larvae of the pear thrips found in the 

 soil during July and August, 1909, San Jose, Cal. 



Landon and Cottle prune orchards. 



Sample 

 Xos. 



Date ex- 

 amined. 



Larva?. 



Pupa?. 



Number. 



Per cent. 



Xumber. 



Per cent. 



30-33... 

 34-37... 

 38-11... 

 42-15... 

 46-49. . . 

 50-53... 

 54-57... 

 53-61... 



1909. 



July 15 

 20 

 28 



Aug. 3 

 17 

 17 

 23 

 23 



556 

 127 

 67 

 44 

 22 

 165 

 65 

 93 



99 

 100 

 86 



94 

 100 

 87 

 80 

 82 



66 



1 



11 

 4 



14 

 6 



22 

 13 

 18 



13 

 20 

 19 



The time of pupation varies considerably with different orchards; 

 for instance, in orchards where irrigation is practiced in the early fall, 

 pupation probably starts at an earlier date than in orchards where 

 this custom is not followed. Furthermore, from a number of exami- 

 nations made the past two years it seems evident that pupation 

 begins earlier in those orchards having a heavy sedimentary soil 

 than in orchards which have a light, gravelly soil. Fall plowing 

 would necessarily be more effective upon orchards which have a 

 gravelly soil on account of this habit of late pupation, which would 

 enable the owners to wait until the fall rains have started before 

 plowing, and also because a larger number of thrips are near the 

 surface. 



EFFECT OF WEATHER CONDITIONS UPON PUPATION. 



It is hardly probable that temperature conditions affect the length 

 of the pupal stage of the pear thrips very greatly, since the ground 

 does not freeze in the winter, except in the Eastern States, and the 

 mean temperature at 6 to 9 inches below the surface for the year 

 around is probably more even than it is above the ground. An early, 

 wet fall would probably cause the thrips to pupate earlier than would 

 be the case in a dry season. 



The time spent in the pupal stage varies from one to four months, 

 while the normal time for most of the pupae is about two months. 



ADULTS IN WINTER. 



The first adults appear in the ground in late October, the number 

 increasing gradually until December to early January, by which time 

 practically all pupae have transformed to adults. The time spent in 

 the ground as adults before emerging and appearing on the trees 

 varies from a minimum of one month to a possible maximum of five 

 months, averaging, however, about three months. 



