14 



BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE. 



not entirely destroyed. Plate III, figure 1, shows a mature Bartlett 

 pear the one-sided appearance of which was caused partly by adults 

 and partly by larvae. When thrips are more numerous a greater 

 amount of the bud surface is injured, consequently there is a greater 

 loss of sap. If this loss is sufficient to cause the cluster buds to 

 "bleed" (sap to drop from the end), fermentation quickly sets in 

 and the entire cluster is soon destroyed. (See fig. 3, in comparison 

 with fig. 2, which shows the cluster buds developing normally.) In 

 many cases blue molds gain a foothold in this fermenting sap and 



greatly accelerate the injury, 

 causing complete destruction 

 of all fruit buds. The dead 

 clusters later dry up without 

 opening. (See PI. Ill, fig. 1, 

 and compare it with PI. Ill, 

 fig. 2, which is from a photo- 

 graph of the sprayed portion 

 of the same orchard, taken on 

 the same day.) These dead 

 buds may remain on the trees 

 for months unless washed off 

 by rain or blown by winds. 

 The writers have seen many 

 orchards so severely injured 

 that it was difficult to find a 

 single healthy blossom, and 

 the entire orchard from a dis- 

 tance presented at blossoming 

 time a brownish color and 

 dead appearance, due to these 

 blasted buds. 



Weather conditions influ- 

 ence to a great extent the de- 

 struction following the injury 

 caused by the thrips. For 

 instance, the weather of 1909 

 hi the interior valleys during late February and the first 20 days 

 of March was open and comparatively dry, with more or less 

 wind blowing, giving quick evaporation throughout the day. Many 

 clusters of buds that were kept under observation throughout the 

 season, with from 10 to 20 thrips in the cluster, developed many of 

 then buds and produced fruit, a large percentage of which was first 

 class. During this period for 1910 there was considerable rain and the 

 atmosphere was warm and humid with very light evaporation. From 

 many observations in Contra Costa and Solano Counties it was shown 

 conclusively that in every case where as many as 10 to 15 thrips 



Fig. 2. — Cluster buds of Bartlett pears developing 

 normally. (Original.) 



