10 



MASS. EXPERIMENT STATION BULLETIN 352 



second to the eighth inch with the greatest number being found in the fourth 

 inch. In short carrots, which were 2 to 5 inches long, averaging 3.3 inches, and 

 were also misshapen by earlier attacks of the rust fly (Plate 2), 70 percent of 

 the maggots were found in the second and third inches. 



TYPE OF PLANTS ATTACKED 



When young carrots develop tops 2 inches high and a root about }>4 inch in 

 diameter, they attract the flies for oviposition. 



After the carrots become mature or well grown and are exposed to the attack 

 of the second generation flies, the greatest injury is most likely to be found on the 

 better and more normal carrots (Plate 4). In two separate plantings in 1928, 

 severe injury by the second generation of the carrot rust fly was found in 81 and 

 83 percent of the large, well-shaped carrots; while in other plantings, in which 

 practically all of the carrots were stunted and misshapen by the attack of the 

 first generation maggots, only 34 and 48 percent were moderately or severely 

 damaged by second generation larvae. There was plenty of food in the smaller, 

 stunted carrots (Plate 4) for a much larger number of larvae, and it was apparent 

 that the flies distinguished between the quality of the carrots when laying eggs. 



SEASONAL ABUNDANCE 



The field infestation of the carrot rust fly varied each season, and, in order to 

 determine the abundance of these insects and the potential infestation, soil from 

 sections of beds where carrots were grown the previous season was examined 

 each spring. 



Samples 1 foot square and 6 inches deep were taken from the garden, both 

 where carrots were left in the ground over winter and where carrots were harvested 

 in the previous fall. Each sample was placed in a tub of water and the soil thor- 

 oughly stirred and washed until the lumps were broken. When the pupae were 

 freed from the soil, they immediately floated to the surface of the water where 

 they were collected and counted. Immersion did not harm the pupae, and many 

 of those washed out in this way were used in the life-history studies. 



The result of these collections is given in Table 5, which shows that the over- 

 wintering population in 1929 was approximately twice as large as in 1930, and 

 ten times as large as in 1931 and 1932. Table 5 also shows that if the carrots are 



Table 5. Number of overwintering carrot rust fly pupae in 1 square 



FOOT OF soil 6 inches DEEP 

 Waltham, Mass. 1929-1932. 



♦Carrots left in soil had a 74 percent infestation in 1928. 

 **Carrots pulled had a 54 percent infestation in 1928. 



