36 



When two or more groups are introduced into the molecule the 

 toxicity is much affected by their relative positions in the ring 

 (see p. 6G). Several of the substances finally obtained are highly 

 toxic both to insects and eggs; some are being tried this year 

 on a field scale. 



This investigation, like that on partial sterilisation, raises 

 the important problem of exploiting a laboratory discovery and 

 applying it on the large scale. Between the Rothamsted Station 

 and the agricultural and horticultural industries there is the 

 important difference that the one is working with a few pounds 

 only, while the other may require in the aggregate thousands of 

 tons. It is not possible for the Research Station to bridge this 

 gap, nor to carry up to the farm stage the methods it may 

 evolve. When superphosphate was discovered at Rothamsted 

 many years ago, Lawes completely separated the factory and 

 exploitation sides from the Rothamsted experiments. In a 

 letter to the Ministry of Agriculture, published in the Journal 

 of the Ministry of Agriculture, February, 1922, Lord Elvedon 

 emphasised the lack of bridging agencies, and offered himself 

 to finance a non-profit making syndicate for the exploitation of 

 the " artificial farmyard manure " process (see p. 32). This 

 is proving a very effective way of securing development. Both 

 the insecticide and partial sterilisation work are now almost ripe 

 for extension to the factory, as also is some of the physical work 

 described above. The most suitable procedure has yet to be 

 decided. 



PLANT PATHOLOGY. 



New laboratories have been erected, to which in September, 

 1924, the Entomological and Mycological Departments migrated ; 

 work is now being done under eminently satisfactory conditions. 



In the Entomological Department Dr. Imms has concen- 

 trated the attention of the Staff on insecticides, on aphids and 

 on the gout fly of barley. The work on insecticides has already 

 been described (p. 35). 



Dr. Davidson's aphid studies have shown the important 

 connection between the nutrition of the host plant and the rate 

 of multiplication of the insects ; contrary to general belief, it is 

 the best nourished beans on which the aphids multiply most 

 rapidly. 



Certain varieties of field beans are only slightly susceptible 

 to aphid attack, and plant breeding experiments suggest that 

 this factor can be transmitted to new varieties. It appears 

 possible, therefore, that a bean might be evolved of commercial 

 value, and, at the same time, possessing considerable resistance 

 to aphid attack. No rapid progress towards the production of 

 such a variety can be expected owing to the laborious nature 

 of the work and the necessity of making detailed tests at every 

 stage. 



A pure line of the bean aphis has been carried on continuously 

 since 1920, over 80 generations having been passed through. 

 The sexual cycle appears with remarkable regularity during early 



