194 



Journal of Agricultural Research 



Vol. XVII, No. s 



of powder to 400 gallons of water, with soap at the rate of i pound to 

 100 gallons. 



OysTER-shelIv scale. — At the rate of i pound of powder to 20 gallons 

 of water, either with or without soap in the proportion of i pound to 100 

 gallons, Derris was ineffective against the crawling young of the oyster- 

 shell scale (Lepidosaphes ulmi L.). 



While taking records of numerous greenhouse tests with Derris against 

 aphids, it was noticed that all the aphids were not killed during the first 

 24 hours but continued to die for several days. Since a contact insecti- 

 cide which continued to kill for a period of five or six days seemed an 

 anomaly, the following experiments were made to determine definitely 

 if this were the case and over how long a period this killing would extend. 



The aphids on small potted plants, were counted, and the plants were 

 then thoroughly dusted or sprayed. Paper disks were placed around the 

 plants to catch the aphids that fell. Careful counts were made ever}- day 

 until all of the aphids were gone. In these counts each aphid was 

 observed through a lens, and when necessary each one was touched wath 

 the point of a knife to determine whether it was still alive. A single 

 untreated plant was used w'ith each series as a control. 



The aphids began falling from the plants wathin an hour, but for the 

 first 24 hours most of those on the paper disks were alive. After this the 

 aphids that fell were practically all dead. In the case of the dusted plants 

 a few dead aphids were found clinging to the leaves the third day, but 

 as a rule only the living ones remained on the plants. 



These tests fully confirmed the earlier observations and, furthermore, 

 showed that some of the aphids did not die until five or six days after 

 the application of the insecticide. The results are presented in Tables 

 VII and VIII. 



Table VII. — Results of tests against nasturtium aphids (Aphis rumicis L.), using Derris 

 powder in spray mixtures at the rate of i pound of powder to 100 gallons of water 



