THE PEAE THRIPS AND ITS CONTROL. 



61 



the others, which then go deeper. Continuous cultivation in June 

 and July, which, however, is not always practicable, would also have 

 the same effect. It should be remembered that these insects are so 

 small that they can easily remain inside of very small clods and be 

 turned over and over again by cultivation without receiving any injury. 



The thrips are passing through their pupal development in the 

 late fall and early winter, and they are then more susceptible to 

 mechanical injury than at any other time. They are only slightly 

 active, and can not build other cells if once they are forced from the 

 old ones. Their new legs, antennse, and wings are sheathed in long, 

 delicate sacs, any one of which may easily be broken or deformed by 

 the least disturbance. 



Several experiments with fall and winter plowing for thrips were 

 carried out in the fall of 1908, and the following records show what 

 results have been obtained in two of these orchards, where special 

 attention was given to securing data. Areas of 20 and 70 acres, 

 respectively, were plowed and harrowed, and all of the first, with 20 

 acres of the second, was cross plowed. This plowing was done 

 mostly during December, a lack of early rains having hindered from 

 doing the work sooner. In each case several samples of soil, 17 by 

 17 inches square by 20 inches deep, were removed from the orchards, 

 both before and after treatment, brought to the laboratory yard, 

 and embedded to their natural depth in the ground. The cages 

 remained open until a time when the adult thrips began to come out. 

 They were then covered over, and thereafter a daily record of the 

 emerging insects was made for each. The blocks of soil were selected 

 from near-by trees and under like conditions, to insure as far as pos- 

 sible a uniform number of thrips in each. 



Cages I, II, III, and IV from the Landon orchard were taken from 

 land which had been plowed and cross plowed in November and 

 December, and cages V and VI, from the same orchard, were taken 

 from under trees where no winter plowing had been done. Cages VII 

 and VIII, from the Hume orchard, were taken from land which was 

 plowed and cross plowed, and cages IX and X from untreated soil. 



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

 in the Santa Clara Galley, California. 



LANDON ORCHARD. 





Plowed and cross plowed. 



Not treated. 





Cage I. 



Cage II. 



Cage III. 



Cage IV. 



Cage V. 



Cage VI. 



Total number of thrips 



475 

 237 



389 

 194 



607 

 303 



115 

 57 



1,175 

 587 



1,474 

 734 



Total number of thrips per square foot of 

 surface 







Average number of thrips per cage: 



Cages I, II, III, and IV 



396 



Cages V and VI 



1,324 



Average number of thrips per square foot of surface in each cage: 



Treated 



198 



Untreated. 



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

 Approximate percentage killed 



662 

 30 

 70 



