62 DECIDUOUS FRUIT INSECTS AND INSECTICIDES. 



Table III. — Experiments with fall and winter plovjing for the pear thrips in two 

 orchards in the Santa Clara Valley, California — Continued. 



HUME ORCHARD. 





Plowed and 



cross plowed. 



Xot treated. 





Cage VII. 



Cage VHI. 



Cage IX. 



CageX. 





421 

 210 



643 

 321 



2,185 1 

 1,092 1 



1,771 



Total number of thrips per s quare foot of surface . 



885 



Average number of thrips per cage: 



Cages VII and VHI 







265 









988 



Average number of thrips per square foot of surface in each ca? 

 Treated 



■e: 





133 



Untreated 494 



Percentage living in treated areas as against the numbers of thrips in untreated ground 27 



Approximate percentage killed '. 73 



Bearing in mind that the larvae penetrate into the ground quickly 

 after they leave the trees; that they remain usually below the loose 

 top soil, going deeper if disturbed, and also that they are most sus- 

 ceptible to injury in the pupal stage, cultivating and plowing should 

 be so arranged as to take best advantage of their habits, to encourage 

 their locating near the surface, planning at the same time to reach 

 them by late fall and early winter plowing. 



The principle of fall plowing is to use a moldboard or disk plow, 

 and by turning the land over to bring the thrips which rest in the 

 lower strata of ground up to the surface. The land should then be 

 thoroughly harrowed or worked over with a disk cultivator. TTith 

 the present methods of plowing, a strip of 2 feet or more of undis- 

 turbed ground is usually left in the tree row. It is necessary also to 

 plow to a less depth close under the trees than in the middle of the 

 rows. The land should therefore be plowed and cross plowed, to 

 insure breaking up all of the ground to a uniform depth, and harrowed 

 after each plowing, to make the treatment thorough. 



The Landon orchard was uniformly plowed to a depth of about 9 

 inches. It will be seen by referring to Table II that SI per cent of 

 all the thrips were above this depth and were therefore disturbed. 

 Table III shows that there were 70 per cent less live thrips in ground 

 which had been plowed and cultivated than .in that which had 

 received no winter treatment. These thrips, about 89 per cent of 

 all which were disturbed, must therefore have been kiUed by the 

 cultivating. 



The Hume orchard was plowed uniformly to a depth of about 7 

 inches. Table II shows that 88 per cent of the thrips were between 

 the surface and this depth, and Table III shows that about 73 per 

 cent of the total number of thrips in this orchard were killed by 

 cultivation. 



