April, '14] HEWITT: THRIPS INJURING OATS 213 



grass and timothy, the "Silver top" appearance is apparently caused 

 by the thrips injuring the stems of the plants and thereby cutting off 

 the supply of sap to the inflorescence. The adults feed upon the leaves 

 and external parts of the plants; the larvse feed chiefly within the 

 leaf sheaths. This type of " Silver top " injury produced by A. striatus 

 in the grasses mentioned is totally different in nature and in manner 

 of origin from the injuries in oats which I am describing in this paper. 



Description of Injury. The injury is chiefly confined to the 

 inflorescence or panicle; the leaves of the plants were not affected to 

 any noticeable degree. The injured spikelets are usually cream-col- 

 ored, shrivelled and thin in appearance, contrasting vividly with the 

 healthy green and plump spikelets, which fact facilitates detection in 

 the field. The sterile ears vary also in length, that is, in development. 

 I have found a similar type of sterility produced by thrips (A. striatus) 

 in Stipa robusta. While the sterile spikelets are usually confined to 

 the lower part of the inflorescence, as shown in Fig. D, for the rea- 

 sons mentioned later, they may occasionally be scattered through the 

 inflorescence or occupy a terminal position (Plate 7, A and B). 



Production of Injury. On July 13, 1911, different varieties of 

 oats in the experimental plots of the Central Experimental Farm, 

 Ottawa, w^ere found showing the characteristic "blighting" of the 

 spikelets. It was possible at this period of growth to make an exami- 

 nation of a large series of plants of different varieties showing all stages 

 of growth from the young plants with the inflorescence completely 

 enclosed to the full-grown plants in which the inflorescence was fully 

 developed and extended. 



Aiiaphothrips striatus Osborn, which has been popularly named the 

 grass thrips, was responsible for the injuries which I am about to de- 

 scribe. The insects, both adult and larvse, feed chiefly within the 

 leaf sheaths; few were found externally. The inflorescence is attacked 

 before it leaves the leaf sheath w^hich encloses it. In consequence of 

 this the thrips were more abundant in proportion to the extent to 

 which the inflorescence was still enclosed by the leaf sheath ; the more 

 the inflorescence was enclosed the greater was the number of the in- 

 sects found. The adult and larval insects feed upon the developing 

 spikelets and produce complete sterility in the same by sucking the 

 ovaries and feeding on the young anthers, in some cases two or three 

 larvse could be found on the base of a single floret. No larvse were 

 found above the edge of the leaf sheath, though they were occasionally 

 found on the stem of the inflorescence between the low^er florets. The 

 injury to the florets is effected while the portion of the inflorescence 

 bearing them is retained within the leaf sheath. In cases where the 

 inflorescence had left the leaf sheath and no sign of injury to the lower 



