THE CARROT RUST FLY 27 



carrot seed one-half pound of calomel will adhere to one pound of seed after 

 shaking together. This should be done just before the seed is placed in the planter 

 for sowing. Results are summarized in Table 24. 



Against the First Generation 



In the experimental plantings when the seed was coated with pure calomel, 

 the protection against the first generation of the carrot rust fly was consistently 

 good. Even when the natural infestation in the untreated rows was light, ranging 

 between 2 and 25 percent, the infestation in the treated carrots did not reach 

 2 percent, and represented an average gain of 94 percent in 1929. When calomel 

 was sprinkled in the seed drill as well as on the seed, the infestation was reduced 

 from 25 percent in the check to 1.5 percent; but the use of additional calomel 

 did not seem worth the extra expense on the earl\- carrots. Treatment of the seed 

 with a mixture containing 1 part of calomel and 3 parts of clay did not give con- 

 sistent protection, being very effective in 1935 but only partially so in 1936 and 

 1937. Greater dilutions of the calomel were much less effective and unsatis- 

 factory. In 1930, 1932, 1933, and 1934, when treated seed was planted, the 

 infestation in the untreated carrots was either none or so little that results of 

 insecticide treatments were valueless, especially in the early plantings. In 1933, 

 1934, and 1935, seed was treated with corrosive sublimate-clay mixture containing 

 15 and 25 percent corrosive sublimate; but this injured the seed and caused very 

 poor germination, being especially injurious in 1934. 



Against the Second Generation 



Seed treatments against the second generation have been much less effective 

 in these plantings, owing partly to a heavier infestation and partly to the longer 

 period from the time the seed was sown until the carrot was attacked. Where 

 the undiluted calomel was used, the infestation was reduced about 20 percent in 

 three of the four years when it was used. On the late carrots calomel in the drill 

 as well as on the seed was by far the most effective seed treatment, and it is val- 

 uable where a heavy infestation is anticipated. For this purpose a pound of 

 calomel for each pound of <:reated seed (in addition to the calomel on the seed) 

 should be placed in the seeder and allowed to drop out with the seed. 



Field Experiments with Contact Insecticides 



Conditions and Methods of Application 



Studies of insecticides to control the carrot rust fly in the field at Waltham 

 have been greatly handicapped by the tremendous variation in the natural in- 

 festation from year to year due to climatic and soil conditions unfavorable to the 

 development and reproduction of the insect. In 1928 when the work was started, 

 field counts showed from 70 to 90 percent moderate and severe injury to un- 

 treated carrots. In 1929 this decreased to 22 percent in the early carrots and 52 

 percent in the late carrots. In 1930, 1932, and 1934, no injured carrots were found 

 in the early plantings, making some extensive insecticide experiments worthless. 

 Between 1930 and 1937 the commercial injury to the check plantings was always 

 less than 20 percent. Under these conditions, it is evident that results of the 

 experiments are applicable only to moderate or light infestations and that the 

 effectiveness of the treatments might be considerably less in heavy infestations. 



Throughout the ten years covered in this report, the carrots used for insecticide 

 studies have been grown in blocks of 3 to 9 rows, 14 inches apart and from 60 to 

 120 feet long. 



All dusts were applied with a plunger type duster holding about four pounds of 

 dust. Application was made from one side only, and the dust was directed onto the 



