214 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 7 



spikelet was noticeable only an occasional thrips could be found, but 

 "where the inflorescence had not left or had only partially left the leaf 

 sheath, in every plant I examined thrips were found in varying num- 

 bers; their activity on disturbance made it impossible to count them. 



It sometimes happens that when the inflorescence is emerging from 

 the leaf sheath, the tip of the inflorescence is accidentally retained in 

 the leaf sheath while the middle portion continues to unfold, the in- 

 florescence assuming the shape of a mark of interrogation (?) with the 

 distal extremity fixed. When this occurs the distal portion of the 

 inflorescence is usually attacked by the thrips within the leaf sheath 

 with the result that when it is ultimately withdrawn the terminal 

 spikelets are sterile (Plate 7, Fig. 1, B). 



A study of the different varieties of oats showed that those varieties 

 in which the inflorescence left the leaf sheath earlier suffered less from 

 thrips injury, that is they had fewer sterile spikelets at the basal region 

 of the inflorescence, than the varieties in which the inflorescence was 

 later in leaving the leaf sheath. I have- already referred to the simi- 

 lar observation of Zimmermann in the case of Thrips cerealium. 



Extent of Injury. In order to determine the actual extent of the 

 injury, as field estimates are rarely within a considerable percentage 

 either way of the actual injury, I made actual counts of the sterile 

 and fertile spikelets on each inflorescence of stems gathered from dif- 

 ferent experimental plots. From these counts the actual percentage 

 of sterile spikelets was obtained. 



With a view to eliminating the possible chance of error due to per- 

 sonal selection of the plants from which the counts were to be made, 

 I had the plants collected from different plots by a person who was 

 unaware of the purpose for which they were intended. 



The following figures ^ give the percentage of sterile florets found on 

 the stems of the oat plants of the different varieties thus collected:— 



"Excelsior." Outside plot, outside row. 12 stems counted. 



Maximum sterile spikelets 52 . 2% 



Minimum sterile spikelets .^ 4.1% 



Average sterile spikelets 30 . 9% 



"Abu7idance A." Outside plot, outside row. 13 stems counted. 



Maximum sterile spikelets 42.2% 



Minimum sterile spikelets 15 . 6% 



Average sterile spikelets 25 . 6% 



Outside plot, inside row. 14 stems counted. 



Maximum sterile spikelets 43.4% 



Minimum sterile spikelets 16 . 8% 



Average sterile spikelets 26 .2% 



>As there is no real necessity for setting out the figures in detail, the maximum, 

 minimum and average percentages only are given, but I shall be pleased to give the 

 -jdetailed counts to any investigators to whom the same would be of value. 



