EFFECTS OF DDT AND OTHER INSECTICIDES 



dered by a small creek, Rock 

 Creek, for a distance of % mile. 

 The creek, which had a flow of 

 only 0.7 cubic foot a second, con- 

 tained 34 beaver ponds in the sec- 

 tion studied and supported a large 

 population of cutthroat trout 

 (Salmo clarkii lewisi) . The area 

 was sprayed by airplane with 

 DDT in the proportion of 5 pounds 

 of the insecticide to an acre ap- 

 plied in two equal treatments. 

 The first spraying of 2.5 pounds 

 of DDT to an acre was done on 

 July 22, and a thorough coverage 

 of the stream resulted. The sec- 

 ond treatment was applied to the 

 area on July 30, at which time 

 almost none of the poison fell on 

 Rock Creek, except for a light 

 drift deposited on some parts of it. 

 After the first spraying only 11 

 dead fish were found. This kill 

 was much lighter than was ex- 

 pected considering the dosage, 

 and actually was not as great as 

 has occurred with DDT dosages 

 of less than half the quantity used 

 in this test. Certain conditions of 

 the stream may have greatly min- 

 imized the loss : a nearly continu- 

 ous chain of beaver dams slowed 

 the flow of water so that little 

 mixing of the spray with the 

 water of the creek occurred, and 

 most of the DDT remained sus- 

 pended at the surface; also, the 

 stream contained much organic 

 debris and was very muddy as a 

 result of activity by the beavers. 

 Studies have shown that the toxic 

 action of DDT may be reduced 

 under such conditions. Another 

 factor would involve the suscepti- 



bility of the cutthroat trout to this 

 poison. Although toxic levels are 

 not known for this species, trout 

 in general have been found less 

 susceptible to DDT poisoning 

 than are a number of other fresh- 

 water species. 



In conformity with the in- 

 creased feeding activities of fishes 

 observed after DDT has been ap- 

 plied to a stream, the average 

 stomach contents of 11 trout 

 taken at this time were four times 

 as great as those of 32 fish taken 

 before spraying. The riffle organ- 

 isms were greatly reduced in 

 numbers but were not entirely 

 wiped out, there being some sur- 

 vivors among all susceptible in- 

 sect larvae and nymphs. 



BLACK HILLS NATIONAL FOREST. 

 WYO. 



The Bureau of Entomology and 

 Plant Quarantine and the United 

 States Forest Service cooperated 

 in experimental attempts to con- 

 trol the Black Hills beetle in the 

 Black Hills National Forest, near 

 Sundance, Wyo. Each of five 

 plots varying in size from about 

 15 to 90 acres was sprayed aeri- 

 ally with one of the following 

 toxicants: 10 pounds of DDT to 

 an acre, 71/2 pounds of DDT to an 

 acre, 5 pounds of DDT to an acre, 



1 pound of gamma isomer of ben- 

 zene hexachloride to an acre, and 



2 pounds of gamma isomer of ben- 

 zene hexachloride to an acre. The 

 Fish and Wildlife Service investi- 

 gated the effects of these experi- 

 ments on wildlife. 



An index to the small mammal 



