PLANT RESPONSES TO INSECTICIDES IN THE SOIL 23 



with the highly sensitive cucurbits, its presence in the soil reduces 

 plant growth to about the same extent that DDT does (table 6). 

 Its interference with germination and stand, however, appeared 

 to be even more severe than that of technical BHC on most crops 

 tested. One hundred pounds per acre produced significant reduc- 

 tions in stand with 10 of the 15 crops observed. On the average, 

 100 pounds reduced stand more than a third below the control, 

 and 400 pounds nearly two-thirds. The cucurbits were extremely 

 hard-hit at 400 pounds. 



Of the plants that became established and grew, the 100-pound 

 treatment significantly reduced yield in but one instance, musk- 

 melon. Treatments of 400 pounds, however, reduced young plant 

 growth significantly in 11 of the 15 sets of comparisons. Beans, 

 corn, cabbage and broccoli, and cotton appeared definitely less 

 sensitive than beets, the cucurbits, and tomatoes. Effects at 400 

 pounds ranged all the way from nonsignificant to complete killing 

 of the plants, with an average reduction of about 50 percent below 

 the control. 



The stability of chlordane is not definitely known. Although 

 believed to be somewhat persistent in soil, there are some indi- 

 cations that it is less stable than DDT. 4 There appears to be a 

 possibility of developing residues that will be harmful to sensi- 

 tive crops, but until more is known of the stability of chlordane 

 in soil, the probability of developing harmful residues cannot be 

 predicted. 



Results With Toxaphene 



Unlike the tests with DDT, which have continued for years 

 after a single treatment of the soil, the few exploratory tests with 

 toxaphene were suspended 3 to 4 months after starting them, 

 Toxaphene, a chlorinated camphene, is generally believed to be 

 far less stable or persistent in the soil than the chlorinated hydro- 

 carbons DDT, BHC, and chlordane (40). Table 7 shows, how- 

 ever, that for a few months at least, large amounts of toxaphene 

 in the soil are toxic to some crops. 



At 100 pounds per acre small, nonsignificant, but rather con- 

 sistent depressions in germination and stand were noted for beans, 

 cucurbits, and tomatoes. At 400 pounds the results were more 

 pronounced, averaging a reduction of about one-third below the 

 control, but they were significant only for tomato and watermelon. 

 Corn was unaffected in germination and stand even at 1,000 

 pounds of toxaphene per acre. 



In growth of young plants high variability within treatments 

 reduced the significance of results, although the means for corn 

 and winter squash suggest toxic effects. Snap beans appeared 

 tolerant. Pumpkin, summer squash, watermelon, and tomato all 

 showed significant reductions in growth at 400 and at 1,000 

 pounds per acre. If toxaphene breaks down in the soil within a 

 year and its decomposition products are nontoxic, these figures 

 will become of only academic interest — no such amount as 400 

 pounds is likely to develop as a residue from normal use. 



4 Unpublished data obtained by R. D. Chisholm, Bureau of Entomology and 

 Plant Quarantine. 



