58 MASS. EXPERIMENT STATION BULLETIN 347 



Plantings at different periods showed great variation in the natural field infesta- 

 tion, the earliest planting being almost a total loss where no treatment was made. 



Percentage of Radishes 

 Not Infested Moderately or 



Severely Damaged 



5 92 



41 41 



20 63 



Treatments with calomel gave some protection but were less effective than on 

 cabbage and generally not commercially practical as applied. Seed treatments 

 with calomel reduced the infestation 20 to 30 percent but permitted moderate or 

 severe damage to 30 to 70 percent of the radishes. Two or three applications of 

 calomel spray (1 ounce in 10 gallons of water) at weekly intervals beginning when 

 eggs were first found reduced severe damage about 45 percent in the May 4 

 planting, but were less effective on later plantings. Due to burning and stunting 

 of the plants, corrosive sublimate solution 1-1280 cannot be used safely on rad- 

 ishes. 



Control of Onion Thrips. (A. I. Bourne.) At the outset of the growing sea.son 

 weather conditions were very favorable for a rapid growth of the onion plants 

 and at the same time served to retard the development and spread of thrips. 

 This was borne out by the excellent appearance of the onion fields throughout 

 the entire Valley up to the last of June and by the fact that at that time there 

 were practically no thrips in evidence. On the college plots, counts made on 

 June 24 showed most of the plants still uninfested, and comparatively few with 

 one or two thrips per plant. Unusually high temperature and an excess of rainfall 

 during May and June were the chief contributing factors to bring about such 

 favorable conditions. 



The early half of July, however, was characterized by extremely hot weather 

 and almost no rain. In the 5-day period from July 7 to 11, the maximum daily 

 temperature was 90° F. or above and reached 96° on the 8th. No rainfall occurred 

 during this period. More ideal conditions for the development of blast could hardly 

 have been produced artificially. Thrips multiplied rapidly during this time and 

 developed the heaviest infestation in recent years. Blast, however, overshadowed 

 all other pests. This began to appear by July 12, within ten days had spread 

 throughout the entire Valley, and by the end of the month plants in many fields 

 had broken down and were dead. 



In the experimental plots the thrips infestation was insignificant until early 

 July. On July 6 it averaged 22 thrips per plant. It increased to 40 thrips per plant 

 by July 13 and by July 19 reached 45 which represented an average of less than 

 3 thrips per linear inch of plant. By July 26, however, the response of the thrips 

 to the excessively hot weather of July 7 to 11 produced an average of 138 thrips 

 per plant. By that time many plants were killed by blast and by August 3 practi- 

 cally all of the plants were dead. 



The intervention of blast caused such a migration of thrips from dying plants 

 to those which were slower to succumb that no authentic data could be secured 

 relative to thrips-resistant varieties, although up to the time blast became prev- 

 alent both the Valencia and Riverside varieties showed less than one half the 

 infestation of Danvers and were making more rapid growth. Blast appeared on 

 these varieties as soon as on Danvers but their vigorous growth resisted its effects, 

 and as late as August 10 some plants were still alive when all the Danvers were 

 dead. 



Blast appeared so early and spread so rapidly through the experimental plots 



