THE CARROT RUST FLY 29 



The first application for late carrots should be made about August 1. The 

 activity of flies of the second generation usually continues throughout the month 

 of August, and consequently four applications at about weekly intervals are 

 advisable. When volatile materials such as naphthalene are used, the applica- 

 tions should be repeated as soon as the crystals have evaporated, and in very 

 warm weather this may occur in less than a week. The dates when insecticides 

 were applied during these studies are shown in Table 25. 



Discussion of Materials 



With derris, cube, pyrethrum, calomel, fluosilicates, oil emulsions, and nicotine, 

 the flies are killed by contact at the time of application, or by creeping through 

 the insecticide while it is fresh on the plants. To a less extent, also, the flies are 

 repelled and the eggs and newly hatched larvae of the carrot rust fly killed by 

 contact with these materials. 



Derris, Cube, and Pyrethrum. — Derris and cube have been the most con- 

 sistently effective materials, especially to control the first generation flies on 

 early carrots. Good results with derris were also obtained by Gorham (corres- 

 pondence) in New Brunswick, Canada. Derris and cube, which contain rotenone 

 and other toxic resins, are primarily fish poisons, being relatively nonpoisonous 

 to warm-blooded animals but toxic to insects and other cold-blooded animals. 



In the first work with derris, the undiluted derris root was superior to any 

 mixture of it with gypsum. In fact, the greatest commercial damage by the 

 flies where this treatment was used was 13.2 percent in 1928, which represented 

 a gain of 64.4 percent over the check; and the average loss in 3 years was only 

 7.73 percent as shown in Table 26. However, ground derris root costs at least 

 40 cents a pound and undiluted it is much too expensive for commercial use on 

 carrots. Since 1933, derris and cube powders have been improved by careful 

 handling and grinding. In 1937, four applications of a cube-clay dust containing 

 0.6 percent rotenone gave complete protection from a 14 percent infestation in 

 an adjacent check; and in 1932 and 1933 a similar commercial dust containing 

 0.55 percent rotenone permitted an average damage of only 3.5 percent, which 

 represented the greatest gain from any treatment with a contact insecticide 

 against the second generation. This dust is now readily available for about 13 

 cents a pound, and used at the rate of 50 to 75 pounds per acre it would cost from 

 $6.50 to $10 for each application. This e.Kpense has not been justified in the last 

 few years; but with a heavier infestation and a yield of 500 bushels or more per 

 acre, it might be practical on early carrots. On small plantings and home gardens 

 it can be recommended. 



Sprays containing rotenone have been quite effective but in most cases are more 

 difficult to apply. 



Pyrethrum as a dust or spray was less effective than derris or cube. 



Mercury and Fluosilicates. — Corrosive sublimate solution, prepared by dis- 

 solving 1 ounce of corrosive sublimate in 73^ gallons of water, has given mod- 

 erately good control in experiments, and has been found effective by Caesar (3) 

 in Ontario. However, a stunting of the growth from this treatment is reported 

 by Caesar and has been observed by the writer. At Waltham, this material also 

 caused rough, blotchy areas on the skin of the carrot, which have been referred 

 to as mercury russet. 



A spray of calomel (mercurous chloride), 1 ounce in 5 gallons of water, did not 

 stunt the growth of the carrots but failed to control the pest with three applica- 

 tions in 1929. Dusts containing 2 and 4 percent calomel in lime or gypsum gave 

 fair commercial control of light infestations in 1930 and 1933, but did not control 

 a heavier infestation in 1929. 



