50 MASS. EXPERIMENT STATION BULLETIN 339 



matured on the vines which live from the nodular or secondary roots after the 

 records are taken. 



Control of Onion Thrips. (A. I. Bourne.) Although weather conditions 

 during the late spring and early summer were favorable for thrips development, 

 the insects were late in appearing and attained only moderate numbers during 

 the season. Many fields throughout the Connecticut Valley were comparatively 

 free from thrips and no instances of severe injury were noted. The submergence 

 of much of the onion-growing area under several feet of water and the erosion 

 caused by the flood in March apparently had little effect upon the hibernating 

 thrips. The insects appeared at approximately the same time and in the same 

 abundance in such areas as in fields above the flood level. 



In the experimental plots the infestation averaged only 4 thrips per plant on 

 July 7 and had increased to only 14 per plant by July 20. The peak of abun- 

 dance occurred from July 27 to August 1 with an average of 40 thrips per plant, 

 or less than 2 thrips per linear inch of plant growth. The numbers dropped 

 rapidly from that date until August 18 when there was a slight increase due to 

 the fact that most of the fields had been killed by blast by mid-August and 

 thrips had necessarily migrated to the comparatively few plants remaining alive. 



The materials tested did not show to best advantage because the infestation 

 in the experimental plots was so light, and none of them proved equal to the 

 nicotine-soap combination in their immediate effects. The rotenone compounds 

 however, showed a very pronounced residual effect over a 7-day interval, as 

 shown in the following table. 



Average number of Average increase in 



Trpatment thrips per plant 1 number of thrips 



day after application during 7- day interval. 



Derris (rotenone) 12.2 8 



Niagron (rotenone) 17.9 7.7 



Nicotine tannate 12.8 25.8 



Oil-Nicotine 17.1 2.9 



Nicotine sulfate-Soap 3.9 17.4 



Unsprayed 22.8 18.4 



The oil-nicotine combination was somewhat slow in becoming effective. It 

 apparently left on the plants a residue that was toxic to thrips and gave greater 

 protection against reinfestation than any material that has yet been tested. 



The thrips-resistant varieties selected for study the past season were grown 

 from seed secured through local distributors and are therefore readily available 

 at the present time. Of the six varieties studied, four proved to be as readily 

 attacked by thrips as did Dan vers Yellow Globe, the standard variety. Two 

 varieties of Sweet Spanish onions, however, showed a consistently lighter infes- 

 tation throughout the season, as shown in the following table. 



Average number of thrips per plant 



Date 



Danvers Yellow Sweet Spanish Sweet Spanish 

 Globe Valencia Riverside 



July 14 10.4 8.5 6.4 



July 21 14.6 4.7 4.2 



July 27 43.4 4. 3.5 



Aug. 4 14.1 6.8 4.6 



Aug. 11 13.1 14.4 2.1 



