346 GROWTH OF PLANTS 



killed with potassium cyanide, indicating that respiratory movements or 

 at least vital activity are necessary for penetration to take place. 



Following the study of the use of soap as a contact insecticide, the 

 toxicity of some of the common fatty acids was determined by Dills and 

 Menusan,^ using Aphis rumicis and the rose aphid (Macrosiphum rosae L.) 

 as experimental insects. Capric and lauric acids were found to be more 

 toxic than oleic, caprylic, myristic, caproic, and palmitic acids, while 

 stearic was the least toxic of the fatty acids tested. The insecticidal value 

 of the potassium soaps in decreasing order of toxicity was found to be: 

 oleate, laurate, caprate, followed by the equally toxic caprylate, myristate, 

 and palmitate, which are more toxic than the stearate and caproate. The 

 addition of nicotine to the soap solutions did not alter the order of toxicity. 

 When the soap and nicotine sulphate were combined, the toxicity due to 

 nicotine was not strictly additive; the better-spreading soaps increased the 

 effectiveness of the nicotine. 



Potassium soaps made from olive, coconut, castor, corn, palm, cotton- 

 seed, and menhaden fish oils were tested on aphids and several other species 

 of insects. Olive-oil soap, containing the highest percentage of oleate, was 

 found to be the most toxic. The phytotoxicity of the fatty acids was found 

 to be in the same order as that for their toxicity to insects. 



Pyrethrum. Although the results obtained by microscopic examina- 

 tion and the application of physical chemistry to nicotine-soap solutions 

 explained at least in part the mechanism by which nicotine acts on insects, 

 these results did not explain the mode of action of pyrethrum {Chrysanthe- 

 mum cinerariaefolmm Vis.), an insecticidal plant. ^' ^ In the case of pyreth- 

 rum sprays, a wetting agent was also found to be desirable. Nevertheless, 

 with pyrethrum, killing can take place without penetration of the tracheae. 

 This is shown by the fact that aqueous emulsions of pyrethrum are quite 

 toxic even without a wetting agent, and also by the fact that concentrated 

 pyrethrum preparations, when placed on portions of insects far removed 

 from the spiracles and other body openings, are able to cause characteristic 

 pyrethrum intoxication and death.^^ In an attempt to explain this phe- 

 nomenon, the possible penetration of the material through the integument 

 was studied by means of dyes dissolved in pyrethrum concentrate. By 

 using Sudan III and other vital stains, evidence was obtained that pyreth- 

 rum could penetrate the integument of insects at least in certain regions.^"- ^^ 



Meal worms {Tenebrio molitor L.) were painted on the dorsal surface with 

 pyrethrum extract colored with Sudan III, care being taken that none of 

 the material came in contact with the spiracles. After the insects were dead, 

 they were sectioned with a freezing microtome and examined immediately 

 under a microscope for the presence of the dye in the tissues. It was noted 

 that the trichogen and hypodermal cells were stained red (Fig. 139B). It 

 would appear that the pyrethrum extract with the dye had entered through 

 the articular membranes and the trichopores and penetrated the cells of the 

 hypodermis. Similar membranes are found between the segments and at the 



