216 MASS. EXPERIMENT STATION BULLETIN 280 



sets were maturing and shortly after the field to the south was mowed and hay 



removed, showed the following infestation. 



Thrips per 

 Section of plot 100 plants 



North side, adjoining sets 1900 



South side, adjoining grassland 1760 



Center 600 



As the season advanced the gradual spread of the insects from each border 



had resulted in a heavy and more nearly uniform infestation throughout the 



entire field, as shown by the following. 



Thrips per 

 Section of plot 100 plants 



North side 4230 



South side 3120 



Center 2175 



Applications of the nicotine-soap combination were sufficient to check thrips 

 on seed onions exposed to infestation from sets. This was due in large part to 

 the early maturity of the sets and the fact that the principal migration of thrips 

 took place somewhat earlier than normal. Infestation was reduced from an 

 average of 1200 thrips per 100 plants to 125 thrips per 100 plants in one field 

 following an application of the above spray. A similar application in a second 

 field reduced the numbers of thrips from an average of 3250 per 100 plants to 

 190 per 100 plants. 



Predacious insects were present in such insignificant numbers that they played 

 little or no part in checking thrips infestation. 



The study of the effect of sprays and dusts on onion blast was continued in 

 cooperation with the Department of Botany, and the work has been reported in 

 Bulletin 279. 



The Spray Residue Problem. (A. I. Bourne). Abundant rainfall through- 

 out the summer and early fall for the most part automatically disposed of the 

 problem of spray residue this past season. Most of the excess precipitation 

 occurred in late August between the last applications of spray or dust and har- 

 vest. Mcintosh and later varieties were practically free from residue, even in 

 those orchards where late July sprays were applied for the control of apple 

 maggot. 



Further studies of the value of dusts for the control of late season pests such 

 as apple maggot, late codling moth, etc., again emphasized the advantage of 

 dusts over sprays for late summer use. Timeliness is a factor of major impor- 

 tance at that season. Dusts allowed a nmch more flexible schedule than did 

 sprays. This advantage, coupled with the fact that dusts may be safely used 

 during August within two to three weeks of harvest, rather more than offsets 

 the undeniably superior persistence of sprays. 



Systematic Study of Oil Sprays. (A. I. Bourne in cooperation with the 

 Department of Chemistry). Nine san;ples of oil sprays prepared by the Chemis- 

 try Department were tested in the orchard to check their effectiveness agninst 

 European red mite and toxicity to plant growth. The samples consisted of three 

 different types of light lubricating oils, each combined with three different 

 emulsifying agents. All of the samples went into solution readily, and once 

 diluted formed a stable solution with no tendency for separation of oil, even upon 



