EFFECT OF SOIL APPLICATIONS OF INSECTICIDES 15 



difference in yield between treatments. Poor yields were obtained 

 in all plots in 1950 owing to a saprophytic growth on the leaves. 5 

 The yields of marketable tubers were significantly lower in the 

 check plots owing to wireworm injury. Injured tubers were also 

 noted in the plots treated with ethylene dibromide. 



To determine the amount of damage, a 20-pound lot of potatoes 

 from each plot was inspected and graded on the basis of the num- 

 ber of entrance holes per tuber. The extensive damage is difficult 

 to explain considering the excellent control of wireworms that has 

 been obtained throughout Ventura County with lower dosages of 

 ethylene dibromide. It appeared that the wireworms were deep 

 in the soil at the time of fumigation and were out of range of the 

 vapor. The other insecticides effectively controlled the wireworms 

 and produced potatoes of good quality. 



Periodic observations of the plants in all plots throughout the 

 1951 season did not reveal any striking differences in the size or 

 appearance of the potatoes. Nevertheless, there was a significant 

 reduction in yield in the toxaphene plots after a total of 60 pounds 

 of toxaphene per acre had been applied. There was no wireworm 

 damage to the 1951 crop of potatoes, because the soil in the entire 

 experimental area had been refumigated with ethylene dibromide 

 prior to planting (table 5). 



RESIDUE ANALYSES 



For the study of the possibility of the absorption and transloca- 

 tion of the toxaphene and aldrin into the edible portions of the 

 crop, random samples were taken from each plot and pooled to 

 make one composite sample for each treatment. In addition, soil 

 samples for residue analysis were taken with a soil auger 1 inch 

 in diameter. A total of 12 borings 8 inches deep were taken at 

 random in each replicate, or 72 per treatment. Check samples 

 were taken in a windbreak adjacent to the treated plots. The soil 

 samples and vegetables were submitted for analysis to the chemical 

 firms that supplied the insecticides. 



Toxaphene 



The analyses for toxaphene residues were made by the Her- 

 cules Powder Company. Their procedure in preparing vegetables 

 and soils for toxaphene analysis was as follows : The shelled lima 

 beans were ground in a food chopper and extracted with benzene. 

 The sweetpotatoes and potatoes were scrubbed, peeled, diced, dried 

 in a forced-draft oven at 70° C, and extracted with benzene; the 

 peels were analyzed separately from the inner portions. The soil 

 was air-dried and extracted with a 2:1 mixture of benzene and 

 isopropanol; the organic-chlorine content of the extract was de- 

 termined in the usual way by treatment with metallic sodium fol- 



5 J. B. Kendrick, Jr., of the University of California, diagnosed this growth 

 as due to the organism Stemphyllium. 



