THE CARROT RUST FLY 



21 



In 1931 and 1933 the effect of moisture was demonstrated in plantings in which 

 a portion of the carrots was kept wet during the oviposition period of both the 

 first and second generation flies by means of an overhead irrigation system. 

 Although the field infestation was extremely light, the greater number of infested 

 carrots was found in the irrigated part of the planting as shown in Table 19. 



Mulch Paper 



From 1928 to 1931 considerable interest was shown in the stimulating effect 

 on the growth of vegetables of paper used as a mulch (14). The paper used for 

 this purpose was impervious and coated with asphalt which gave ofT an odor 

 distinctly repellent to insects. 



In these experiments with carrots the seed was sown in rows 14 inches apart, 

 and mulch paper 12 inches wide was laid between the rows, leaving a 2-inch 

 strip uncovered along the row. In most of these plantings the paper was laid 

 when the carrots had grown about an inch high, or, at least, when they had 

 grown sufficiently to mark the rows distinctly; but in a few of the plantings 

 the paper was laid at the time the seed was sown. In 1928 several rows of carrot 

 seed were sown on November 23 and paper was laid between the rows about 

 Mav 1. 1929. 



Table 20. Effect of mulch paper on infestation by the carrot rust fly 

 Waltham, Mass. 1928-1931. 



As shown in Table 20, the mulch paper gave considerable protection from first 

 generation injury, especially in 1928 and 1929, but there was more injury by the 

 second generation where the paper was used. Apparently, the repellent odor of 

 the asphalt provided the protection against the first generation of the flies, but 

 the odor had disappeared by August 1 when the flies of the second generation 

 were ovipositing, and instead of repelling the flies the paper actually provided 

 a protected, favorable location in which to lay eggs. A comparison of carrots 

 grown with and without paper is shown in Table 21. 



Mulch paper is too expensive and its use too difficult to be practical in com- 

 mercial carrot plantings, but it may be satisfactory in the home garden where 

 the cost and labor are less important. 



In connection with the studies of mulch paper as a repellent to the carrot 

 rust fly, observations made on the stimulation to growth by this paper showed 

 a gain of 20 to 33 percent in the weight of carrots grown under paper; but this 

 advantage does not seem enough to offset the cost and labor involved in using it. 



