48 



BULLETIN 173, U, S. DEPARTMENT OF AGRICULTURE. 



In Contra Costa County the greater portion of the orchard area 

 is on the distinctively adobe soil. It is a noticeable fact that the 

 larvse penetrate this soil to a greater depth than they do the hard 

 gravelly soils, probably owing to the greater prevalence of cracks. 

 An examination of Table XII, which is the record of the results of 

 soil examinations from five pear orchards and one prune orchard 

 during the winter of 1908-09, shows that all of the larvae in the hard 

 gravelly soils were within 8 inches of the surface, while in the adobe 

 soil only 79 per cent were found at this depth, the other 21 per cent 

 being between 8 and 13 inches below the surface. 



Table XII. 



-Comparative depth of larvae of the pear thrips in various soils near Walnut 

 Creek, Contra Costa County, Col. 



Pear and prune orchards. 



Anderson, F. A. Bancroft, and Whit- 

 man (pear), and Jones (prune) orch- 

 ards. Heavy loam to adohe. 



WescottandH. H. 

 Bancroft (pear) 

 orchards. Hard, 

 sandy, gravelly 

 soil. 



Xumber 

 of layer. 



Depth. 



24 samples. 



12 samples. 



Xumber 

 of thrips. 



Per cent 

 above. 



Xumber 

 of thrips. 



Per cent 

 above. 



2 



3 



4 



5 



6 



7 



8 



9 

 10 

 11 

 12 

 13 



Inches. 

 1-2 

 2-3 

 3-4 

 1- 5 



5- 6 



6- 7 

 7-8 

 8-9 

 9-10 



10-11 

 11-12 

 12-13 















76 



276 



152 



82 



48 



32 



42 



24 



4 





3 

 3 

 9 



18 

 33 

 18 

 6 

 

 

 

 

 



3.33 

 6.66 

 14.44 

 36.66 

 73.33 

 93.33 

 100.00 







10.33 

 47.83 

 68.70 

 79.98 

 86.23 

 90. 57 

 96. 28 

 99. 55 

 100. 00 















Total number of 



736 





90 

 30 





Average number 

 of larvae per 









"' 









AREA AROUXD DLTFEREXT TREES IX WHICH THRIPS ARE HOST XTXMEROUS. 



The area around trees in which thrips are most numerous would 

 usually be within a radius of 6 to 8 feet of the base in prune orchards 

 where the trees are from 22 to 24 feet apart. Under prune trees 

 which are from 18 to 20 feet apart, and where the branches overlap, 

 the area infested will be more uniform, and more thrips will be 

 present midway between the rows than nearer the base, as such trees, 

 growing close together, usually do not have so many smaller limbs 

 in the center of the tree as nearer the end of the branches. Pear 

 trees are more upright and compact in growth; hence the greater 

 percentage of the larva? are near the trunk of the tree, and in the 



