352 GROWTH OF PLANTS 



the pyrethrins killed 51.1 per cent of the flies by the Peet-Grady method. 

 Fly sprays containing 0.05 per cent piperine and 0.01 per cent pyrethrins 

 were more toxic than sprays containing pyrethrins alone at a concentra- 

 tion of 0.10 per cent. 



Acetone extracts of black pepper are unsuited for use in fly sprays, since 

 they contain an oil with a very sharp, pungent odor. Piperine is almost 

 without odor and causes little or no nasal irritation. 



Tests were made of solutions of various substituted amides and pyreth- 

 rum. The presence of a methylenedioxyphenyl group increased the effec- 

 tiveness of the amides. Increasing the side-chain attached to the methylene- 

 dioxyphenyl group increased the effectiveness of the amide. 



Piperine was found to produce characteristic effects on the central ner- 

 vous system and muscles of the housefly,^^ usmg the Bodian technique. 

 The foremost effect was the destruction of the fiber tracts and vacuolation 

 of the nerve tissue of the brain, but the widespread clumping effect of the 

 chromatin of the nuclei characteristic for pyrethrum was not observed with 

 piperine. 



The action of piperine on the muscles of the housefly also appears to 

 be different from that of pyrethrum. The head muscles of houseflies that 

 were sprayed with piperine (0.5 per cent) showed evidence of tetanus and 

 Krause's membrane appeared to be enlarged so that it stood out promi- 

 nently. With pyrethrum there is fenestration of the cytoplasm, clumping 

 of the chromatin of the nuclei, and loss of striation of the muscles. 



FUMIGANTS 



Naphthalene. A severe infestation of red spider {Tetranychus telarius L.) 

 in the Institute greenhouses focused attention on a chemical means of con- 

 trolling this pest. At that time the only means known to eradicate red 

 spider were cultural practices and hosing the plants with water from a high- 

 pressure nozzle. Naphthalene was tried as a fumigant. This was probably 

 the first time that it was successfully employed for that purpose in the 

 United States, although it had been used in England a few months pre- 

 viously.^- ^^ 



At first, flake naphthalene was volatilized by means of heat, either over 

 lamps or on electric hot plates.^ It was soon found, however, that if the 

 volatilization was too rapid, the foliage of the plants was injured and the 

 mites were not killed. A slow, even distribution of naphthalene vapor 

 results in a satisfactory kill of the mites with little or no injury to the 

 plants, but this is difficult to accomplish when naphthalene is volatilized by 

 means of heat. The aid of two physical chemists was enlisted to devise a 

 method that would overcome this difficulty. 



The first suggestion was to pass a current of air from an outside source 

 over naphthalene balls arranged on shelves in a metal box.^^ The naphtha- 

 lene-laden air was blown into the greenhouse to be fumigated. By substi- 



