28 



MASS. EXPERIMENT STATION BULLETIN 352 



crown of the carrot and the soil immediately adjacent to it. On small seedlings 

 less than 2 inches high, the dust was usually applied from above and the foliage 

 of the plant was covered, but on the larger plants no attempt was made to cover 

 the tops thoroughly. The insecticidal dusts were used in these experiments at 

 approximately Y^ pound to 100 feet of row or about 75 pounds per acre. More 

 dust was used in the late applications to larger plants, but no comparison of heavy 

 and light dosages was made. 



Sprays were applied from compressed-air knapsack sprayers with the nozzle 

 held close to the ground so that the crown of the plant and the adjacent soil was 

 thoroughly wet. Materials for spraying were less expensive than for dusting, but 

 application with the necessary thoroughness was very laborious and impractical 

 on large plantings. 



In summarizing the results of treatments which were applied in more than one 

 season, the percentage of carrots injured is averaged for all seasons in which the 

 treatment was used. Likewise, the percentage gain over check is calculated from 

 the average injury to the treated carrots and the average infestation in the un- 

 treated carrots for the corresponding years. 



Time of Application 



Insecticides for combating the carrot rust fly should be applied while the 

 flies are active and eggs are being laid. Based on the life history studies at 

 Waltham, the first insecticide treatments on early carrots have been applied dur- 

 ing the last week in May or the first week in June. Because of the variation in 

 spring development from year to year, the emergence period of the flies should 

 be determined; but in the absence of such information, an average date for the 

 first application of insecticides is June 1. Three or four applications at approx- 

 imately weekly intervals appear to be necessary, and a heavy rain soon after 

 treatment usually necessitates the repetition of the application. 



Table 25. Time of application of insecticides and repellents in field 

 experiments for control of carrot rust fly 



Waltham, Mass. 1928-1937. 



Year- 



DATE OF APPLICATIONS 



First 



Second 



Third 



Fourth 



Fifth 



*Extra application August 8. 



