62 MASS. EXPERIMENT STATION BULLETIN 355 



the development of thrips, so that at the beginning of July fields of seed onions 

 throughout the Valley were in excellent condition and showing promise of a 

 heavy crop. 



The 10-day period from July 8 to 17 was marked by high temperature and very 

 little rainfall. These conditions gave considerable impetus to the seasonal devel- 

 opment of thrips, and the infestation became general and increased rapidly in 

 severity. Fortunately the plants by that time had made excellent top growth 

 and no serious damage was caused. In the experimental plots the infestation 

 reached its maximum on July 26 with an average of 108 thrips per plant, and 

 declined rapidly from that point. 



Blast appeared soon after the period of very warm weather which culminated 

 in the 87° maxinmm on July 17, and it spread rapidly throughout the Valley. 

 While the thrips were not abundant enough to cause serious damage to normal, 

 healthy plants, their presence in fields stricken b>' blast served to intensify the 

 damage and hasten the death of the plants. 



Field tests were made of a commercial spray containing 2 percent rotenone; 

 ground derris and cube, each with a 4 percent rotenone content; a pyrethrum-sul- 

 f ur compound ; a fixed nicotine-oil combination ; and the standard nicotine sulfate- 

 soap mixture. In addition 3 dusts were tested: a pyrethrum-sulfur mixture, 

 cube-vatsol, and dual-fixed nicotine. Two applications were made 7 days apart. 

 The relative efTectiveness of these materials is shown in the following table. 



Average Number of Thrips per Plant 



7 days 

 Treatment Before 1 day after 1st 1 day 



1st after 1st and just after 2d 



treatment treatment before 2d treatment 



treatment 



Sprays 



Commercial 2 percent rotenone. . . 67 18.4 20.7 2.5 



Derris 34 3.4 8.7 1.6 



Cube 69 3.5 9.2 11.1 



Ortho nicotine-oil 56 8 8.1 1.2 



Pyrethrum-sulfur 60 9.1 13 3 



Nicotine sulfate-soap 54 0.6 8.7 0.5 



Dusts 



Pyrethrum-sulfur 71.5 31.5 15.3 1.6 



Cube-vatsol 51 19.5 26.5 6.6 



Dual-fixed nicotine 49 7.9 22.7 4.5 



Check 54 — 25 — 



Superior coverage and penetrating qualities made the sprays more effective 

 than the dusts, although dual-fixed nicotine compared very well with the control 

 furnished by the sprays. Dusts also showed little residual effect or retardation 

 of reinfestation. Nicotine sulfate was superior to other sprays in its immediate 

 effects in both applications. The rotenone sprays were somewhat slower in 

 killing action, but showed a pronounced residual effect which checked reinfesta- 

 tion. The ortho nicotine-oil combination was superior to all others in its pro- 

 longed protection of the plants from attack, as shown in the table. 



Studies on varietal resistance of different strains of onions to thrips attack 

 were cut short by the early appearance and rapid spread of blast in the area where 

 the variety plots were located. Selzers Sweet Spanish, Nebraska, Grano, and an 

 unnamed variety designated McL No. 1, were noticeably resistant to thrips 

 attack with an average of only 6 to 9 thrips per plant, while the infestation on 

 less resistant varieties varied from 70 to 80 thrips per plant. Also Nebraska 



