12 CIRCULAR 862, U. S. DEPARTMENT OF AGRICULTURE 



hydrocarbon insecticide was used or required for insect control 

 on any of the plots. 



The seedlings or small plants were usually harvested from an 

 entire bench or set of benches at the time the plants on the check 

 plots were large enough to become a little crowded in the rows. 

 Total number and weight of seedlings per row per plot were 

 recorded, and notes were made of any visible symptoms asso- 

 ciated with treatment. Original stand counts were made soon 

 after seedling emergence and at time of harvest. With the 

 exception of potato tubers all plants were lifted from the soil 

 with a hand trowel, the soil was shaken from roots, and the entire 

 plant weight was taken. This procedure facilitated examination 

 of the roots for any effects of the DDT treatment. 



Any given operation for a single species or variety in an entire 

 bench was carried out by a single operator to minimize error, 

 but despite precautions some instances of inequalities in water- 

 ing or localized attacks by pathogenic organisms caused undesir- 

 ably large experimental errors. To correct partially for loss of 

 plants by damping-off, the yields were expressed as weight per 

 plant. 



Several tests were conducted by methods as described above 

 to compare the relative toxicity of the principal constituents of 

 technical DDT: o,p' DDT, p,p' DDT, p,p' TDE. These substances 

 were each applied to duplicate plots at rates per acre correspond- 

 ing to the approximate amounts present in 100 pounds and in 

 400 pounds of technical DDT. (See p. 20 and table 4 for the 

 amounts.) Other duplicates received a corresponding amount 

 of bis(p-chlorophenyl) sulfone from 100 pounds of DDT, and a 

 last treatment involved the sulfone and the o,p' DDT equivalent to 

 400 pounds of DDT per acre. 



The tests on substances other than DDT were confined to the 

 Chester loam, and w T ere on a far less extensive scale. Usually 

 only two benches were used for a given series of rates per acre 

 of one substance or to a group of substances at a constant rate 

 per acre. Although many crops were grown, there were fewer 

 repeated runs of a single species or variety. There also were 

 duplicates on only one soil instead of on each of three or four 

 soils. Thus, the data on these other substances are only explora- 

 cory and are less precise than those on DDT. 



Methods of growing the test crops were essentially the same 

 as for the DDT work. 



The several isomers of different substances and the technical 

 grades of toxaphene, chlordane, and BHC were applied by spray- 

 ing onto the soils in acetone solution with an atomizer, in suc- 

 cessive portions as the soil was mixed and re-mixed. Check soils 

 were sprayed with acetone alone. Parathion was applied as a 

 25 percent dust in talc. 



Varietal responses of some crops were marked enough to be 

 significant and to justify specific mention regarding their reaction 

 to DDT. Although numerous varieties of several crops were 

 grown on the several treatments with the several substances, 

 most of the varieties of a single crop were too few, tested too 

 few times, or showed too little difference among them to justify 



