24 



MASS. EXPERIMENT STATION BULLETIN 318 



A similar field planted on the same day but located next to set onions showed 

 an infestation of 35 thrips per plant by July 7. By July 14 the plants were becom- 

 ing 1 adly scored. The maximum infestation was reached on July 28 with an 

 average of more than 80 thrips per plant. At that time the damage was so severe 

 that the field was sprayed in the hope of securing at least a partial crop. The 

 plants in this field were badly scarred and stunted. Their maximum growth 

 amounted to little more than half that of the plants in the field isolated from sets 

 and they matured a light crop of inferior-sized onions. 



Susceptibility of Different Varieties and Types of Onions to Thrips Attack 



It has been recognized for soma years that extreme differences exist in sus- 

 ceptibility of different varieties of plants to insect injury. During the past year 

 investigations of this phase of onion thrips control carried on in Massachusetts 

 were of two types: (1) a comparative study of six varieties — Ohio Yellow Globe, 

 Ebenezer, Harris No. 58, Utah Experiment Station Sweet Spanish, Harris No. 

 597 Riverside Sweet Spanish, and Winegar Strain Sweet Spanish — in their resis- 

 tance to thrips attack, and (2) a similar study of different strains of Danvers 

 Yellow Globe, the standard variety now being grown in the Connecticut Valley. 



The different varieties were all planted the same day and were so located 

 that they had equal opportunities for thrips infestation. Weekly counts were 

 made of thrips abundance throughout the season. The observations are presented 

 in the accompanying chart. 



CoMPftRFIT IVE SuSCEPT IBI LITY 



or Different Varieties of 

 Onions toThrips Attach 



CBCH CZTR 



h*»b<s sa 



HARRIS i"9 7 



\JTHH EYPrRiriCNT STftTIO 



WINrGfiR STRHIN 



From these data it will be observed thai the infestation on the two varieties, 



Utah Experiment Station and Winegar Sweet Spanish, was consistently lower 

 than on the other varieties. Harris No. 5 ( >7 ranked next, followed by Harris 

 No. 58. It may be pointed f.ut that throughout the growing season these four 

 varieties had fewer thrips per plant than was the case in an adjacent field of 



