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



blood is probably what causes the protrusion of the bladder. Vari- 

 ous agencies have been used in experiments to hinder the thrips 

 in walking about on the surfaces of the plants they are attacking, 

 with the view that if in some way the mechanism of the bladder was 

 affected, either by causticity or by absorption, the bladder would 

 not be able to perform its function, and the insects would fall from 

 any surfaces that were so treated. Tins has not been successful 

 from the writers' experience, as they have observed on numerous 

 instances thrips crawling around on sticky surfaces, even on tangle- 

 foot, which was to all appearances and to the touch very sticky. 

 This bladderlike formation is probably so delicate that surfaces 

 which appear smooth or sticky or caustic to the naked eye and human 

 touch are rough and uneven to the thrips and are neither adhesive 

 nor caustic. The writers have never seen thrips stuck to any sur- 

 face by the ends of their tarsi, but only by their bodies, legs, or 

 wings. It is apparent that they are able to walk on practically 

 every kind of surface, especially after this treated surface has been 

 exposed to the atmosphere for a few hours. 



LIFE HISTORY AND HABITS. 



ADULTS IN SPRING. 



EMERGENCE FROM GROUND. 



The first form of the pear thrips to be seen by the orchardists during 

 the growing season is the adult (PI. I, fig. 1), which emerges from the 

 ground during the last winter months and the early spring. The 

 period in which they first appear upon the trees in Santa Clara, 

 Contra Costa, Solano, and Sacramento Counties is variable. Certain 

 sections in each territory are earlier than others and some orchards 

 are in advance of others in regard to blossoming conditions. 



In the Santa Clara Valley during the year 1909 the first adult 

 thrips were collected February 15. (See Table IV.) By February 

 18 they were quite numerous in one of the orchards under observa- 

 tion and were common in all orchards by February 25. Maximum 

 emergence began about February 19 and lasted until March 18. 

 They continued to emerge until the first three days in April. In 

 Contra Costa County first thrips were out at the laboratory February 

 12 and in the field February 16, emerging in numbers by February 

 20. Maximum emergence was over by March 15 and all were out by 

 March 27. During the season of 1910 the first thrips taken in the 

 field in Santa Clara County were observed on February 7, while the 

 first in emergence cages appeared on February 9. They were common 

 in the field from February 15 on. Thrips appeared in maximum 

 numbers from the cages (see fig. 5) beginning February 22 and 

 ending March 10, with the last stragglers coming out as late as 

 March 20. The emergence season for 1911 at first gave promise 



