46 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE. 



average depth to which larvae will penetrate in gravelly and sandy 

 loam soils is usually less than in heavy sedimentary loam. In those 

 soils which incline toward the adobe type and in the distinctly adobe 

 soil the larvae usually go deeper. On account of the cracking of this 

 latter type of soil as it dries out in the spring, and the texture, which 

 is such as to prevent the making of a perfect soil mulch, suitable 

 places for making the cell are not found so near the surface. In soils 

 which can be worked readily except in cases of silt deposits or an 

 abnormal amount of vegetable matter below the surface, very few 

 larvae, as a rule, penetrate to an unusual depth below the surface; 

 for this reason practically all the soils in the Santa Clara Valley that 

 are badly infested by thrips are such as render possible the obtaining of 

 practicable results from early fall plowing. Table XI shows the com- 

 parative depth of larvae in a number of samples of soil taken from 10 

 orchards in Santa Clara County. While no sandy soil is present, these 

 samples represent fairly well the different types of soil of the Santa 

 Clara Valley. 



