Sibyl T. Jewson and F. Tattersfield 215 



3. The Problem of Control, 



The sine qua non of any method of control is that the treatment 

 should kill 100 per cent, of the mites and their eggs and have a minimum 

 detrimental effect upon the fungus cultures. It should not be harmful 

 to the operator and it should be easy to apply. If a chemical method is 

 to be used it is essential that the substance be volatile, not too dis- 

 agreeable, and that in its toxic action it should be reasonably speedy. 

 In flour mills it is customary to keep mites under control by scrupulous 

 cleanliness and where necessary by the application of heat. The lowest 

 lethal temperature was found by Newstead and Duvall(2) to be 49° C. 

 appHed for at least 12 hours. This latter method was not available in 

 our case as the temperatures likely to be effective against the parasite 

 would have a seriously detrimental action upon the fungus culture. A 

 fairly extensive Hst of volatile organic compounds was therefore tried 

 and their effect studied upon mites and their eggs and upon fungi. 



Ammonia was found to be the most rapidly toxic substance to mites 

 and their eggs. It had, however, a definite toxic action on fungi and 

 although it may prove of great value for ridding laboratory apparatus, 

 such as incubators, of these pests, its vapour should not be allowed to 

 play upon the cultures of fungi for any length of time. 



Pyridine was the next most rapidly toxic compound tested and 

 although it is many times less toxic than ammonia vapour, it has the 

 added advantage of not being poisonous to fungi, except in doses not 

 likely to occur in practice. As its vapour is rather disagreeable it is 

 hardly suitable for the purposes for which ammonia is recommended, 

 but for freeing fungus cultures of mite pests it can be so easily applied 

 that it should not prove in any way obnoxious to the operator. A 

 detailed description of both methods is given on p. 239. 



4. Experimental. 



The compounds tested were : 



/ Ammonia 



Pyridine 



Ammonia bases-' Anihne 



Monomethylaniline 



, Dimethylaniline 



("Benzene 



/T*'\ Toluene 



hydrocarbons [j^^pj^^halene 



{Para-dichlorbenzene 

 Carbon tetrachloride 

 Carbon bisulphide 



