Sept. 3, 1917 Quassia Extract as a Contact Insecticide 521 



leaves, and when within the interior of the leaves it might have been 

 imbibed through the proboscides of the aphids and then passed into their 

 alimentary canals. To test this probability, 58 aphids on tulip-tree 

 leaves were gently brushed into a wire-screen case, and at 8.30 a. m. they 

 were sprayed with the quassiin solution; at 12.30 p. m. only a few of them 

 were dead, but at 4.30 p. m. 88 per cent of them were dead. This experi- 

 ment shows that the quassiin did not act as a stomach poison by passing 

 from the interior of the leaves through the proboscides and then into the 

 alimentary canals of the aphids, and it is unlikely that the aphids imbibed 

 some of the solution on their bodies before they died. While tracing 

 insecticides inside aphids, the senior writer has never seen the poisons 

 either in the proboscis or in the alimentary canal. Sections also show 

 that quassia-extract solutions do not pass into the interior of plants. 



In the foregoing it is shown that neither the exhalations nor vapor from 

 the quassiin solution kill aphids, and this insecticide when applied as a spray 

 also does not act as a stomach poison ; but there yet remain the two fol- 

 lowing possible ways in which it may exert its effects upon aphids: (i) 

 The solution may enter the spiracles and come into contact with the 

 nervous system or it may cause death by suffocation, or (2) some of the 

 fine spray may be taken into the respiratory system while the aphids are 

 being sprayed. 



Regarding the first view, the senior writer (22, p. 103) has recently 

 shown that nicotine-spray solutions (not containing soap) do not enter 

 the spiracles of aphids, and it is shown on page 525 that quassia-extract 

 solutions (not containing soap) also do not enter the spiracles. If it is 

 granted that this solution does gain entrance through the spiracles but 

 does not come into contact with the nerve cells, would it kill aphids 

 within a limited time by cutting off the supply of oxygen ? To test this 

 possibility one sweet-pea plant and four nasturtium leaves, each bearing 

 many aphids, were submerged in water for ^}4 hours. A film of air sur- 

 rounded the leaves, stems, and parts of the aphids, but it was gradually 

 absorbed by the water, so that at the time at which the insects were 

 removed from the water practically all of the air had been absorbed, 

 except several minute bubbles which still adhered to the aphids. When 

 removed from the water, one-half of the pea aphids had fallen from the 

 plant to the bottom of the jar containing the water; they were apparently 

 lifeless, while those remaining on the plant moved when touched. All 

 of the nasturtium aphids exhibited signs of life when removed from the 

 water. Within a short time the inactive aphids revived, and all of the 

 submerged ones soon became normal. It is therefore evident that aphids 

 are not easily suffocated. 



To support the second view, three nasturtium leaves and two sweet-pea 

 plants, each bearing many aphids, were submerged in the quassiin solu- 

 tion for half a minute. The solution did not adhere well to the leaves and 



