THE PEAR THRIPS IN" CALIFORNIA. 45 



HABITS OF LARVAE IN THE GROUND. 



After the larvae have pentrated the soil to a sufficient depth they 

 hollow out for themselves a small oblong cell, the inner surface of 

 which is a hard, smooth wall, the cell proper being about one-half 

 inch long. These cells are made for safe places in which the larvae 

 may pupate or transform to adults. It is here they spend most of 

 the year. 



DEPTH TO WHICH LARVAE GO IN THE GROUND. 



The depth that larvae will penetrate the ground depends largely 

 upon the type of soil. Practically all of the larva? go below the 3 

 or 4 inches of a loose topsoil mulch and establish themselves at 

 various depths in the harder soil below. The depths at which larvae 

 are found in soils vary from 1 to 26 inches. Both of these are extremes 

 and very rarely contain many thrips. In Contra Costa, Solano, and 

 Santa Clara Counties from 50 to 95 per cent of the thrips do not go 

 below 9 to 10 inches, the gravelly soil having the highest percentage 

 of the larvae nearest the surface. Some of the sedimentary soils 

 along the Sacramento River are very open and porous — a recent 

 alluvial containing a great deal of decaying vegetable matter. The 

 larvae in such soil may go much deeper, and in many cases they were 

 found in numbers 24 to 26 inches below the surface when none could 

 be found above this depth. In other cases where these light soils 

 have a good heavy sod, thrips have been found in large numbers from 

 1 to 3 inches below the surface in the cells constructed among the 

 grass roots. 



DEPTH TO WHICH LARVAE GO IN DIFFERENT SOILS. 



An absolutely definite statement as to how deep larvae will go in 

 the various soils, such as gravelly, sandy, sandy loam, sediment loam, 

 and adobe, can not be made, and only comparisons can be given from 

 samples taken from these various soils. On account of the local 

 character of thrips infestation it is important, when one is trying 

 to ascertain the depth of most of the larvae in an orchard, that several 

 samples be taken, to insure accuracy. The samples should come not 

 only from different parts of the orchard but also from various distances 

 and locations in the vicinity of the same trees. Soil samples for 

 determining the number of thrips per square foot and the depth to 

 which the larvae go in the soil should be taken at about 2 to 4 feet 

 from the base of the tree. 



The samples from which the records given in Table XI were made were 

 taken by sinking galvanized-iron cages into the soil and removing them 

 to the laboratory. The cages had a sliding fourth side which could be 

 be removed so that each layer could be examined by cutting off the 

 desired thickness and sifting the dirt upon a piece of black paper. The 



