THE CARROT RUST FLY 35 



Eggs hatch in 6 to 9 days under norma! conditions but required 20 days at 55° F. 

 In constant-temperature studies no larvae hatched from eggs held at 85° F., and 

 65° and 75° were the most favorable temperatures for incubation of the eggs. 



The feeding period of the larvae covers about four weeks, varying from 27.55 

 days in 1928 to 30.1 days in 1929; and approximately the same length of time is 

 spent in the soil as prepupae and pupae. In 1930 there were indications that 

 unfavorable soil conditions e.Ktended their period in the soil to seven weeks. 

 In these studies 75 to 85 percent of the flies of the first generation emerged between 

 August 10 and August 25. 



The life cycle is completed in 9 to 1 1 weeks. The time required for the develop- 

 ment of the second generation corresponds ver\ closely with the same period for 

 the first generation. No studies of a third generation have been made, but it is 

 evident that one develops in some seasons. 



Cultural practices which provide unfavorable conditions for development and 

 reproduction are important factors in controlling this pest. In Massachusetts 

 seed sown before June 1 will be attacked by the first generation, and seed sown 

 after June 15 is likely to be damaged by the second generation. The growing of 

 host plants on the same area year after year, and the growing of both earl}- and 

 late crops of these host plants near-by in the same season favor the establishment 

 and increase of the carrot rust fly, and these practices should be avoided. Moist 

 soil favors the carrot rust fly, and irrigation at the time the flies were laying eggs 

 increased the infestation. Mulch paper gave good protection on early carrots, 

 but on late carrots it was obviously the cause of a greater infestation by providing 

 protection for the flies during oviposition. Carrots grown with mulch paper 

 were 20 to 33 percent heavier than those grown without paper. Carrots grown in 

 alternate rows with onions were less infested than those grown in solid beds. 

 All infested carrots and parsnips should be pulled, regardless of their value, 

 and treated to kill any insects in them. 



Treatment of the seed with one-half its weight of calomel protected early 

 carrots from damage by the first generation of the carrot rust fly, but was not so 

 effective on late carrots against the second generation. Undiluted calomel was 

 more effective than calomel diluted with clay. 



In laboratory e.xperiments with insecticides, eggs and adults of this insect were 

 killed by derris sprays and dusts more effectively than with other insecticides. 

 Ground derris root killed all the flies in 24 hours when the flies were caged with 

 dusted plants four days after the dust was applied. 



Spraying or dusting carrots for pest control is not generally considered practical 

 or economical in Massachusetts; but where yields of 500 bushels per acre are 

 obtained, or in seasons when severe damage by this insect places a premium on 

 uninfested carrots, insecticides should be worth while. When used, insecticide 

 applications should begin about June 1 on early carrots and about August 1 on 

 late carrots. 



In the early experiments, undiluted ground derris root was the most effective 

 insecticide, but this is too expensive for use on carrots. Later, a cube-clay dust 

 containing 0.6 percent rotenone gave good results against a light infestation, and 

 its use is recommended. Mercury compounds and fluosilicate mixtures have not 

 been consistently effective, and Bordeaux-oil emulsion and nicotine were un- 

 satisfactory. Naphthalene, soot, and ground tobacco have been effective as 

 r-epellents. Naphthalene is preferred when used at the rate of 1 pound to 100 feet 

 of row, and is recommended in three or four applications on late carrots. 



