ANNUAL REPORT, 1934 43 



control of the onion thrips and also because of the fact that a resistant strain of 

 onions would probably offer an excellent means of control of this pest, investiga- 

 tions were begun on several thrips-resistant varieties of onions this year. Through 

 the courtesy of investigators at Cornell University, six varieties of onions were 

 secured, including Utah Experiment Station Sweet Spanish, Winegar Strain 

 Sweet Spanish, Harris 597 Riverside Sweet Spanish, Harris No. 58, Ohio Yellow 

 Globe, and Ebenezer. The onions were planted in 13-inch rows about May 15. 

 Due to the late development of thrips, counts were taken first on July 16 and at 

 weekly intervals thereafter. The average number of thrips per plant is recorded 

 in the following table. 



Average number of thrips per plant 

 Variety July 16 July 23 July 31 Aug. 6 Aug. 14 Aug. 20 



Utah Exp. Station Sweet 



Spanish 14.2 18.6 14.6 9.0 13.0 14.0 



Winegar Strain Sweet 



Spanish 4.2 12.4 12 9.2 12.0 15.4 



Harris No. 597 Riverside 



Sweet Spanish 10.4 14 23.4 14.0 18.0 49.0 



Harris No. 58 Prizetaker.. 17 20.6 53.6 35.4 47.0 41.0 



Ebenezer or Japanese 23.6 44.4 77.8 52.0 62.0 71.0 



Ohio Yellow Globe 14.4 40 74.4 57.6 91.0 70.0 



It is quite apparent that all of the Sweet Spanish type of onions consistently 

 showed the smaller number of thrips. The population curve of the Ohio Yellow 

 Globe was very similar to that obtained over a period of years for the Dan vers 

 Yellow Globe so commonly grown here in the Connecticut Valley. 



The type of growth of the different varieties would appear to have a direct 

 correlation with the abundance of thrips. The varieties Ohio Yellow Globe and 

 Ebenezer, both of which show a marked susceptibility to thrips injury, seem to 

 offer the most favorable condition for protection of the thrips from adverse condi- 

 tions. In the case of the variety Ebenezer, it was noted that the leaves have a 

 strong tendency to curl, thus offering much more protection to the thrips than 

 the markedly upright type of growth shown by Winegar Strain Sweet Spanish. 

 The type of growth in the variety Ohio Yellow Globe is such that the growing tip 

 (the chit) is more or less completely enclosed by the older leaves. In some strains 

 grown by the Department of Agronomy of this station, it was noted, in an exam- 

 ination of three strains which otherwise had the same characteristics, that two of 

 the three plots had an open chit. The average number of thrips per plant for these 

 plots was as follows: 



Plants with Open Chit Plants with Tight Chit 



Average number Average number 



Date Plot of thrips Plot of thrips 



Aug. 1 B1+B2 16 B3 103 



Aug. 8 B1+B2 26 B3 05 



Aug. 14 B1+B2 40 B3 119 



From these data it would seem that the open type of growth is a character to 

 be sought for in breeding strains of onions resistant to thrips. Whether or not 

 the character of an open chit is of value in a thrips-resistant strain, however, it 

 is apparent that this type of growth is desirable if the thrips are to be controlled 

 by either a dust or a spray. With the open type of growth, the growing tip (where 

 the majority of young thrips are usually found) can be kept covered with an in- 



