J. Davidson 01 



of aphids have adapted themselves in dift'erent countries; this being an 

 expression of local races and local conditions. For instance, although 

 my figures of reproduction on sugar beet are comparatively low, it is 

 feasible to expect that, in districts where sugar beet is the most widely 

 distributed favourable food plant, this reproduction figure may be 

 higher. In this respect the reproduction figures on sugar beet for 1914 

 and 1920 are interesting. To quote an example; Davis (1909) has shown 

 that Afhis maidis Fitch, prefers broom corn to Indian corn or sorghum. 

 In the struggle for existence against adverse conditions, the chances of 

 survival are greater for the strain of aphis produced on plants which 

 cause a high reproduction figure. 



It seems probable, although the data obtained from my experiments 

 are not sufficient to draw conclusions, that reproduction on certain 

 intermediate hosts may be affected by the nature of the previous host 

 on which the aphids were reared. In Tables II and IV it will be seen that 

 mangolds, red beet and sugar beet, infected with a strain of Aphis 

 rumicis which had been reared for several generations on EuonyniKs 

 europaeus, gave a higher reproduction figure than when infected with 

 a strain reared on Rumex. Similarly, higher figures were obtained on 

 these plants when they were infected with strains of Aphis rumicis 

 reared on beans and poppies. This question, however, requires further 

 investigation with reference to the factors concerned, especially the con- 

 dition of the plant and temperature conditions. 



There are the further questions of the influence of chmate, varying soil 

 conditions, agricultural methods, and the manurial treatment of crops, 

 on the constitution of the cell sap of plants. The importance of manurial 

 treatment in relation to the susceptibihty of varieties of wheat to attacks 

 of rust fungus is well known. According to Comes, nitrogenous fertihzers 

 stimulate the cells of the plants, causing the tissues to be more juicy and 

 diminishing the acidity of the cell sap. 



The ten varieties of broad beans experimented with (Table V) showed 

 little difference in the degree of susceptibihty to attacks of Aphis rumicis. 

 One might infer that the varieties Sutton's Improved Windsor and 

 Early Mazagan are shghtly more susceptible than the other varieties, 

 and that Dwarf Koyal Cluster, Sutton's Green Giant and Taylor's 

 Windsor are shghtly less susceptible. It is clear however that these 

 strains of beans are too closely alhed to give any striking difference. 

 In any case, they only represent a very hmited number of the varieties 

 obtainable. Experiments are being continued with varieties of field 

 beans. 



