ANNUAL REPORT, 1933 31 



Weather conditions during June and July proved to be unusually favorable 

 for the rapid multiplication of thrips. Weather during June was considerably 

 warmer than normal, with very high temperatures on several days. With the 

 exception of two heavy storms, the precipitation of the month was comparatively 

 insignificant. July with its hot weather and accompanying drought was even 

 more favorable for the development of thrips. This and the early maturing and 

 harvesting of sets resulted in an unusually heavy infestation on seed onions. 

 Although the peak of abundance occurred at the usual time (the last week of 

 July and first of August), the rise to this point was more gradual and began much 

 earlier than normal. Consequently seed onions particularly were subject to 

 an unusually heavy infestation over a long period and the damage was correspond- 

 ingly heavy. 



While the sprays applied for thrips control gave as high a percentage of kill 

 as formerly, migration from surrounding fields was so constant that heavy rein- 

 festation took place very rapidly, demonstrating that in any similar season at 

 least one and probably two extra applications would be necessary to give protec- 

 tion over the unusually long period of danger. 



Several insecticides and combinations were tested and their effectiveness 

 compared with the standard nicotine sulfate-soap combination. The best 

 control was obtained from the nicotine sulfate-fish-oil soap spray. Lethane 

 and Penetrol proved to be a close second in effectiveness. Its cost is, however, 

 somewhat prohibitive. 



The plots treated with naphthalene flakes showed comparativ^ely slight damage 

 from thrips, and this treatment appears to be very promising. The combined 

 spray of Paris green and molasses, so successful in the control of gladiolus thrips, 

 caused considerable injury to onion foliage after each application. The material 

 DAO, a dry powder, broadcasted along the rows checked the thrips but its cost 

 is prohibitive for use in large acreages. The pyrethrum dust was shipped so 

 late that only one application could be made. The results were so promising 

 it is worthy of further study. 



The fungous disease of thrips which was so generally distributed throughout 

 the Valley during 1932 and caused a rapid decrease in the numbers of thrips 

 was present this season, but was much less abundant and was later in appearing. 

 Its appearance coincided with the normal drop in thrips abundance which occurs 

 early in August, so that it was of much less benefit than in 1932. It was, however, 

 present over practically the same area as in the preceding year, and leads to the 

 hope that it might survive an unfavorable season and build up a heavy infection 

 in a year when conditions were more favorable. 



The Spray Residue Problem. (A. I. Bourne.) The recent activity of govern- 

 mental agencies and the establishment of limits of tolerance for lead and for 

 fluorine compounds upon harvested fruit and vegetables, in addition to those 

 already fixed for arsenic, have combined to reemphasize this problem and nec- 

 essitate additional investigation. 



The limit of tolerance first proposed for lead was so low that further study o' 

 arsenical residue became necessary, and no tolerance on fluorine residue was 

 recognized. These regulations were subsequently so modified for the 1933 season 

 that drastic changes in the spray schedule were not necessary, and a limit of 

 tolerance for fluorine residue, equal to that for arsenic, was established. 



The strict limits first proposed for lead residue necessitated a study of possible 

 substitutes for lead arsenate. Tests were made of seven different brands of 

 calcium arsenate, a manganese arsenate, and a zinc arsenate. All of these caused 



