258 MASS. EXPERIMENT STATION BULLETIN 271 



I'JoO, Wealtliy and siiiiilnr varieties shoiiid not he sprayed later tlian about 

 July 10. If further protection is necessary, dusts siiould be used. Mc- 

 intosh sprayed as late as the third week of July showed a sutficient mar- 

 gin of safety unless heavy, drenchin.tf api)lioations were made. Baldwins 

 could be sprayed through July with safety. In all cases where sprays are 

 jil^plied after July l-i, any spreading- or sticking agent should be omitted 

 and lighter strength of lead arsenate should be used. 



Results showed that dusts, if properly applied to Mcintosh or Baldwin, 

 could be used as late as August 15 without encountering any difiiculty. 



The Biology and Control of the Carrot Rust Fly. (W. D. Whitcomb. 

 Waltham). The average over-wintering population of the carrot rust fly 

 at Waltham in 1930 was 39 pupae per square foot where infested carrots 

 remained in the soil, compared with 79 per square foot in 1929, and this 

 should have produced a normal moderate infestation in early carrots. 

 However, the injury by the first generation was negligible, apparently 

 due to tiie dry, iiot weatlier during the oviposition period in early June. 

 Under laboratory conditions twenty eggs kept in the greenhouse at tem- 

 peratures always above 70° failed to hatch, while a similar number of eggs 

 in a root cellar maintained at 60°-70° F. hatched perfectly in twelve and 

 thirteen days. 



The normal second generation infestation was consequently very light, 

 but a moderate amount of injury was caused in October by a combination 

 of third generation and late second generation maggots. Flies emerged as 

 late as November 10. 



Carrots planted May 1 or later were free from injury by the first genera- 

 tion, and the variety plantings showed no definite degree of immunity be- 

 cause of the general light infestation. 



Early carrots were very free from injury when grown with or without 

 paper mulch, but those under paper were 20 per cent heavier. 



The results of the insecticide treatments in the field were inconclusive 

 because of the light infestation. Carrots grown from seed treated with 

 calomel showed considerable mercury russeting. 



Derris dust, both pure and diluted 1-2 with gypsum, was very effective 

 in killing the adult flies, and prevented oviposition, while eight other ma- 

 terials were much less etfective, in the laboratory. 



All larvae in infested harvested carrots were killed by fumigation with 

 paradichlorobenzine and carbon disulfide. The carbon disulfide used at 

 the rate of 4 ounces for each 100 cubic feet for eighteen hours was the 

 better treatment. 



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

 tiic I")epartmcnt of Chemistry). During the winter, laboratory tests 

 were made of the stability of samples of oil sjirays prepared by the 

 Chemi.stry Department, to determine the relative value of different emul- 

 sifying agents as well as of various proportions of oil and emulsifier. 



Three samples of oil sprays were submitted for preliminary orchard 

 tests. These consisted of a miscilile oil (83% per cent oil), and two types 

 of oil emulsions (66 per cent oil). Two of these gave iiromising results 

 and indicated the direction for further development of the work. 



Apple Maggot Control. (A. I. Bourne). The serious nature of this 

 pest, its abundance in recent years, and the threatened loss of foreign 



