Aug. 15, 1919 



Derris as an Insecticide 



195 



Table VIII. — Results of tests against aphids (Myzus persicae Sulz.) on cabbage plants, 

 using Derris powder as a dust 



Reference to Tables VII and VIII shows that the percentage of un- 

 treated aphids gradually increased from the first day of the tests onward ; 

 This increase was due to the birth of aphids on the untreated plants, 

 aphids were born likewise on the treated plants from the time the insecti- 

 cide was applied until all the reproducing females had died. Since 

 practically all of the aphids on the treated plants were dead at the close 

 of the tests, the newly born young ones must have been killed by coming 

 in close proximity to the particles of powder still remaining on the plants. 



EFFICIENCY OF DERRIS AS A STOMACH POISON AGAINST VARIOUS INSECTS 



Potato-beetle larv^. — Derris powder as a stomach poison was 

 tested on a small scale against potato-beetle larvae {Leptinotarsa decem- 

 lineata Say) at several strengths, ranging from i pound of powder to 16 

 gallons of water up to i pound to 128 gallons and was found to be very 

 effective. Practically all of the larvae were killed within 48 hours and 

 the plants were little eaten. 



Since these spray mixtures might have acted as contact poisons, 

 because the larvae were already on the plants when the latter were sprayed, 

 a second series of tests was arranged to eliminate this factor. The same 

 plants were used and from 20 to 40 larvae were placed on them one or 

 two days after they had been sprayed. The results obtained were prac- 

 tically the same as in the first series of tests. Very few living larvae 

 were found three days later and the plants were little eaten. 



When applied as a dust, Derris was equally efficient against potato- 

 beetle larvae. 



Tent caterpillars. — Derris was tested against young tent cater- 

 pillars {Malacosoma americana Fab.) in a series of strengths ranging 

 from I pound of powder to 8 gallons of water to i pound to 200 gallons. 

 All the mixtures were found to be effective. 



