INSECTICIDES 353 



tuting solid solutions of naphthalene-sulphur mixture for naphthalene balls 

 on the shelves, the air could be recirculated through the apparatus rather 

 than drawn in from an outside source. This was a decided advantage when 

 fumigating in cold weather. Both methods of fumigation would maintain 

 practically a constant concentration under ordinary greenhouse tempera- 

 tures. This apparatus unfortunately proved to be too clumsy to pass along 

 the narrow aisles of a greenhouse. 



The next development was to dissolve naphthalene in a motor oil of 

 definite physical properties and spray the oil by means of a motor-driven 

 pump in a cylinder open at the top protected mth suitable baffles. ^^ In 

 this way the naphthalene was volatilized from the oil and dispersed in the 

 greenhouse air and could be maintained at a constant concentration. This 

 machine was small enough to be rolled along the greenhouse aisles. It is 

 still in use. 



Naphthalene fumigation suffers from the disadvantage that fumigations 

 repeated at too short intervals cause the tissues of plants to harden. Many 

 varieties of roses also are intolerant to naphthalene vapor. In spite of 

 these limitations naphthalene has come into general use as a greenhouse 

 fumigant both in England and in the United States. 



/3, j8'-Dichloroethyl ether. This compound was tested as a greenhouse 

 fumigant. ^"^ It was found to control Aphis rumicis, red spider, gladiolus 

 thrips {Taeniothrips simplex M orison), and adult white fly (Trialeurodes 

 vaporarium Westw.). The use of the pure compound m shallow pans with 

 vertical porous plates dipped in the liquid to give increased evaporation 

 was found to be the most practical method. An electric fan was used to 

 maintain circulation of the air. Overnight fumigations of from 14 to 17 

 hours were used. Among 44 species and varieties of plants tested, rose {Rosa 

 [hybrid tea] sp. vars. Briarcliff, Hollywood), peach (Prunus persica [L.] 

 Stokes), carnation (Dianthus sp.), and castor bean (Ricinus communis L.) 

 were among the most susceptible. 



Gases. The toxicity of ammonia, chlorine, hydrogen cyanide, hydrogen 

 sulphide, and sulphur dioxide to insects has been discussed in Chapter 5. 



Search for New Insecticides 



Colloidal sulphur. The earliest work on insecticides at Boyce Thompson 

 Institute was on sulphur ^^ under a joint project financed by the Crop Pro- 

 tection Institute and Boyce Thompson Institute. A new method of making 

 colloidal sulphur by distilling flowers of sulphur into a soap and water solu- 

 tion was discovered. 



The search for new insecticides has followed two main lines of attack. 

 Taking advantage of man's long experience with plants possessing potent 

 chemicals, particularly with drug plants, many of these have been tested 

 for insecticidal properties. Another line of attack has been the preparation 

 and testing of synthetic products. 



