58 MASS. EXPERIMENT STATION BULLETIN 369 



apparently due to the beneficial effects of a fungicide in two of the dusts, and to 

 the fact that with favorable growing conditions the damage caused by an in- 

 festation averaging about three borers per vine in August does not materially 

 reduce the yield of marketable squash. 



Control of Onion Thrips. (A. I. Bourne.) Weather conditions in 1939 were 

 not so favorable for the onion crop as in the previous year. The first part of the 

 season was about 7 to 10 days later than normal. Seed onions were late in appear- 

 ing and made slow progress during the early summer. Thrips were correspond- 

 ingly late in making their appearance even on set onions and increased but little 

 throughout June and early July. The drought which persisted from late June 

 until the closing days of July, coupled with the high temperature and frequent 

 hot, drying winds, combined to cause onions, as well as most other field crops, to 

 suffer severely. Unfortunately these conditions were also ideal for the rapid 

 development of thrips, so that by the third week of July the insects were increasing 

 rapidly, and by the end of the month there was one of the heaviest infestations of 

 recent years. 



In the experimental plots in early July the plants were only 6 to 8 inches high 

 and showed an average infestation of only 5 to 6 thrips per plant. A 7-day period 

 of extremely hot weather from July 4 to 10 stimulated the development of thrips 

 and more than doubled the infestation. The initial infestation was, however, 

 so low that this increase was not serious although the drought was slowing down 

 the growth of the plants. From July 24 to 29, however, there occurred a second 

 period of abnormally' high temperature, and development of thrips reacted quickly 

 to bring the infestation to the peak of its abundance in late July and early August 

 when the plots showed an average infestation of 132 to 134 thrips per plant. 

 This heavy infestation and the continued drought, which persisted with only 

 slight relief until the heavy rain of August 4, caused serious injury to the crop 

 and led to premature death of the plants. 



Studies of insecticidal control included field tests of the following sprays: 

 pyrethrum-oil, rotenone-oil, nicotine-oil, derris alone and with talc or cherokee 

 clay as adhesives, a pyrethrum-sulfur mixture, and the standard nicotine sulfate— 

 soap combination. Dust applications were made with pyrethrum-sulfur and with 

 calcium cyanamid. 



All sprays containing nicotine or rotenone gave good to e.xcellent control and 

 were noticeably superior to pyrethrum combinations. The efTectiveness of 

 derris was somewhat improved by the addition of talc or clay. Derris also showed 

 a marked residual effect which retarded reinfestation. Calcium cyanamid applied 

 as a light dust to plants and soil caused a great reduction in the population of 

 thrips but unfortunately killed the plants. When this material was used in an 

 amount that was nontoxic to the plants, it failed to kill thrips. The nicotine 

 sulfate-soap combination was again superior to all other sprays or dusts used, 

 giving 95 percent effective control, and appears to be the most effective spray 

 for this particular species. 



The Spray Residue Problem. (A. I. Bourne ) The drought of midsummer 

 which was not compensated for by the early autumn rains made the spray residue 

 problem a serious one in most orchards, and only by employing a dust schedule 

 or by a strict adherence to the recommended spray schedule with its progressively 

 lighter dosage of lead arsenate in the late sprays was trouble avoided. Increasing 

 abundance of codling moth and apple maggot in late summer made the problem 

 more difificult, in spite of the new limits of tolerance announced in 1938. Records 

 taken at the college showed that the deficiency in precipitation for the months 

 of July, August, and September up to the time the Mcintosh crop was harvested 

 was nearly 3}^ inches, and from the date of the first cover spra>- in eaily June the 



